A Review on Power Electronics Technologies for Electric Mobility

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Review A Review on Power Electronics Technologies for Electric Mobility

Joao L. Afonso , Luiz A. Lisboa Cardoso , Delﬁm Pedrosa, Tiago J. C. Sousa, Luis Machado, Mohamed Tanta and Vitor Monteiro * Centro ALGORITMI, University of Minho, 4800-058 Guimaraes, Portugal; [email protected] (J.L.A.); [email protected] (L.A.L.C.); [email protected] (D.P.); [email protected] (T.J.C.S.); [email protected] (L.M.); [email protected] (M.T.) * Correspondence: [email protected]

Received: 7 October 2020; Accepted: 26 November 2020; Published: 1 December 2020

Abstract: Concerns about greenhouse gas emissions are a key topic addressed by modern societies worldwide. As a contribution to mitigate such effects caused by the transportation sector, the full adoption of electric mobility is increasingly being seen as the main alternative to conventional internal combustion engine (ICE) vehicles, which is supported by positive industry indicators, despite some identified hurdles. For such objective, power electronics technologies play an essential role and can be contextualized in different purposes to support the full adoption of electric mobility, including on-board and off-board battery charging systems, inductive wireless charging systems, unified traction and charging systems, new topologies with innovative operation modes for supporting the electrical power grid, and innovative solutions for electrified railways. Embracing all of these aspects, this paper presents a review on power electronics technologies for electric mobility where some of the main technologies and power electronics topologies are presented and explained. In order to address a broad scope of technologies, this paper covers road vehicles, lightweight vehicles and railway vehicles, among other electric vehicles.

Keywords: power electronics; electric mobility; electrical power grid; battery charging systems; uniﬁed traction and battery charging systems; inductive wireless power transfer; railway systems; smart grids

1. Introduction Throughout the 20th century, scientific development radically altered societies in different sectors. In particular, the expansion of vehicle traffic has been intense, generating serious environmental concerns, such as the increase in greenhouse gas emissions, urban air pollution and dependence on fossil fuels [1]. Worldwide, 29% of final energy consumption is due to the transport sector, representing 23% of the carbon dioxide (CO2) emissions [2]. Specifically, in the European Union, 33% of the final energy consumption is due to the transportation sector, where 82% are associated with road transport [3]. To overcome this concern, electric mobility is pointed out as a prominent answer to diminish CO2 emissions, as well as to contribute to the sustainability of the transportation sector [4]. A global review focused on electric mobility in a smart grid environment is presented in [5], where future perspectives regarding industrial information technologies are also established. A broad investigation concerning the electrification of the transportation sector is presented in [6], where key topics are addressed, namely electric machines, powertrain architectures, power electronics technologies, and energy storage systems. A comprehensive overview of the status and issues of electric mobility is presented in [7,8], highlighting and exploring its challenges and opportunities. Despite the positive industry indicators, the full adoption of electric mobility is facing some hurdles, as investigated in [9]. Indeed, the shifting to

Energies 2020, 13, 6343; doi:10.3390/en13236343 www.mdpi.com/journal/energies Energies 2020, 13, 6343 2 of 61 electric mobility is already underway, with several electric vehicle (EV) models commercially available worldwide, embedding diverse scientific and technological developments, as presented in [10–12]. Moreover, electric mobility is also underway for military use, either in standalone applications [13] or in association with military-based microgrid systems, as discussed in [14]. The paradigm of electric mobility is only feasible due to advances in different technologies, mainly regarding energy storage systems and electric machines. As examples, an investigation about how lithium transformed electric mobility is presented in [15], the future of batteries in an electrified fleet is explored in [16], the present state and forthcoming aims in propulsion technologies for electric mobility is investigated in [17], and a survey regarding electric machines contextualized with electric mobility is presented in [18]. Despite all the advantages, a key point to assure the sustainability of electric mobility is the source of the electrical energy, which should preferably come from renewable resources, such as wind, solar, and hydro. These renewable energy sources, together with environmentally friendly energy storage technologies, are the pillars of a profound and exciting revolution toward smart grids, contributing to reducing costs and greenhouse gas emissions, as well as to achieving power optimization [19]. Based on these technologies, the role of electric mobility in smart grids in scrutinized in [20], opportunities and challenges for smart grids are studied in [21,22], demand-side power management strategies are presented in [23], an integrated solution for electric mobility and renewables is proposed in [24], the electric mobility integration with energy storage systems and power grids is investigated in [25], a broad methodology for exploiting the flexibility and benefits offered by electric mobility is presented in [26], and the key challenges and advancements regarding electric mobility are discussed in [27]. As the power grids were not planned to deal with a large-scale introduction of electric mobility, aspects of power quality cannot be ignored. The integration and consequent impact of electric mobility on the power grid is addressed in several works in the literature, as well as the incorporation in different and specific parts of the world, as demonstrated in [28–32]. From this point of view, electric mobility can be seen as a problem for the power grid, however, by adopting convenient storage policies, it is possible to establish smart charging strategies [33]. This represents an important added value, since most private vehicles are parked during the major part of their useful lifetime [34]. Therefore, by using bidirectional EV battery-charging systems (EVBC), it is possible to obtain a bidirectional power flow, between the EV and the power grid, allowing the grid-to-vehicle (G2V) mode and also to return part of the energy stored back to the power grid, i.e., the vehicle-to-grid (V2G) mode. In fact, the V2G operation mode is an important contribution in order to balance power demand in the power grid, as demonstrated in [35,36], however, it can also contribute to degrading the EV battery. This operation mode requires communication with the regulator of the power grid and will have a predominant role in smart grids. Moreover, electric mobility can also be fundamental to provide ancillary services, both from the smart grid and smart home perspectives, as experimentally validated in [37] with different and cooperative operation modes. Specifically, the possibility of using electric mobility to provide power quality services is validated in [38], while the possibility of providing uninterruptible power supply (UPS) functionalities is validated in [39]. By analyzing all these aspects in a broad perspective, power electronics systems are recognized as a common denominator [40], providing new challenges between sustainable development and scientific development [41]. Several review papers can be found in the literature, but focusing only on specific topics of power electronics for electric mobility, e.g., in [42] are presented advancements and challenges for electric mobility, addressing the battery chargers, wireless solutions and with a special focus on energy storage technologies; in [43,44] are presented extended reviews about the state-of-the-art of battery technologies and respective management systems; in [45] is presented a review of on-board EVBC, but only in the perspective of integrated systems; and in [46] is also presented a review focusing only on EVBC. In [47] are presented global trends of EVBCs, but only focusing on the fast battery charging; in [48,49] are presented reviews of technologies for electric mobility, but only focusing on wireless battery charging systems; and in [50] is presented a review exclusively about electric motor Energies 2020, 13, 6343 3 of 61 drivelines in commercial EVs. Throughout the paper, when relevant, other references for review papers are also introduced. By contrast with the other review papers, this paper aims to provide an overview of the variety of technologies, covering: (i) the aggregation and accurate explanation, in a single paper, of the main technologies and opportunities of electric mobility; (ii) a broad review of power electronics topologies and architectures for electric mobility (including on-board and off-board EVBC, stationary and dynamic inductive wireless EVBC, unified traction and EVBC, operation modes for supporting the power grid, and innovative solutions for electrified railway systems); (iii) the framework of power electronics systems with future perspectives in the scope of electric mobility. The following sections of this paper are organized as follows. Section2 introduces road vehicles, where an overview of EVBC (on-board and off-board), inductive wireless charging systems, and unified systems is presented. Section3 presents a review regarding power electronics for railway systems, specifically highlighting power quality compensators, power management converters, and converters for machine drives and for regenerative braking. Section4 introduces a review of power electronics for other EVs, such as ships, aircraft, trucks, buses, forklifts, motorcycles and bicycles. Section5 finalizes the paper with the main conclusions.

2. Power Electronics for Road Electric Vehicles This section presents a review regarding the different technologies of road EVs (i.e., vehicles equipped with an electric power-train, which are normally used to transport people or goods by road). In such road EVs, power electronics systems are used to drive the electric machine and charge the battery. In this manner, reviews are presented regarding technology and power electronics structures for EVBC (including the auxiliary battery), traction systems, inductive wireless charging systems, and uniﬁed traction and EVBC.

2.1. Power Electronics for Battery Charging Systems EV battery chargers (EVBC) are classified into two main groups in relation to their position to the EV, namely on-board or off-board, i.e., inside or outside the EV. By comparing both approaches, it is possible to identify advantages and disadvantages of each charging architecture. For instance, an on-board EVBC offers more flexibility in terms of the possibility of charging the battery at different places, but currently has its charging power limited to a few kW, besides representing an additional weight to the EV. On the other hand, an off-board EVBC is installed in a specific place (public or private) and the EV must travel there to perform the battery charging, but the charging power can be higher, up to dozens or hundreds of kW. Both on-board and off-board EVBCs are constituted by power electronics stages in order to guarantee a proper interface between the power grid and the EV battery. Therefore, and considering the wide variety of power electronics converters, several perspectives can be adopted. The most common structures are based on alternating current–direct current (ac-dc) converters guaranteeing a sinusoidal grid current (i.e., preserving power quality on the power grid side) and a dc–dc converter guaranteeing controlled current and voltage on the EV battery (i.e., preserving the battery lifetime), in both on-board and off-board cases [51–53]. However, other structures can be adopted, including the possibility of single power stage structures. A classification of the different structures, based on single or double power stages, is presented in Figure1. As emphasized, this classification considers the on-board and off-board possibilities. As an illustrative example, Figure2 shows an EV considering both on-board and off-board EVBCs and the respective interface with the power grid. As shown, the on-board EVBC is connected to the power grid through a single-phase interface with the possibility of a bidirectional power flow, while the off-board EVBC is connected to the power grid through a three-phase interface, also considering the possibility of a bidirectional power flow, allowing both G2V and V2G operation modes. This is an increasingly more important feature for smart grids and smart homes, although it normally implies in a more complex implementation of the power stages. Energies 2020, 13, x FOR PEER REVIEW 4 of 63

When looking to the internal constitution of the power converters, it is possible to recognize that several topologies can be adopted, including multilevel and interleaved topologies [54–59]. It is also possible to have topologies with or without galvanic isolation [60–62]. An ample comparison among off-board EVBCs based on current-source and voltage-source structures is presented in [63], showing details of the operation principle for both structures, as well as a comparison in terms of the total harmonic distortion of the power grid currents, estimated losses distribution, and estimated efficiency for a maximum operating power of 50 kW. New architectures of on-board EVBCs can be found in [64,65]. The topology proposed in [64] has a reduced number of components, four metal oxide semiconductor field effect transistor (MOSFETs) and four diodes, for operating with a multilevel feature (five voltage levels) and sinusoidal current; however, it only allows unidirectional operation, which is its main disadvantage. The topology proposed in [65] allows bidirectional power operation (i.e., G2V and V2G modes), where the bridgeless structure and the possibility to operate in interleaved mode are its main advantages; however, the maximum voltage applied to each semiconductor is double when compared with bridge topologies. A power electronics topology based on a single-switch is proposed in [66], allowing the operation with three voltage levels. The topology is controlled with a model predictive control strategy model, guaranteeing a sinusoidal current and unitary power factor, but only permitting unidirectional power flow, representing the main limitation for modern EVBCs. A high-density, high-efficiency, isolated on-board EVBC is presented in [67], where the introduction of silicon carbide (SiC) devices is its main contribution, since the topology is based on two power stages constituted by traditional power converters. The topology guarantees sinusoidal current, but only permits unidirectional power flow. A new SiC integrated power module for EV applications is proposed in [68], where the possibility to integrate multiple functional elements, as dc link capacitors or gate-drivers, is highlighted as a key contribution. A comprehensive review regarding the state-of-the-art and the future trends for high-power on-board EVBCs is presented in [47], considering both unidirectional and bidirectional power flow, and both integrated and non-integrated structures. A global perspective, considering state-of-the-art and future trends and challenges, of power and industrial electronics for EVs is investigated in [69], where different possibilities for the EVBC structures are presented. Taking into account the need to perform the EVBC as fast as possible, more specific requirements are necessary regarding power electronics. A technology overview of fast battery charging for EVs is presented in [70], where are discussed Energiesdesign2020, 13 considerations, 6343 of fast EV battery charging stations and the most conventional power4 of 61 electronics topologies.

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2.1.1. WhenOn-Board looking Battery to theCharging internal Systems constitution of the power converters, it is possible to recognize that several topologies can be adopted, including multilevel and interleaved topologies [54–59]. It is alsoRegarding possible to on-board have topologies EVBCs, more with orspecifically, without galvanic for power isolation electronics [60– converters62]. An ample for the comparison front-end stage,among when off-board focusing EVBCs on unidirectional based on current-source converters and, the voltage-source main topology structures is based on is presented the diode inac-dc [63], convertershowing detailsfollowed of theby operationthe boost-type principle dc–dc for both converter structures, (i.e., as the well unidirectional as a comparison power in terms factor of correction).the total harmonic This topology distortion has of thethe power main gridadvantage currents, of estimated using only losses one distribution, switching and device, estimated still guaranteeingefficiency for the a maximum operation operatingwith sinusoidal power current of 50 kW. and Newunitary architectures power factor. of on-boardThis topology EVBCs is shown can be infound Figure in 3a. [64 On,65]. the The other topology hand, in proposed a future inperspective [64] has a of reduced electric number mobility of as components, a support for four the smart metal grid,oxide a semiconductorbidirectional operation field effect is transistoressential, (MOSFETs)in which the and four-quadrant four diodes, forac-dc operating converter with is used a multilevel as the mainfeature topology (five voltage (i.e., levels)a bidirectional and sinusoidal full-bridge) current;. however,This converter, it only allowsshown unidirectional in Figure 3b, operation, allows bidirectionalwhich is its main operation disadvantage. (G2V and The V2G topology modes) proposed without inneglecting [65] allows the bidirectional sinusoidal powercurrent operation and the unitary(i.e., G2V power and V2G factor. modes), These where two topologies the bridgeless are structuremore commonly and the possibility used. However, to operate when in interleaved the main objectivemode are is itsto reduce main advantages; the need for however,passive filters the maximum coupled to voltagethe power applied grid and to eachthe maximum semiconductor current is indouble each semiconductor, when compared to with maintain bridge the topologies. characteristic A power of bidirectional electronics topology operation, based the on main a single-switch choice lies inis proposedinterleaved in type [66], allowingtopologies. the Figure operation 3c shows with three the voltagemost common levels. Thebidirectional topology isinterleaved controlled full- with bridgea model ac-dc predictive topology, control consisting strategy of two model, four-quadr guaranteeingant ac-dc a converters sinusoidal (i.e., current dual-phase and unitary interleaved power topology).factor, but onlyWhen permitting the main unidirectional objective is power to re flow,duce representingthe maximum the mainvoltage limitation applied for to modern each semiconductor,EVBCs. A high-density, as well as to high-e reducefficiency, the requiremen isolatedts on-board of passive EVBC filters is coupled presented to the in power [67], where grid, the the mainintroduction choice lies of silicon in multilevel carbide (SiC)topologies. devices Figure is its main3d shows contribution, one of the since most the topologycommon ismultilevel based on bidirectional ac-dc topology (i.e., a topology of five voltage levels).

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Energies 2020, 13, 6343 5 of 61

Energies 2020, 13, x FOR PEER REVIEW 5 of 63 two power stages constituted by traditional power converters. The topology guarantees sinusoidal current, but only permits unidirectional power flow. A new SiC integrated power module for EV Smart Grid applications is proposed in [68], where thePower possibility Management to integrate multiple functional elements, as dc link capacitors or gate-drivers, is highlighted as a key contribution. A comprehensive review regarding the state-of-the-art and the future trendsSmart for high-power on-board EVBCs is presented in [47], Grid considering both unidirectional and bidirectional power flow, and both integrated and non-integrated structures. A global perspective, considering state-of-the-art and future trends and challenges, of power and industrial electronics for EVs is investigated in [69], where different possibilities for the EVBC structures are presented. Takingon-board into account theEV need tooff-board perform the EVBC as fast as possible, battery more specific requirements are necessaryEVBC regarding power electronics.EVBC A technology overview of fast battery charging for EVs is presented in [70], where are discussed design considerations of fast EV battery charging stations and the most conventional power electronics topologies. Communication Power (slow charging) Power (fast charging) 2.1.1. On-BoardFigure 2. Battery EV considering Charging both Systems on-board and off-board battery chargers and the respective interface with the electrical power grid, allowing bidirectional power flow and communication. Regarding on-board EVBCs, more specifically, for power electronics converters for the front-end stage,2.1.1. when On-Board focusing Battery on unidirectional Charging Systems converters, the main topology is based on the diode ac-dc converter followed by the boost-type dc–dc converter (i.e., the unidirectional power factor correction). Regarding on-board EVBCs, more specifically, for power electronics converters for the front-end This topologystage, when has focusing the main on unidirectional advantage of converters using only, the onemain switching topology is device, based on still theguaranteeing diode ac-dc the operationconverter with followed sinusoidal by currentthe boost-type and unitary dc–dc powerconverter factor. (i.e., Thisthe topologyunidirectional is shown power infactor Figure 3a. On thecorrection). other hand, This intopology a future has perspective the main advantage of electric of mobilityusing only as one a support switching for device, the smart still grid, a bidirectionalguaranteeing operation the operation is essential, with sinusoidal in which current the four-quadrantand unitary power ac-dc factor. converter This topology is used is shown as the main topologyin Figure (i.e., 3a. a bidirectional On the other hand, full-bridge). in a future This perspective converter, of electric shown mobility in Figure as a support3b, allows for the bidirectional smart operationgrid, (G2Va bidirectional and V2G operation modes) is withoutessential, neglectingin which thethe four-quadrant sinusoidal ac-dc current converter and theis used unitary as the power main topology (i.e., a bidirectional full-bridge). This converter, shown in Figure 3b, allows factor. These two topologies are more commonly used. However, when the main objective is to reduce bidirectional operation (G2V and V2G modes) without neglecting the sinusoidal current and the the needunitary for passivepower factor. filters These coupled two to topologies the power are grid more and commonly the maximum used. currentHowever, in when each semiconductor,the main to maintainobjective the is to characteristic reduce the need of for bidirectional passive filters operation,coupled to the the power main grid choice and the lies maximum in interleaved current type topologies.in each Figuresemiconductor,3c shows to themaintain most the common characteristic bidirectional of bidirectional interleaved operation, full-bridge the main ac-dcchoice topology,lies consistingin interleaved of two four-quadrant type topologies. ac-dc Figure converters 3c shows (i.e.,the most dual-phase common interleavedbidirectional topology).interleaved full- When the main objectivebridge ac-dc is totopology, reduce theconsisting maximum of two voltage four-quadr appliedant ac-dc to each converters semiconductor, (i.e., dual-phase as well interleaved as to reduce the requirementstopology). of passiveWhen the filters main coupled objective to theis powerto reduce grid, the the maximum main choice voltage lies in applied multilevel to topologies.each semiconductor, as well as to reduce the requirements of passive filters coupled to the power grid, the Figure3d shows one of the most common multilevel bidirectional ac-dc topology (i.e., a topology of main choice lies in multilevel topologies. Figure 3d shows one of the most common multilevel five voltagebidirectional levels). ac-dc topology (i.e., a topology of five voltage levels).

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Figure 3. Main single-phase power electronics converters used as front-end power stage of on-board electric vehicle battery-charging system (EVBC): (a) unidirectional power factor correction; (b) bidirectional full-bridge; (c) bidirectional interleaved full-bridge; (d) bidirectional multilevel. EnergiesEnergies2020, 132020, 6343, 13, x FOR PEER REVIEW 6 of 636 of 61

Figure 3. Main single-phase power electronics converters used as front-end power stage of on-board Whenelectric focusing vehicle on unidirectionalbattery-charging converterssystem (EVBC): for the(a) back-endunidirectional stage, power the factor main correction; topology is(b) based on the buck-typebidirectional dc–dc full-bridge; converter. (c) Thisbidirectional converter, interleaved shown full-bridge; in Figure (d)4 bidirectionala, is easily multilevel. controllable, as it only has one switching device, and allows the fundamental requirements of battery charging to be met, When focusing on unidirectional converters for the back-end stage, the main topology is based i.e., chargingon the buck-type with constant dc–dc converter. current (CC)This converter, and constant show voltagen in Figure (CV) 4a, is stages. easily controllable, Additionally, as it when only the objectivehas one is to switching guarantee device, operation and allows in bidirectional the fundamenta powerl requirements mode, the mainof battery topology charging that to can be met, be used is thei.e., half-bridge charging with dc–dc constant converter. current This (CC) converter and cons istant shown voltage in (CV) Figure stages.4b. This Additionally, topology when operates the in buckobjective mode when is to itguarantee is necessary operation to charge in bidirectional the batteries power (i.e., mode, G2V the mode), main andtopology operates that can in boost be used mode whenis it the is necessaryhalf-bridge to dc–dc discharge converter. the batteriesThis converter (i.e., V2Gis shown mode). in Figure Thistopology 4b. This topology has as an operates added in value the possibilitybuck mode of when guaranteeing it is necessary the batteryto charge charging the batte inries CC-CV (i.e., G2V mode, mode), as welland operates as controlling in boost the mode battery discharge,when it which is necessary can use to CCdischarge control the algorithms batteries (i.e or., constantV2G mode). power This topology discharge has algorithms. as an added As value is easy to observe,the possibility only one of guaranteeing switching device the battery is controlled charging in in CC-CV each operating mode, as well mode as controlling (i.e., one inthe buck battery mode discharge, which can use CC control algorithms or constant power discharge algorithms. As is easy and another in boost mode), providing added value for the optimization of the converter eﬃciency. to observe, only one switching device is controlled in each operating mode (i.e., one in buck mode However, this converter is non-isolated. Focusing on this last point, when it is necessary to guarantee and another in boost mode), providing added value for the optimization of the converter efficiency. galvanicHowever, isolation, this theconverter main topologyis non-isolat thated. can Focusing be used on isthis shown last point, in Figure when4 itc. is This necessary topology to guarantee is simple to control,galvanic but only isolation, allows the unidirectional main topology operation. that can be With used this is shown topology, in Figure it is possible4c. This topology to control is thesimple battery chargingto control, process, but mainly only allows ensuring unidirectional controllability operation. in CV With mode. this Ontopology, the other it is hand, possible when to control it is pertinent the to guaranteebattery charging the operation process, in mainly bidirectional ensuring mode, controllability the logical in CV option mode. is On to usethe other the topology hand, when presented it is in Figurepertinent4d, which to guarantee is similar the tooperation that of Figurein bidirectional4c, but in mode, which the the logical converters option is are to fullyuse the controlled topology on both dcpresented sides, beingin Figure called 4d, awhich dual activeis similar bridge. to that With of Figure this last 4c, topology,but in which it is the possible converters to either are fully charge (i.e., G2Vcontrolled mode) on or both discharge dc sides, (i.e., being V2G called mode) a dual the active battery bridge. in aWith similar this last way topology, to the topology it is possible shown to in Figureeither4b. charge (i.e., G2V mode) or discharge (i.e., V2G mode) the battery in a similar way to the topology shown in Figure 4b.

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FigureFigure5a presents 5a presents an example an example of a developedof a develope on-boardd on-board EVBC. EVBC. In this In system,this system, the topologythe topology shown in Figureshown3d wasin Figure used 3d as was the front-endused as the converter front-end and converter the topology and the showntopology in shown Figure 4inb Figure was used 4b was as the back-endused converter.as the back-end This converter. ﬁgure also This shows figure some also shows results some obtained results during obtained the during experimental the experimental validation validation of this on-board EVBC [37]. These results are merely illustrative of the operation principle, of this on-board EVBC [37]. These results are merely illustrative of the operation principle, having been having been obtained for the front-end converter. The result of Figure 5b shows the current on the obtained for the front-end converter. The result of Figure5b shows the current on the power grid side, power grid side, allowing to verify that it has a sinusoidal waveform and it is in phase with the power allowinggrid voltage. to verify This that result it has is related a sinusoidal to the G2V waveform operation and mode. it is The in phase result within Figure the power5c also shows grid voltage. the This result is related to the G2V operation mode. The result in Figure5c also shows the current and the voltage on the power grid side, but, in this case, during the V2G operation mode (i.e., the current and the voltage are in phase opposition). Energies 2020, 13, x FOR PEER REVIEW 7 of 63

Energiescurrent2020, 13and, 6343 the voltage on the power grid side, but, in this case, during the V2G operation mode (i.e.,7 of 61 the current and the voltage are in phase opposition).

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FigureFigure 5. On-board 5. On-board EVBC: EVBC: (a ()a example) example ofof aa developeddeveloped laboratory laboratory prototype; prototype; (b) (ebxperimental) experimental results results duringduring the the grid-to-vehicle grid-to-vehicle (G2V) (G2V mode) mode ( v(vgg:: 100 V/div V/div/i/gi:g 10: 10 A/div A/div | 5| ms/div);5 ms/div); (c) (Experimentalc) Experimental results results duringduring the the vehicle-to-grid vehicle-to-grid (V2G) (V2G mode) mode ( v(vgg:: 100 V/div V/div/i/gi:g 10: 10 A/div A/div | 5| ms/div).5 ms/div).

2.1.2.2.1.2. Off -BoardOff-Board Battery Battery Charging Charging Systems Systems RegardingRegarding off -boardoff-board EVBC, EVBC, as as for for the the on-boardon-board counterpart, counterpart, there there are are several several options, options, where where the the mostmost common common are are based based on on aa double-stagedouble-stage structure. structure. For Forthe front the front-end,-end, as these as are these fast areEVBC, fast the EVBC, the topologiestopologies that interface interface with with the the power power grid grid are are of ofthe the three three-phase-phase type. type. Common Common unidirectional unidirectional topologiestopologies are are based based on on VIENNA-type VIENNA-type structures, structures, as as the the exampl examplee shown shown in inFigure Figure 6a.6 Thisa. This topology topology consists of a three-phase diode ac-dc converter and an arrangement of switching devices consists of a three-phase diode ac-dc converter and an arrangement of switching devices (bidirectional (bidirectional and bipolar cell) connected between each phase of the power grid and the common and bipolar cell) connected between each phase of the power grid and the common point at the midpoint point at the midpoint of the dc link. This topology only allows unidirectional operation, which may of thebe dca disadvantage link. This topology for off-board only charging allows unidirectional systems in the operation,future. On the which other may hand, be the a disadvantage three-phase for off-boardtopology charging that guarantees systems inbidirectional the future. operation On the other and sinusoidal hand, the currents three-phase on the topology power grid that sid guaranteese, in bidirectionalboth G2V operation and V2G and modes, sinusoidal is the full currents-bridge on topology, the power as shown grid side, in Figure in both 6b. G2V This and topology V2G modes, is is thecomposed full-bridge of three topology, legs. asThe shown dc link in is Figure divided,6b. where This topology the midpoint is composed is connected of three to the legs. power The grid dc link is divided,neutral. where The maximum the midpoint voltage is connectedapplied to each to the semiconductor power grid neutral.is half the The voltage maximum of the voltagedc link and applied to eachthe semiconductormaximum current is halfin each the semiconductor voltage of the dccorresponds link and the to the maximum maximum current current in in each each semiconductor phase of correspondsthe power to grid. the In maximum this context, current when it in is each desired phase to minimize of the power the maximum grid. In current this context, to which when each it is desiredsemiconductor to minimize is subjecte the maximumd, one possible current solution to which is to eachuse interleaved semiconductor topologies. is subjected, Figure 6c onepresents possible a three-phase topology of the interleaved type (double-phase). This topology consists of using, in solution is to use interleaved topologies. Figure6c presents a three-phase topology of the interleaved parallel, two topologies equal to that shown in Figure 6b. Thus, the current of each phase of the power type (double-phase). This topology consists of using, in parallel, two topologies equal to that shown grid is divided by the two legs of each phase, allowing, at the expense of more hardware, the in Figuremaximum6b. Thus, current the to currentbe decreased of each in each phase switching of the power device.grid In addition, is divided by applying by the two an adequate legs of each phase,modulation allowing, strategy at the (with expense two carriers of more wit hardware,h a phase-shift the of maximum 180°), it is possible current to to significantly be decreased reduce in each switchingthe ripple device. of the In addition, resulting current by applying in each an phase adequate of the modulation power grid. strategy This characteristic (with two carriers is very with a phase-shiftimportant, of since 180 it◦), can it isalso possible be useful to to significantly reduce the need reduce for passive the ripple filters of in the the resultingcoupling to current the power in each phasegrid. of the As power mentioned, grid. This with characteristic the interleaved is very topology, important, the maximum since it can current also be applied useful to to reduce each the needsemiconductor for passive filters is reduced, in the couplinghowever, tothe the maximum power grid. voltage As applied mentioned, to each with semiconductor the interleaved remains topology, the maximumhalf the voltage current of the applied dc link. toIn eachthis sense, semiconductor when the object is reduced,ive is to decrease however, the the maximum maximum voltage voltage appliedapplied to each to each semiconductor switching device, remains the logical half the solution voltage consists of the dcof using link. Inmultilevel this sense, topologies. when the Figure objective 6d shows the most frequently used multilevel three-phase topology. As with the three-phase is to decrease the maximum voltage applied to each switching device, the logical solution consists of topologies previously presented, this one also has a split dc link. This topology allows the generation using multilevel topologies. Figure6d shows the most frequently used multilevel three-phase topology. of more voltage levels, and so it is possible to reduce the requirements of the passive filters coupled As withwith the the three-phasepower grid. topologies previously presented, this one also has a split dc link. This topology allows the generation of more voltage levels, and so it is possible to reduce the requirements of the passive filters coupled with the power grid. On the other hand, in relation to power electronics converters for the back-end stage of off-board EVBC, the unidirectional topology based on an interleaved structure is the one that allows the performance of the off-board EVBC to be optimized, because the currents are high. Multi-phase interleaved topologies can be employed, however, the simplest is the double-phase, as shown in Figure7a. As mentioned, this topology allows the operation in interleaved mode, which is a very important feature. In addition to this topology, Figure7b shows a multilevel topology that also allows the operation in a very similar way to a double-phase interleaved converter. This converter has as its Energies 2020, 13, 6343 8 of 61 main differentiating factor the possibility of operation with different voltage levels, i.e., the voltage produced by the converter is dependent on the voltage at each dc interface. This converter allows operation in bidirectional mode and, as shown, it consists of four semiconductors, with only two semiconductors being used in each operation mode (i.e., two in buck mode for G2V and two in boost mode for V2G). When galvanic isolation is required, the most common base topologies used are those shown in Figure4c,d. The latter, however, is designed for higher-order powers. In addition to these possibilities, it is also possible to use three-phase topologies, as shown in Figure7c. The principle of operation of this topology is very similar to the isolated topologies previously presented, but as an added value, the operating current is divided by the three-phases, each of them feeding a separated isolation transformer, that handles one third of the total operating power. Another possibility of isolated topology that also allows dividing the operating power is shown in Figure7d. This topology is constituted by the parallel association of several topologies. In addition to allowing to divide the operating power, this topology can also be readjusted for other configurations, e.g., keeping one of the dc interfaces common to all converters (i.e., with a parallel arrangement) and the other dc interface with a serial arrangement of all converters. Energies 2020, 13, x FOR PEER REVIEW 8 of 63

D1 D3 D5 S1 S3 S5 ieva vdc ieva vdc C1 C1 2 2 S1 S2 L1 L1

L2 L2 S3 S4 L vdc L vdc 3 C2 3 C2 2 S2 S4 S6 2 D2 D4 D6 S5 S6 (a) (b)

S1 S3 S5 S7 S9 S11 S1 S5 S9

iga1 vdc vdc C1 C1 2 S2 D1 S6 D3 S10 D5 2 L1 ieva

L2 L1

L3 L2 L4

L5 L3 vdc S3 D2 S7 D4 S11 D6 vdc L6 C2 C2 2 2

S2 S4 S6 S8 S10 S12 S4 S8 S12

FigureFigure 6. Main 6. Main three-phase three-phase power power electronics electronics converters converters used as as front-end front-end power power stage stage of off-board of oﬀ-board EVBC:EVBC: (a) unidirectional(a) unidirectional VIENNA-type VIENNA-type topology; topology; ((bb)) bidirectionalbidirectional full-bridge; full-bridge; (c) (cbidirectional) bidirectional interleavedinterleaved full-bridge; full-bridge; (d) ( bidirectionald) bidirectional multilevel. multilevel.

FigureOn8a the presents other hand, an illustrative in relation to example power electronic of a developeds converters o ﬀ-board for the EVBCback-end and stage Figure of off-board8b shows a detailEVBC, of the powerthe unidirectional electronics converter.topology based In this on system, an interleaved the topology structure shown is inthe Figure one that6c was allows used the as the performance of the off-board EVBC to be optimized, because the currents are high. Multi-phase front-end converter and the topology shown in Figure7b was used as the back-end converter. Figure8b interleaved topologies can be employed, however, the simplest is the double-phase, as shown in presents some results obtained during the experimental validation of this oﬀ-board EVBC [52]. As for Figure 7a. As mentioned, this topology allows the operation in interleaved mode, which is a very the on-boardimportant EVBC, feature. these In addition results to are this also topology, merely Figu illustrativere 7b shows of a themultilevel operation topology principle, that also having allows been obtainedthe operation for the front-end in a very similar converter. way to The a double-phase result of Figure interleaved8c shows converter. the three This currents converter on has the as power its grid side,main allowingdifferentiating to verify factor that the possibility they have of a operation sinusoidal with waveform different andvoltage that levels, they i.e., are the in phasevoltage with the voltages.produced by the converter is dependent on the voltage at each dc interface. This converter allows operation in bidirectional mode and, as shown, it consists of four semiconductors, with only two semiconductors being used in each operation mode (i.e., two in buck mode for G2V and two in boost mode for V2G). When galvanic isolation is required, the most common base topologies used are those shown in Figure 4c,d. The latter, however, is designed for higher-order powers. In addition to these possibilities, it is also possible to use three-phase topologies, as shown in Figure 7c. The principle of operation of this topology is very similar to the isolated topologies previously presented, but as an added value, the operating current is divided by the three-phases, each of them feeding a separated isolation transformer, that handles one third of the total operating power. Another possibility of isolated topology that also allows dividing the operating power is shown in Figure 7d. This topology is constituted by the parallel association of several topologies. In addition to allowing to divide the operating power, this topology can also be readjusted for other configurations, e.g., keeping one of the dc interfaces common to all converters (i.e., with a parallel arrangement) and the other dc interface with a serial arrangement of all converters.

Energies 2020, 13, x FOR PEER REVIEW 9 of 63

S1 vdc1 S1 S2 C1 S2 2

L1 C v vdc1 C1 3 dc2

L2 vdc1 C2 vdc2 C2 S3 D1 D2 2 S4 Energies 2020, 13, 6343 L2 9 of 61 Energies 2020, 13, x FOR PEER REVIEW(a) (b) 9 of 63

L1 S1 S3 S5 S7

n1 : n2 S1 S3 S5 S7 S9 S11 S1 vdc1 L1 S1 S2 C1 S2 n1 : n2 2 vdc1 C1 C2 vdc2 L1 L1 C v vdc1 C1 S2 S4 S6 S8 3 dc2

n1 : n2 L2 vdc1 C1 C2 vdc2 vdc1 L2 C2 vdc2 C2 S3 D1 D2 2 S4 S9 S11 S13 L2 S15 n1 : n2 n1 : n2 L3 (a)L2 (b)

S2 S4 S6 S8 S10 S12

S10 S12 S14 S16 S1 S3 S5 S7

n1 : n2 S1 S3 S5 S7 S9 S11 (c) L1 (d)

n1 : n2 vdc1 C1 C2 vdc2 L1 Figure 7. Main power electronics converters used as back-endS2 powerS4 stage of offS6 -boardS8 EVBC: (a)

n1 : n2 C1 C2 vdc2 vdc1 unidirectional interleavedL2 buck-type; (b) bidirectional multilevel; (c) bidirectional three-phase

isolated; (d) bidirectional multiple dual active bridge. S9 S11 S13 S15 n1 : n2 n1 : n2 L3 L2 Figure 8a presents an illustrative example of a developed off-board EVBC and Figure 8b shows S2 S4 S6 S8 S10 S12

a detail of the power electronics converter. In this system, theS10 topologyS12 shownS 14in FigureS16 6c was used as the front-end converter and the topology shown in Figure 7b was used as the back-end converter. (c) (d) Figure 8b presents some results obtained during the experimental validation of this off-board EVBC [Figure52]. FigureAs 7.for Main7.the Main on power- boardpower electronicsEVBC,electronics these converters converters results areused used also as asback-endmerely back-end illustrativepower power stage of stageof the off-board operation of oﬀ -boardEVBC: principle, ( EVBC:a) having(a) unidirectionalunidirectional been obtained interleavedinterleaved for the frontbuck-type; buck-type;-end converter. (b ()b )bidirectional bidirectional The result multilevel; multilevel; of Figure (c 8) c (c bidirectionalshows) bidirectional the three three-phase three-phase currents on theisolated; powerisolated; (d grid) bidirectional(d) side,bidirectional allowing multiple multiple to verify dual dual active thatactive theybridge. bridge. have a sinusoidal waveform and that they are in phase with the voltages. Figure 8a presents an illustrative example of a developed off-board EVBC and Figure 8b shows a detail of the power electronics converter. In this system, the topology shown in Figure 6c was used as the front-end converter and the topology shown in Figure 7b was used as the back-endvga vgb converter.vgc Figure 8b presents some results obtained during the experimental validation of this off-board EVBC [52]. As for the on-board EVBC, these results are also merely illustrative of the operation principle, having been obtained for the front-end converter. The result of Figure 8c shows the three currents on iga igb igc the power grid side, allowing to verify that they have a sinusoidal waveform and that they are in phase with the voltages.

(a) (b) (c) FigureFigure 8. O 8.ﬀ -boardOff-board EVBC: EVBC: (a )(a example) example of of aa developeddeveloped laboratory laboratory prototype; prototype; (b) (dbetail) detail of the of power the power electronics converters; (c) experimental results showing the voltages (vga, vgb, vgc: 100 V/div|5 ms/div) electronics converters; (c) experimental results showing the voltages (vga, vgb, vgc: 100 V/div|5 ms/div) and the currents (iga, igb, igc: 10 A/div|5 ms/div) on the power grid side. and the currents (iga, igb, igc: 10 A/div|5 ms/div) on the power grid side. 2.1.3.2.1.3. Auxiliary Auxiliary Battery Battery Charging Charging Systems Systems BesidesBesides the the aforementioned aforementioned EVBC,EVBC, the the EV EV must must also also be equipped be equipped with witha charging a charging system systemfor the for auxiliary battery. In the same way as a conventional vehicle, the internal electronic loads of an EV the auxiliary battery. In the same way as a conventional vehicle, the internal electronic loads of an must be powered by an auxiliary battery (i.e., a low power battery). In such electronic loads are EV must be powered by an auxiliary battery (i.e., a low power battery). In such electronic loads are includedFigure systems 8. Off-board that guaranteeEVBC: (a) example driving of safety a developed (e.g., lights, laboratory windshield prototype; wipers, (b) detail and of horn), the power comfort included systems that guarantee driving safety (e.g., lights, windshieldga wipers,gb gc and horn), comfort (e.g., (e.g., electronicsair conditioning converters; and ( cmedia) experimental systems), results and showingdriving thesupport voltages (e.g., (v sensors, v , v : 100and V/div|5 global ms/div)positioning air conditioningand the currents and media(iga, igb, igc systems),: 10 A/div|5 andms/div) driving on the power support grid (e.g.,side. sensors and global positioning system) [71]. Conventionally, in an internal combustion engine (ICE) vehicle, the auxiliary battery is charged2.1.3. Auxiliary via an electricBattery Charging generator, Systems named as alternator, which is coupled to the ICE. In contrast, in electricBesides and hybrid the aforementioned vehicles, this EVBC, charging the operationEV must al takesso be equipped place through with a acharging dc–dc converter system for which the is connected,auxiliary on battery. the higher In the voltage same way side, as to a theconventional traction battery.vehicle, Forthe internal this task, electronic isolated loads dc–dc of convertersan EV mustmust be used be powered because, by according an auxiliary to the battery IEC 61851-1 (i.e., a low standard, power it battery). is mandatory In such that electronic the traction loads batteries are are isolatedincluded from systems the that vehicle guarantee chassis. driving In [72 safety] is presented (e.g., lights, a windshield power electronics wipers, and solution horn), basedcomfort on an (e.g., air conditioning and media systems), and driving support (e.g., sensors and global positioning integrated converter to enable the energy exchange between two batteries with diﬀerent voltage levels. A three-port converter topology based on the triple active bridge converter is addressed in [73], a system that allows auxiliary battery charging from the traction battery with galvanic isolation ensured by a three winding high-frequency transformer. In [74], a method for controlling the auxiliary Energies 2020, 13, x FOR PEER REVIEW 10 of 63

system) [71]. Conventionally, in an internal combustion engine (ICE) vehicle, the auxiliary battery is charged via an electric generator, named as alternator, which is coupled to the ICE. In contrast, in electric and hybrid vehicles, this charging operation takes place through a dc–dc converter which is connected, on the higher voltage side, to the traction battery. For this task, isolated dc–dc converters must be used because, according to the IEC 61851-1 standard, it is mandatory that the traction Energiesbatteries2020, 13 are, 6343 isolated from the vehicle chassis. In [72] is presented a power electronics solution based10 of 61 on an integrated converter to enable the energy exchange between two batteries with different voltage levels. A three-port converter topology based on the triple active bridge converter is batteryaddressed charging in via[73], the a system traction that battery allows is auxiliary patented, battery where charging the auxiliary from the battery traction charging battery iswith carried out, whenevergalvanic isolation possible, ensured during by thea three standby winding of the high-frequency EV, and according transformer. to the In state [74], ofa method charge offor both auxiliarycontrolling and traction the auxiliary batteries. battery In [ 75charging], an integrated via the traction solution battery based is onpatented, power electronicswhere the auxiliary is presented for chargingbattery charging the auxiliary is carried battery out, whenever from the tractionpossible, battery,during the identified standby asof the traction-to-auxiliary EV, and according to mode. In thethe presented state of charge on-board of both solution, auxiliary shown and traction in Figure batteries.9, a unified In [75], converter an integrated topology, solution the based EVBC on can power electronics is presented for charging the auxiliary battery from the traction battery, identified be configured in an isolated full-bridge dc–dc converter the auxiliary battery charging, thus making as traction-to-auxiliary mode. In the presented on-board solution, shown in Figure 9, a unified the traction-to-auxiliary mode possible. In [76], a topology based on the combination of a boost-type converter topology, the EVBC can be configured in an isolated full-bridge dc–dc converter the dc–dcauxiliary converter battery and acharging, flyback thus converter making is presentedthe traction-to-auxiliary for auxiliary mode battery possible. charging, In [76], where a topology the primary sidebased of the on transformer the combination of the of flyback a boost-type converter dc–dc is co usednverter as and the inputa flyback inductor converter of ais boost-typepresented for dc–dc converterauxiliary where battery the charging, EV battery where is connectedthe primary to. side In of [ 77the], transformer it is presented of the a flyback strategy converter without is auxiliaryused battery,as the where input the inductor loads areof a suppliedboost-type by dc–dc the EVconverte battery.r where An intermediatethe EV battery 48 is connected V dc link wasto. In considered [77], it and implementedis presented a strategy by means without of a buck-typeauxiliary battery, dc–dc where converter, the loads to which are supplied the EV by battery the EV is battery. connected An on the higherintermediate voltage 48 V side. dc link In was this considered situation, and a transformer implemented is by used means to ensureof a buck-type galvanic dc–dc isolation. converter, In the createdto which 48 Vdc the link, EV severalbattery is dc–dc connected converters on the are higher placed, voltage also side. based In onthis the situation, buck-type a transformer dc–dc converter, is whichused result to ensure in voltages galvanic of 12 isolation. V to supply In the the create variousd 48 V auxiliary dc link, loads.several The dc–dc availability converters of are a 48placed, V dc link also based on the buck-type dc–dc converter, which result in voltages of 12 V to supply the various is considered to be more efficient when compared to the traditional 12 V dc link, since, for the same auxiliary loads. The availability of a 48 V dc link is considered to be more efficient when compared power, a higher voltage allows the use of electrical cables of smaller section that are, consequently, to the traditional 12 V dc link, since, for the same power, a higher voltage allows the use of electrical lightercables [76]. of smaller section that are, consequently, lighter [76].

Smart ac-dc dc-dc EV Grid (full-bridge) (reversible) battery

SW1

SW2

isolation ac-dc auxiliary (diodes) battery

full-bridge isolated dc-dc converter

FigureFigure 9. Uniﬁed 9. Unified converter converter topology topology forfor chargingcharging the the EV EV battery battery and and the the auxiliary auxiliary battery. battery.

2.2. Inductive2.2. Inductive Wireless Wireless Battery Battery Charging Charging CoredCored inductive inductive power power transfer transfer is is a a popular popular technologytechnology that that has has been been extensively extensively used used since since the the electricelectric transformers transformers have have been been invented invented [78], [78], and itan isd basedit is based on the on magnetic the magnetic induction induction effect effect observed and exploredobserved byand Michael explored Faraday by Michael in 1831 Faraday [79,80 in]. 1831 When [79,80]. two When coils aretwo wound coils are around wound a around body of a body magnetic materialof magnetic (the “core”), material their (the electrical “core”), behaviors their electr are mutuallyical behaviors interdependent. are mutually This interdependent. happens in such This a way that energyhappens can in such be exchanged a way that between energy can the be coils exchan and,ged ultimately, between the between coils and, the ultimately, circuits they between are connected the to, bycircuits means they of a are commonly connected shared to, by magneticmeans of a field. commonl Thisy is shared not itself magnetic an invention, field. This but is rather,not itself a naturalan invention, but rather, a natural phenomenon that is easier to observe when the core material has a phenomenon that is easier to observe when the core material has a high magnetic permeability, but that high magnetic permeability, but that also occurs even if the core is constituted by air or, simply, by also occurs even if the core is constituted by air or, simply, by vacuum. In these latter cases, where there is no solid material to serve as a path for the magnetic field all the way between the two coils, the pair of coils form an “air-gapped” magnetic coupling, and the phenomenon is often referred to as “wireless” power transfer (WPT) or, more precisely, inductive wireless power transfer (IWPT). Soon it was realized that, in IWPT, the transmission of energy was not efficient to 100%. The energy losses were later explained by: (i) the energy dissipated in the electrical resistance of the conductors of both coils; (ii) the energy dissipated in the core material (if not air-gapped), that can be due to either induced parasitic (eddy) currents or magnetic reluctance; (iii) the energy that is irradiated to free-space by the coils. All these losses vary with the working frequency. Noticeably, the frequency dependency of conductive losses in (i) is due to skin and proximity effects [81], which results from the interaction of the conductor with the magnetic fields respectively generated by the current passing on that same Energies 2020, 13, x FOR PEER REVIEW 11 of 63

vacuum. In these latter cases, where there is no solid material to serve as a path for the magnetic field all the way between the two coils, the pair of coils form an “air-gapped” magnetic coupling, and the phenomenon is often referred to as “wireless” power transfer (WPT) or, more precisely, inductive wireless power transfer (IWPT). Soon it was realized that, in IWPT, the transmission of energy was not efficient to 100%. The energy losses were later explained by: (i) the energy dissipated in the electrical resistance of the conductors of both coils; (ii) the energy dissipated in the core material (if not air-gapped), that can be due to either induced parasitic (eddy) currents or magnetic reluctance; (iii) the energy that is irradiated to free-space by the coils. All these losses vary with the working frequency. Noticeably, Energies 2020, 13, 6343 11 of 61 the frequency dependency of conductive losses in (i) is due to skin and proximity effects [81], which results from the interaction of the conductor with the magnetic fields respectively generated by the conductorcurrent and passing other on nearby that same conductors, conductor making and other the nearby current conductors, distribution making uneven the current along distribution the conduct cross-sections,uneven along and thus,the conduct increasing cross-sections, the apparent and resistance thus, increasing of the coilthe apparent windings. resistance of the coil windings. 2.2.1. Principles of Inductive Wireless Power Transfer 2.2.1. Principles of Inductive Wireless Power Transfer In both “cored” or “wireless” versions, when submitted to sinusoidal oscillations of suﬃciently In both “cored” or “wireless” versions, when submitted to sinusoidal oscillations of sufficiently low frequencieslow frequencies (negligible (negligible irradiated irradiated power), power), the the magnetic magnetic couplingcoupling voltages voltages and and currents currents can can be be modeledmodeled in terms in terms of phasors of phasors by idealby ideal linear linear circuit circuit components, components, asas shownshown in Figure Figure 10. 10 .

(a) (b)

FigureFigure 10. Magnet 10. Magnet coupling coupling model withmodel negligible with negligible irradiated irradiated power: ( a)power: two non-irradiating (a) two non-irradiating magnetically coupledmagnetically coils; (b) simplified coupled coils; circuit (b) modelsimplified of the circuit coupled model coils. of the coupled coils.

In this model, 𝜔 2𝜋𝑓 , and 𝑀 is the mutual inductance between the coils. 𝑅 and 𝑅 In this model, ω = 2π f0, and M is the mutual inductance between the coils. R1 and R2 depend depend on both skin and proximity effects. For any fixed frequency operation 𝑓, the skin losses can on both skin and proximity effects. For any fixed frequency operation f0, the skin losses can be taken be taken into account by recalculating these 𝑅 for that frequency, and the model will still conserve into account by recalculating these R for that frequency, and the model will still conserve its linearity. its linearity. In this manner, withi no irradiation assumed for the magnetic coupling, the power In this manner, with no irradiation assumed for the magnetic coupling, the power transferred from transferred from Coil 1 to Coil 2 is given. by the phasor 𝑆, (1): Coil 1 to Coil 2 is given by the phasor S1,2 (1): ∗ ∗ ∗ ∗ 𝑆, 𝑉𝐼 𝑉𝐼 𝑗 𝜔 𝑀 𝐼 𝐼 𝑗 𝜔 𝑀 𝐼 𝐼 , (1) . . . . . . . . . ∗ ∗ ∗ ∗ and the averageS real =powerVM 𝑃I = transferredVM I is then= j givenω M Iby:I = j ω M I I , (1) 1,2 1 ,1 2 − 2 1 2 − 1 2 𝑃, 𝜔 𝑀 𝐼 𝐼 sin 𝜑 𝜑 , (2) and the average real power P1,2 transferred is then given by: where 𝜑 𝜑 is the phase angle between the current in Coil 1 (primary coil) to Coil 2 (secondary coil). If 𝑃, 0, the power is Pactually= ωtransferredMIrms Irms backwards,sin(ϕ thatϕ ) is,, from Coil 2 to Coil 1. (2) 1,2 1 2 1 − 2 In order to increase 𝑃,, 𝐼 and 𝐼 have to be increased while keeping the phase delay of where𝐼ϕ withϕ respectis the phaseto 𝐼 angleas close between as possible the current to 𝜋2⁄ . in This Coil is 1 achieved (primary by coil) introducing to Coil 2 (secondarycompensation coil). 1 − 2 If P1, 2networks< 0, the powerin primary is actually and secondary transferred circuits. backwards, The simplest that is, fromand first Coil engineered 2 to Coil 1. compensation network is constituted by a single capacitor in series with the load, as shown in Figure 11b, a In order to increase P1, 2, Irms1 and Irms2 have to be increased while keeping the phase delay . configuration already. used by Nikola Tesla in the late 19th century [82], which is called “series of I2 with respect to I1 as close as possible to π/2. This is achieved by introducing compensation compensation”. In this configuration, the reactive component of the equivalent impedance 𝑍, as networks in primary and secondary circuits. The simplest and first engineered compensation network seen by 𝐸, is canceled at 𝜔2𝜋𝑓 when 𝐿𝐶 1⁄2𝜋𝑓 . is constituted by a single capacitor in series with the load, as shown in Figure 11b, a configuration already used by Nikola Tesla in the late 19th century [82], which is called “series compensation”. In this configuration, the reactive component of the equivalent impedance Ze2, as seen by E2, is canceled at 2 ω = 2πEnergiesf0 when 2020L, 132C, x2 FOR= 1 PEER/(2 πREVIEWfo) . 12 of 63

(a) (b)

FigureFigure 11. Impedance 11. Impedance compensation compensation networks networks in: in: (a ()a primary) primary circuit;circuit; (b)) secondary secondary circuit. circuit.

Conditioned to the choice of the phase difference 𝜑 𝜑 𝜋⁄ , 2 the use of a similar serial compensation network in the primary side, as shown in Figure 11, can on the other hand maximize

the current 𝐼 and, consequently, maximize the power dissipated on 𝐸 , which is in fact transferred to the secondary and delivered to the load via 𝑅 by 𝐸. Having the capacitor 𝐶 as the free variable, 𝑍 is minimized, and 𝐼 is maximized (resonant condition on the primary circuit), when 𝐿𝐶 1⁄2𝜋𝑓 . The serial compensation, when used in both the primary and secondary circuits, originates what is called the serial-serial (SS) compensation topology, one the most widely used configurations for IWPT. It has been shown [83,84], that in a passive two-port network, such as an SS-compensated

magnetic coupling, when both primary and secondary circuits “tuned” to 𝑓 , the maximum

achievable electrical efficiency  in the power transference from Coil 1 to Coil 2 at 𝑓 can be calculated as: 𝑘 𝑄𝑄  , (3) 1 1𝑘 𝑄𝑄

where the 𝑄 are the quality factors (or “Q-factors”) of the coils 𝐿, defined by:

ω 𝐿 𝑄 , 𝑖 ∈ 1, 2 (4) 𝑅

and this happens when the load resistor 𝑅 assumes the value:

𝑅 𝑅 1𝑘𝑄 𝑄  (5)

For values of 𝑄 𝑘 𝑄 𝑄 much greater than 1,  can be approximated by: 2 2  ≅  1 1 (6) 𝑘𝑄𝑄 𝑄

The behavior of  as function of the magnetic coupling quality factor, 𝑄 𝑘 𝑄𝑄, is shown in Figure 12a, revealing for instance that in order for a magnetic coupling to allow electric

efficiencies over 85% it necessary that 𝑄 12.3. Typical behavior patterns of the efficiency as a function of the normalized 𝑅 to 𝑅 ratio are shown in Figure 12b, where the color of the plotted curves, ranging from red (lower curve) to blue,

correspond respectively to 𝑄 values ranging from 1 to 100 in the set 1, 2, 5, 10, 20, 50, 100 . Noticeably, the maximum efficiency for each curve in Figure 12b is always achieved when condition

(5) is met, that is, when 𝑅⁄𝑅 ≅𝑘 𝑄𝑄.

Energies 2020, 13, 6343 12 of 61

Conditioned to the choice of the phase difference ϕ ϕ = π/2, the use of a similar serial 1 − 2 compensation network in the primary side, as shown in Figure 11, can on the other hand maximize the current Irms1 and, consequently, maximize the power dissipated on E1, which is in fact transferred to the secondary and delivered to the load via R2 by E2. Having the capacitor C1 as the free variable, Ze1 is 2 minimized, and Irms1 is maximized (resonant condition on the primary circuit), when L1C1 = 1/(2π fo) . The serial compensation, when used in both the primary and secondary circuits, originates what is called the serial-serial (SS) compensation topology, one the most widely used configurations for IWPT. It has been shown [83,84], that in a passive two-port network, such as an SS-compensated magnetic coupling, when both primary and secondary circuits “tuned” to fo, the maximum achievable electrical efficiency ηmax in the power transference from Coil 1 to Coil 2 at fo can be calculated as:

k2Q Q η = 1 2 , (3) max p 2 2 1 + 1 + k Q1Q2 where the Qi are the quality factors (or “Q-factors”) of the coils Li, deﬁned by:

ω0 Li Qi = , i 1, 2 (4) Ri ∈ { } and this happens when the load resistor RL assumes the value: q 2 Rηmax = R2 1 + k Q1Q2 (5)

For values of QF = k √Q1 Q2 much greater than 1, ηmax can be approximated by:

2 2 ηmax eηmax = 1 = 1 (6) − k √Q1Q2 − QF

The behavior of ηmax as function of the magnetic coupling quality factor, QF = k √Q1Q2, is shown in Figure 12a, revealing for instance that in order for a magnetic coupling to allow electric eﬃciencies over 85%Energies it necessary 2020, 13, x FOR that PEERQF REVIEW> 12.3. 13 of 63

(a) (b)

Figure 12.FigureEffi 12.ciency Efficiency in a in serial-serial a serial-serial (SS)-compensated (SS)-compensated inductive inductive wireless wireless power power transfer transfer (IWPT): ( (IWPT):a) maximum achievable efficiency with optimal load, as a function of 𝑘 𝑄 𝑄 ; (b) efficiency as a (a) maximum achievable efficiency with optimal load, as a function of k √Q1 Q2;(b) efficiency as a function of the normalized load. function of the normalized load. Once established, the IWPT overall efficiency and the required efficiency for the magnetic Typical behavior patterns of the efficiency as a function of the normalized R to R ratio coupling, the desired power level can set by adjusting the voltage source 𝑉. If this cannot beL directly2 are shownaccomplished in Figure because12b, where the power the colorsource of is thefixed, plotted a voltage curves, transformation ranging should from redbe used (lower in either curve) to blue, correspondthe primary respectively or secondary to circuit.QF values If the ranging load 𝑅 fromrequires 1 to dc 100 current in the instead, set 1, it 2, will 5, 10,be usually 20, 50, 100 . connected to the secondary circuit through a rectifier and a dc filter. In this case,{ a dc–dc converter } can be used to match the load for optimal efficiency. However, as a rectifier is a non-linear element, the characteristic output response curve will be not linear as well, and a new optimum 𝑅 , 𝑅 ,  will arise, which is potentially different from 𝑅 (5), as observed in [85].  The task of building a magnetic coupling with a reasonably high efficiency translates into the

problem of building resonant coil-capacitor pairs with high-quality factors 𝑄, as high as necessary to compensate for the low magnetic coupling coefficient 𝑘 between these coils.

2.2.2. Frequency Splitting in Serial-Serial (SS)-Compensation Topology It is long known that, in a double-tuned coupled circuit, when the mutual inductive coupling coefficient is increased above a critical value, two resonance peaks in frequency may appear instead

of one, when 𝑅 is lowered (highly loaded secondary). This phenomenon, early observed and used in the design of communication circuits, was called frequency splitting [86].

These frequencies are often referred to in the literature as 𝑓 , which is lower than the resonance frequency 𝑓, and 𝑓, which is higher, and can be approximated by: 1 𝑓 ≅ 𝑓 , (7) √1𝑘

1 𝑓 ≅ 𝑓 , (8) √1𝑘

as long as 𝑘 𝑘 , where 𝑘 is a critical value that is a function of the loaded quality factor of the secondary of the magnetic coupling [87]. This behavior and its simplified formulation, as given by (7) and (8), have been characterized by many other authors since Terman [81], and also need to be considered in IWPT design, either to avoid the frequency splitting operation [88,89] or to exploit it, and tune circuits to the most adequate operating frequency [90]. Frequency splitting, however, is not unique to SS-compensated circuits. This phenomenon can occur, and be exploited, in other circuit topologies, due to the presence of multiple poles in the circuit transfer function and the joint dependence of their location on the coupling coefficient and the load. For all basic IWPT configurations, as displayed in Figure 13, it has

Energies 2020, 13, 6343 13 of 61

Noticeably, the maximum efficiency for each curve in Figure 12b is always achieved when condition (5) is met, that is, when RL/R2 k √Q1Q2. Once established, the IWPT overall efficiency and the required efficiency for the magnetic coupling, the desired power level can set by adjusting the voltage source V0. If this cannot be directly accomplished because the power source is fixed, a voltage transformation should be used in either the primary or secondary circuit. If the load RL requires dc current instead, it will be usually connected to the secondary circuit through a rectifier and a dc filter. In this case, a dc–dc converter can be used to match the load for optimal efficiency. However, as a rectifier is a non-linear element, the characteristic DC output response curve will be not linear as well, and a new optimum RL, Rηmax , will arise, which is potentially different from Rηmax (5), as observed in [85]. The task of building a magnetic coupling with a reasonably high efficiency translates into the problem of building resonant coil-capacitor pairs with high-quality factors Qi, as high as necessary to compensate for the low magnetic coupling coefficient k between these coils.

2.2.2. Frequency Splitting in Serial-Serial (SS)-Compensation Topology It is long known that, in a double-tuned coupled circuit, when the mutual inductive coupling coeﬃcient is increased above a critical value, two resonance peaks in frequency may appear instead of one, when RL is lowered (highly loaded secondary). This phenomenon, early observed and used in the design of communication circuits, was called frequency splitting [86]. These frequencies are often referred to in the literature as fodd, which is lower than the resonance frequency f0, and feven, which is higher, and can be approximated by:

1 feven fo , (7) √1 k − 1 fodd fo , (8) √1 + k as long as k > ks, where ks is a critical value that is a function of the loaded quality factor of the secondary of the magnetic coupling [87]. This behavior and its simplified formulation, as given by (7) and (8), have been characterized by many other authors since Terman [81], and also need to be considered in IWPT design, either to avoid the frequency splitting operation [88,89] or to exploit it, and tune circuits to the most adequate operating frequency [90]. Frequency splitting, however, is not unique to SS-compensated circuits. This phenomenon can occur, and be exploited, in other circuit topologies, due to the presence of multiple poles in the circuit transfer function and the joint dependence of their location on the coupling Energies 2020, 13, x FOR PEER REVIEW 14 of 63 coefficient and the load. For all basic IWPT configurations, as displayed in Figure 13, it has been shown that frequencybeen shown splitting that frequency is prone splitting to be observedis prone to whenever be observed the whenever loaded qualitythe loaded factor quality of thefactor primary of is lowerthe than primary the loaded is lower quality than the factor loaded of quality the secondary factor of the [91 ].secondary [91]. 1

2 3 (a) (b)

4 (c) (d)

FigureFigure 13. Summary 13. Summary of basic of basic circuit circuit compensation compensation topologies topologies used used in in IWPT IWPT configurations: configurations: ( (aa)) serial serial-serial- (SS); (bserial) serial-parallel (SS); (b) serial (SP);-parallel (c) parallel-series (SP); (c) parallel (PS);-series (d ()PS parallel-parallel); (d) parallel-parallel (PP). (PP).

2.2.3. Other Circuit Compensation Topologies Another similar compensation topology, which is also of simple construct and often used in IWPT applications, is the serial-parallel (SP) compensation [92], where on the secondary side there is a capacitor in parallel with the load. Different compensation topologies yield different characteristics that will influence the choice of the compensation type to use in a design. Formulations found in the literature can help the design in the simpler cases [93], as exemplified in this work for the SS-compensation circuit, with coils of high-quality factors that are not heavily loaded. However, in general, exact algebraic analysis is impractical, and numerical simulation is the prevalent tool for both IWPT evaluation and design. The SS- and SP-compensated topologies are possibly the most popular so far observed in academic studies and most real-world implementations. Excited by an ac voltage source in the primary side, they are simpler to implement, easier to analyze than the more complex topologies, and still can successfully handle a large number of applications. For wireless vehicular battery charging, with power ranging from a few kilowatts to a few tens of kilowatts, a number of laboratory implementations report the use the SS- and SP-compensation schemata, such as [94], with some authors having identified efficiency advantages of the SS over the SP-compensated circuits for this type of application [95]. This advantage can be relativized by the fact that the parallel compensation may be the preferable topology to be used if high currents and power levels are to be achieved, and either the input or output voltage is restricted not to be very high. This will often be the case in circumstances where the mutual inductance between the coils is low. Concerning efficiency, it will improve if the passive components of the compensation circuits and the magnetic coupling exhibit losses altogether, for the same output power handled. This is more related to the quality factor of the components themselves and the choice of the more favorable operation point, which may be different for each topology, similarly to what happens with the output power itself. This is exemplified by a case study [96] where the SP efficiency is reported to be better than that of the SS-compensation when lower magnetic coupling coefficients, resulting from asymmetry in the coils, are observed, but worse otherwise. Similar to the SS- and SP-compensated circuits, parallel-parallel (PP) and parallel-series (PS) compensation circuits are defined, by alternatively using a capacitor in parallel in the primary as a compensation device, a configuration that requires, instead, excitation from an ac current source. The SS, SP, PP and PS topologies are summarized in Figure 13, where, for simplicity, the resistors corresponding to the losses in the coil windings have been omitted from the models. Other topologies using arbitrarily more complex passive circuits can also be used to match the load to the power source through a magnetic coupling. There have been many authors investigating

Energies 2020, 13, 6343 14 of 61

2.2.3. Other Circuit Compensation Topologies Another similar compensation topology, which is also of simple construct and often used in IWPT applications, is the serial-parallel (SP) compensation [92], where on the secondary side there is a capacitor in parallel with the load. Different compensation topologies yield different characteristics that will influence the choice of the compensation type to use in a design. Formulations found in the literature can help the design in the simpler cases [93], as exemplified in this work for the SS-compensation circuit, with coils of high-quality factors that are not heavily loaded. However, in general, exact algebraic analysis is impractical, and numerical simulation is the prevalent tool for both IWPT evaluation and design. The SS- and SP-compensated topologies are possibly the most popular so far observed in academic studies and most real-world implementations. Excited by an ac voltage source in the primary side, they are simpler to implement, easier to analyze than the more complex topologies, and still can successfully handle a large number of applications. For wireless vehicular battery charging, with power ranging from a few kilowatts to a few tens of kilowatts, a number of laboratory implementations report the use the SS- and SP-compensation schemata, such as [94], with some authors having identified efficiency advantages of the SS over the SP-compensated circuits for this type of application [95]. This advantage can be relativized by the fact that the parallel compensation may be the preferable topology to be used if high currents and power levels are to be achieved, and either the input or output voltage is restricted not to be very high. This will often be the case in circumstances where the mutual inductance between the coils is low. Concerning efficiency, it will improve if the passive components of the compensation circuits and the magnetic coupling exhibit losses altogether, for the same output power handled. This is more related to the quality factor of the components themselves and the choice of the more favorable operation point, which may be different for each topology, similarly to what happens with the output power itself. This is exemplified by a case study [96] where the SP efficiency is reported to be better than that of the SS-compensation when lower magnetic coupling coefficients, resulting from asymmetry in the coils, are observed, but worse otherwise. Similar to the SS- and SP-compensated circuits, parallel-parallel (PP) and parallel-series (PS) compensation circuits are defined, by alternatively using a capacitor in parallel in the primary as a compensation device, a configuration that requires, instead, excitation from an ac current source. The SS, SP, PP and PS topologies are summarized in Figure 13, where, for simplicity, the resistors corresponding to the losses in the coil windings have been omitted from the models. Other topologies using arbitrarily more complex passive circuits can also be used to match the load to the power source through a magnetic coupling. There have been many authors investigating this topic and extensive reviews on compensation topologies and their main properties can be found for instance in [97,98] for a general design orientation.

2.2.4. Power Electronics Topologies for Inductive Wireless Power Transfer (IWPT) IWPT operation can be either unidirectional or bidirectional. In unidirectional operation mode, an active power electronics topology should be used in the primary-side and a passive power electronics topology could be used in the secondary-side, while in bidirectional operation mode, it is indispensable to consider active power electronics topologies in both the primary side and secondary sides. Another important specification is related to the interface with the power grid, since both single-phase and three-phase interfaces are valid. The same power electronics converter topology can be used in both the primary side or secondary side, or a mixture of circuit topologies can be used for the same IWPT application (i.e., distinct topologies on both sides). A comprehensive and broad review of ac-dc power electronics converters, both unidirectional and bidirectional topologies, as well as power control theories, is presented in [99], showing good candidates for IWPT applications. Specific reviews of ac-dc power electronics converters with galvanic isolation are presented in [100,101]. The covered topologies are relevant to prevent power quality issues on the power grid side and due to the galvanic isolation, however, the complexity is a discouraging factor Energies 2020, 13, 6343 15 of 61 for IWPT applications. Analyzing in more detail the most convenient power electronics topologies, the main option is the full-bridge, shown in Figure 14a. This topology is constituted by two legs, where each one is constituted by two power devices. This topology is very relevant since it can operateEnergies in 2020 four-quadrants, 13, x FOR PEER andREVIEW with three voltage levels, depending on the control strategy,15 which of 63 is an important characteristic when it is necessary to diminish the necessities of the coupling passive filters. Alignedthis topic with and the extensive full-bridge reviews power on electronicscompensation topology, topologies the and half-bridge, their main shownproperties in Figurecan be found14b, is also for instance in [97,98] for a general design orientation. pertinent for IWPT applications. As is possible to verify, it is constituted by a single leg (i.e., half of the2.2.4. power Power devices Electronics and half Topologies of the associated for Inductive hardware), Wireless Power but it Transfer requires (IWPT) a split dc link. Due to this fact, the maximum voltage of the dc link is double when compared with the full-bridge, as well as the voltageIWPT applied operation to thepower can be devices.either unidirectional Moreover, or this bidirectional. topology only In unidirectional operates with operation two voltage mode, levels, an active power electronics topology should be used in the primary-side and a passive power demanding more in terms of coupling passive filters. electronics topology could be used in the secondary-side, while in bidirectional operation mode, it is Besides the aforementioned power electronics topologies, multilevel converters (i.e., more than indispensable to consider active power electronics topologies in both the primary side and secondary thesides. three ofAnother the full-bridge) important arespecification also relevant is related for IWPT to the applications, interface with since the itpower is conceivable grid, since toboth obtain a voltagesingle-phase waveform and withthree-phase improved interfaces quality, are valid. allowing The same the requirements power electronics of the converter coupling topology passive can filters to bebe reduced.used in both However, the primary the hardwareside or secondary and software side, or a necessities mixture of arecircuit more topologies relevant can (e.g., be used more for power switchingthe same devices, IWPT application drivers, and (i.e., more distinct sensors). topologies In counterpart, on both sides). the maximum voltage applied to each power deviceA comprehensive is reduced and in proportionbroad review to of the ac-dc voltage power levels electronics obtained converters, with theboth multilevel unidirectional topology. Moreover,and bidirectional besides reducingtopologies, the as necessitieswell as power of control the coupling theories, passive is presented filters, in this [99], is showing also important good to improvecandidates the effiforciency. IWPT applications. Specifically, Specific in [102, 103revi]ews are presentedof ac-dc power extensive electronics reviews converters regarding with power electronicsgalvanic topologiesisolation are with presented multilevel in [100,101]. characteristics. The covered In this topologies context, are Figure relevant 14c to presents prevent apower multilevel quality issues on the power grid side and due to the galvanic isolation, however, the complexity is a topology capable of producing five voltage levels [104], and suitable for IWPT applications. As shown, discouraging factor for IWPT applications. Analyzing in more detail the most convenient power thiselectronics topology topologies, is composed the of main a traditional option is full-bridgethe full-bridge, converter shown andin Figure by a specific14a. This bidirectional topology is and bipolarconstituted structure by betweentwo legs, thewhere neutral each wireone is and constituted the middle by two point power of the devices. split dc This link. topology This converter is very also allowsrelevant four-quadrant since it can operate operation. in four-quadrants The cascade and h-bridge with three [105 voltage] is also levels, an interesting depending on power the control electronics topologystrategy, to which guarantee is an aimportant multilevel characteristic characteristic, when however, it is necessary it can to bediminish applied the only necessities for specific of the IWPT applications,coupling passive since filters. it requires Aligned independent with the full-bridge dc links. power Depending electronics on topology, the voltage the half-bridge, of the dc links, twoshown distinct in Figure structures 14b, is of also the pertinent cascade for h-bridge IWPT applications. can be used: As symmetric is possible to and verify, asymmetric it is constituted [106]. It is importantby a single to noteleg (i.e., that half by of using the power an asymmetric devices and structure, half of the it associated is possible hardware), to obtain but more it requires voltage a levels withoutsplit dc changing link. Due the to hardware.this fact, theAlso maximum within voltage multilevel of thestructures, dc link is double the neutral when compared point clamped with the and the flyingfull-bridge, capacitor as arewell traditional as the voltage solutions applied [to107 the,108 po]wer both devices. with potentialMoreover, forthis IWPT topology applications. only operates with two voltage levels, demanding more in terms of coupling passive filters.

vdc S1 S3 C1 S1 2 compensation compensation circuit circuit vdc C L C L C

vdc S2 S4 C2 S2 2

(a) (b)

vdc C1 S1 S3 2 compensation to load or to power circuit power source or source via load L S S C magnetic 5 6 coupling vdc C2 S S 2 2 4

(c)

FigureFigure 14. 14.Main Main power power electronics electronics topologies used used in in IW IWPT,PT, both both in primary in primary side sideand secondary and secondary side: side: (a) full-bridge(a) full-bridge converter; converter; (b (b)) half-bridge half-bridge converter; (c ()c )multilevel multilevel topology. topology.

Energies 2020, 13, 6343 16 of 61

Additional to the aforementioned topologies, other specific topologies can be considered for IWPT. For instance, a cascaded dc–dc converter based on a boost-buck structure is proposed in [109] for the dc-side of a unidirectional IWPT, allowing to control both the input as well as the output current. A new pulse-density modulated for the full-bridge converter operation with zero-voltage switching is presented in [110] for IWPT, which is proposed as an advanced technique for improving the efficiency. An ac-dc converter based on a single-stage three-level structure is proposed in [110] for unidirectional IWPT, operating with fixed switching frequency and incorporating, as an innovative feature, power-factor-correction features for the power grid side. A single-phase ac-dc converter based on a matrix structure is proposed in [111] for the primary-side of a bidirectional IWPT, operating as a single power stage and designed for electric mobility applications. A single-stage resonant converter based on a bridgeless boost-type structure is proposed in [112] for unidirectional IWPT, allowing combining in a single equipment the necessary functionalities for the power transfer, as well as for the power grid side with a sinusoidal current and unitary power factor. A novel control strategy for bidirectional IWPT in proposed in [113], which is specially dedicated to controlling structures based on the full-bridge dc–ac and ac-dc converters. A bidirectional IWPT using self-resonant PWM for applications in electric mobility is proposed in [114], permitting to obtain constant frequency accomplished with a big air-gap and without the required supplementary circuit. A three-phase single-stage ac-dc converter for unidirectional IWPT is proposed in [115], allowing it to operate as a power factor correction topology for the power grid interface. A direct ac–ac converter for unidirectional IWPT is proposed in [116], permitting to obtain a single-stage structure with an active source current control. A Z-source resonant converter is proposed in [117] for unidirectional IWPT, guaranteeing both the functionalities of power transfer and power quality on the power grid side.

2.2.5. Stationary Battery Charging of Electric Vehicles with IWPT Because of the large number of possible design solutions involving coil geometry and circuit implementations, and in pursuit of interoperability, SAE International (previously known as the Society of Automotive Engineers) sponsored a workgroup that produced a report establishing some common guidelines for IWPT design of vehicular wireless battery charging systems, the SAE Recommended Practice J2954 [118]. The SAE solution is basically an under-the-vehicle installation, as shown in Figure 15. Energies 2020, 13, x FOR PEER REVIEW 17 of 63

FigureFigure 15. 15.Typical Typical Society Society of Automotive of Automotive Engineers Engineers (SAE)-compatible (SAE)-compatible IWPT conﬁguration IWPT configuration for vehicular for batteryvehicular recharging, battery recharging, with the magnetic with the coupling magnetic positioned coupling positioned underneath un thederneath vehicle. the vehicle.

AsAs defined defined in in SAE SAE J2954, J2954, the secondarythe secondary coil is coil included is included in a structure in a structure denominated denominated “Vehicle Assembly” “Vehicle (VA),Assembly” and the (VA), primary and coil the in theprimary “Ground coil Assembly” in the “Ground (GA), as Assembly” marked in Figure(GA), 15as. marked Without in determining Figure 15. speciﬁcWithout electronic determining implementation, specific electronic the SAE implementa J2954 establishestion, powerthe SAE transfer J2954 classes,establishes and ﬁxespower minimum transfer performanceclasses, and parametersfixes minimum regarding performance the geometry parameters and alignment regarding tolerances the geometry between and primary alignment and secondarytolerancescoils. between It also primary provides and examples secondary of viable coils. circuit It also implementations. provides examples Tables1 ofand viable2 provide circuit a summaryimplementations. of SAE proposed Tables 1 classesand 2 provide of IWPT a power summ andary clearanceof SAE proposed distances classes from VAof IWPT to ground. power and clearance distances from VA to ground.

Table 1. SAE J2954 recommended power levels, efficiency and frequency band.

SAE J2954 Recommendations IWPT Power Classes (as of November 2017) WPT1 WPT2 WPT3 Maximum Input Power 3.7 kW 7.7 kW 11 kW Frequency Band 81.38 kHz to 90 kHz Maximum Transfer Efficiency >85% @ full alignment

Table 2. SAE J2954 ground clearance ranges as per defined Z-Classes.

SAE J2954 Z-Class Ground Clearance Range (mm) Z1 100–150 Z2 140–210 Z3 170–250

In SAE J2954, the efficiency is required to be greater than 85% in the best possible alignment condition (“full alignment”). The base IWPT frequency can be adjusted within the established band from 81.38 kHz to 90 kHz. The feedback for this adjustment can come from several different sources, from direct impedance and power signal measurements, at the primary side, to explicit message passing through a wireless communication link established in between the primary and secondary control circuits, as proposed in SAE Recommended Practice, SAE J2847-6 [119], that establishes requirements and specifications for communication messages between wirelessly charged EVs and the wireless battery charger. This is a complex and relatively new technology that has been recently proposed, but implementations using it are already reported [120]. Although not enforcing any particular circuit implementation, SAE 2954J suggests some viable circuit topologies. In particular, it exemplifies and provides actual circuit parameters for PP-compensation and for the so-called LCC-compensation [121], that can be used either on primary or secondary sides, or on both [122], as shown in Figure 16. The LCC term comes from “inductor-capacitor-capacitor”, which is exactly the sequential order of the passive components that are introduced in between the power source (or the load) and the magnetic coupling. LCC exemplary circuits are provided in SAE 2954J with specific component values assigned, to facilitate an implementation where the required performance levels are assured to be met.

Energies 2020, 13, 6343 17 of 61

Table 1. SAE J2954 recommended power levels, eﬃciency and frequency band.

SAE J2954 Recommendations IWPT Power Classes (as of November 2017) WPT1 WPT2 WPT3 Maximum Input Power 3.7 kW 7.7 kW 11 kW Frequency Band 81.38 kHz to 90 kHz Maximum Transfer Eﬃciency >85% @ full alignment

Table 2. SAE J2954 ground clearance ranges as per deﬁned Z-Classes.

SAE J2954 Z-Class Ground Clearance Range (mm) Z1 100–150 Z2 140–210 Z3 170–250

In SAE J2954, the efficiency is required to be greater than 85% in the best possible alignment condition (“full alignment”). The base IWPT frequency can be adjusted within the established band from 81.38 kHz to 90 kHz. The feedback for this adjustment can come from several different sources, from direct impedance and power signal measurements, at the primary side, to explicit message passing through a wireless communication link established in between the primary and secondary control circuits, as proposed in SAE Recommended Practice, SAE J2847-6 [119], that establishes requirements and specifications for communication messages between wirelessly charged EVs and the wireless battery charger. This is a complex and relatively new technology that has been recently proposed, but implementations using it are already reported [120]. Although not enforcing any particular circuit implementation, SAE 2954J suggests some viable circuit topologies. In particular, it exemplifies and provides actual circuit parameters for PP-compensation and for the so-called LCC-compensation [121], that can be used either on primary or secondary sides, or on both [122], as shown in Figure 16. The LCC term comes from “inductor-capacitor-capacitor”, which is exactly the sequential order of the passive components that are introduced in between the power source (or the load) and the magnetic coupling. LCC exemplary circuits are provided in SAE 2954J with specific component values assigned, to facilitate an implementation where the required performanceEnergies 2020, levels13, x FOR are PEER assured REVIEW to be met. 18 of 63

FigureFigure 16. 16.One One of of the the suggested suggested circuit circuit topologiestopologies (double-sided LCC LCC (inductor-capacitor-capacitor)) (inductor-capacitor-capacitor)) to meetto meet the the performance performance parameters parameters proposed proposed inin SAESAE 2954J.2954J.

TheThe circuit circuit used used to to implementimplement the rectifier rectifier can can be be made, made, by bydesign, design, the same the same as that as used that in used the in theinverter, inverter, such such as as the the full-bridge full-bridge converter/rectifier, converter/rectifier, case case when when only only the the control control of ofthe the gates gates of ofthe the convertersconverters will will di differ,ffer, from from primary primary to to secondary. secondary. ThisThis facilitates the the implementation implementation of of bidirectional bidirectional IWPTIWPT [123 [123–125]–125] which which may may be be very very useful useful inin V2GV2G or vehicle-to-home (V2H) (V2H) applications applications [126,127]. [126,127 ]. AsAs vehicular vehicular bidirectional bidirectional battery battery charging charging itself itself generates generates more more possibilities possibilities forfor applications,applications, eveneven in conductivein conductive charging, charging, it can it can be expectedbe expected that that bidirectional bidirectional IWPT IWPT alsoalso becomes popular. popular. Honda Honda and and WiTricityWiTricity particularly particularly have have already already demonstrated demonstrated thethe feasibility of of a a SAE SAE J2954-compatible J2954-compatible wireless wireless bidirectionalbidirectional battery battery charger charger [128 [128].].

2.2.6. Limiting Human Exposure to Electromagnetic Fields (EMF) in IWPT Systems The concern about human health in the design of electric apparatuses is not unique to IWPT devices. However, due to the very nature of near field energy transmission in IWPT, the frequency and intensity of electromagnetic fields (EMF) in the vicinity of the coils can be very high, above what is currently considered safe for human exposure. The International Commission on Non-Ionizing Radiation Protection (ICNIRP), considering the knowledge available on the subject, periodically reviews and updates some guidelines for limiting exposure to time-varying electric and magnetic fields. In the frequency band recommend by SAE J2954 for wireless charging, the limit rms electric field and magnetic flux density are respectively 83 V/m and 27 µT [129]. SAE J2954 requirements, however, are stricter than the general ICNIRP recommended safety margins: in some positions of the space, inside and around the car, due to the consideration that some humans exposed to the automotive application may have an implanted medical device (IMD), such as cardiac pacemakers, the AAMI/ISO 14117-2012 standard should also be applied. Because of that, inside the cabin, and at any point in space outside the vehicle that is located higher than 70 cm above ground, the required maximum rms values of the magnetic field strength is further limited to 15 µT, with the peak magnetic field limited to 21.2 µT. SAE J2954 admits that conformity may still be observed if this additional requirement is not met in these critical spaces, but in this case, steps should be taken to warn pacemaker wearers to avoid this region, that is, to stay away from the car. In practice, in order not to restrict in any manner the use of cars by passengers with IMD, this additional requirement has to be observed [127].

Energies 2020, 13, 6343 18 of 61

2.2.6. Limiting Human Exposure to Electromagnetic Fields (EMF) in IWPT Systems The concern about human health in the design of electric apparatuses is not unique to IWPT devices. However, due to the very nature of near field energy transmission in IWPT, the frequency and intensity of electromagnetic fields (EMF) in the vicinity of the coils can be very high, above what is currently considered safe for human exposure. The International Commission on Non-Ionizing Radiation Protection (ICNIRP), considering the knowledge available on the subject, periodically reviews and updates some guidelines for limiting exposure to time-varying electric and magnetic fields. In the frequency band recommend by SAE J2954 for wireless charging, the limit rms electric field and magnetic flux density are respectively 83 V/m and 27 µT[129]. SAE J2954 requirements, however, are stricter than the general ICNIRP recommended safety margins: in some positions of the space, inside and around the car, due to the consideration that some humans exposed to the automotive application may have an implanted medical device (IMD), such as cardiac pacemakers, the AAMI/ISO 14117-2012 standard should also be applied. Because of that, inside the cabin, and at any point in space outside the vehicle that is located higher than 70 cm above ground, the required maximum rms values of the magnetic field strength is further limited to 15 µT, with the peak magnetic field limited to 21.2 µT. SAE J2954 admits that conformity may still be observed if this additional requirement is not met in these critical spaces, but in this case, steps should be taken to warn pacemaker wearers to avoid this region, that is, to stay away from the car. In practice, in order not to restrict in any manner the use of cars by passengers with IMD, this additional requirement has to be observed [127].

2.2.7. Dynamic Inductive Wireless Power Transfer In addition to wirelessly charging the batteries of vehicles that are stationary over a charging pod by means of IWPT, there is an increasingly active research line worldwide to implement dynamic inductive wireless power transfer (DIWPT) [130], that is, to transfer power to the vehicles while they are moving along the road. This allows the battery size capacity and cost to be reduced, or even the battery suppression in some specific applications. This idea was described in a 1891 patent from Maurice Hutin and Maurice Leblanc [131], only about 10 years after the introduction of the first electric cars running on tracks, in 1881 [132,133] although it is not known to have been actually implemented at that time. Pursuing the same ideals of Hutin and Leblanc, George Ilyich Babat designed, prototyped and, in June of 1943, successfully demonstrated what he called the “High-frequency Transport” [134,135] which was the first DIWPT system currently reported to have been publicly demonstrated. It included all the electronics for powering modular inductive lanes, as well as harvesting and rectifying the electric current on board of a vehicle. However, Babat’s prototype operated at very low efficiency levels, due to technological limitations of that time. Possibly because of this, the DIWPT applications remained forgotten for decades. In 1974, the idea was rediscovered and a system was proposed by John Bolger [136,137]. Following Bolger’s work, several other pioneering designs and projects with the same goal were established: the roadway powered electric vehicle (RPEV) [138,139] from 1979 to 1986; the developments at the University of Auckland [140,141] in the 1990s, the on-line electric vehicle (OLEV) at the Korea Advanced Institute of Science and Technology (KAIST), since 2009 [142–145]. In the OLEV, for instance, the system architecture was developed in sequential generations, the first in 2009. The system uses ferrite-cored oblong primary coils buried beneath ground level. These coils are energized with ac currents of about 200 A at 20 kHz, in the first development phases, then 60 kHz was used. The SS-compensation topology was adopted [146], in the same manner as it would be in a standard stationary IWPT. The long primary coils create a planar symmetry that keeps the coupling characteristics independent of the particular position of the vehicle along these coils, except if the vehicle is very close to the coil extremities. The coupling coefficient may vary as the vehicle drifts transversally to the lane, but since the magnetic coupling is not high, the resonance frequency is kept approximately constant, thus facilitating the IWPT implementation [147]. Common to all other DIWPT designs where the vehicle can freely move on the plane of displacement, the lateral misalignment is Energies 2020, 13, 6343 19 of 61

one of the most critical parameters to be controlled. If it is too large, an excessive reduction of the magnetic coupling coefficient occurs, causing the efficiency to drop and, ultimately, the transmitter power as well. Despite all these technical difficulties, DIWPT was soon recognized as a promising research effort, attracting the contribution of many groups around the world, among them [148–152]. More recent projects, involving both scientific and industrial partners, such as the FABRIC European project [153–156] have further advanced the technology. However, the proposed solutions are complex, and so are their cost, which is still being fully assessed, and represents a major challenge for the widespread adoption of the technology. Because of that, new enterprises are trying to develop DIWPT systems that are more accessible, but at the same time, up-scalable. One of them, Electreon Wireless, from Israel, is deploying and evaluating, along 2020, the first DIWPT pilot project over one kilometer, in the Isle of Gotland, Sweden [157]. The system is built around a 20 kW IWPT module that is reported to achieve 88% to 90% efficiency and works with a large air-gap, of about 24 cm to 27 cm, allowing the primary coils to rest 8 cm beneath the ground surface, a characteristic that shall increase safety and durability [130]. Most of the research and development efforts in DIWPT are directed to standard size road vehicles, targeting the electrification of the urban fleet with minimum impact to the composition of the service matrix delivered by the currently in use ICE vehicles. However, the application of DIWPT to soft mobility transportation is also being investigated. A DIWPT lane module for e-bikes and similar low-cost lightweight EVs, as shown in Figure 17, has been developed using a PS-compensated IWPT architecture. The primary, on the lane side, is driven by a resonant inverter and wirelessly energizes the power train of an electrically assisted bicycle adapted with a pick-up coil around the rear wheel [158]. The activation of the module power is triggered by the detection and positive identification of a radio frequency identification (RFID) tag placed at the pick-up coil [159,160]. The excess energy transferred during the transit over the lane modules is stored on-board, and it is enough to keep the vehicle running on electricity also over the non-electrified inter-module gaps, provided that consecutive modules are not too far apart from Energieseach other.2020, 13, x FOR PEER REVIEW 20 of 63

(a) (b)

FigureFigure 17. 17. Dynamic inductiveinductive wireless wireless power power transfer transfer (DIWPT) (DIWPT) for e-bike: for e-bike: (a) prototype (a) prototype jointly developed jointly developedby the University by the University of Minho andof Minho the University and the ofUniv Vigo,ersity during of Vigo, tests during on workbench; tests on workbench; (b) experimental (b) experimentalcurves of input curves power of (blue)input andpower net (blue) transferred and net power transferred to powertrain power (red) to powertrain vs. lateral (red) misalignment. vs. lateral misalignment. Overall, the DIWPT can be considered a promising technology, still in its early stages, and the identiﬁcationOverall, the of allDIWPT its possible can be applications considered toa promising transports technology, is yet to be unveiled. still in its The early stationary stages, and wireless the identificationbattery charging of all with its possible IWPT, on applications the other hand, to transports was commercially is yet to be unveiled. introduced The only stationary about three wireless years batteryago [161 charging], but can with already IWPT, be on regarded the other as hand, a newly was consolidatedcommercially technology, introduced withonly aabout growing three market, years agothat [161], is predicted but can already to expand be regarded at a compound as a newly annual consolidated growth rate technology, estimated with between a growing 36% [162market,] and that60% is [ 163predicted] during to the expand next years,at a compound reaching aannual total market growth share rate onestimated the order between of USD 36% 200 million[162] and by 60% 2025. [163] during the next years, reaching a total market share on the order of USD 200 million by 2025.

2.3. Power Electronics for Traction Systems Over the past few years, the technological advances in the field of power electronics have created new opportunities for the use of various types of electric machine for automotive applications, where characteristics such as high starting torque, high power density and high efficiency have always been regarded as fundamental. The dc machine was, at the beginning of the 20th century, the most used electric machine for applications related to traction systems due to its ability to produce high starting torque and ease of speed or torque control [164]. However, the presence of brushes and mechanical commutators requires recurrent maintenance and consequently increases the costs of this solution. As an alternative to these machines, brushless dc (BLDC) machines appeared, which are characterized by their high starting torque, high efficiency and absence of mechanical commutators [165]. In addition to these characteristics, they are especially suitable for applications that require high power density and, therefore, are recommended for electric mobility applications [166,167]. Regarding BLDC machine types, these can be divided into two categories [168,169]: out-runner type electric BLDC machine and in-runner type BLDC machine. An example of an EV that uses a BLDC machine is the Honda-e [170]. Similar to the BLDC machines, permanent magnet synchronous machines (PMSMs) have been around for several decades for numerous applications, as presented in [171–178]. These types of machine, also known in the literature as brushless ac and permanent magnet ac motors, uses permanently magnetized magnets to create the magnetic flux, thus avoiding the need for a dc source, as well as the respective slip rings and brushes. This increases the reliability as well as the power density of the electric machine, making viable its application in EVs [165,179]. Although they have existed for several decades, PMSMs started to gain more popularity during the last decade due to the emergence of new metals used in the manufacture of permanent magnets, as well as due to the innovative construction alternatives for manufacturing this machine as demonstrated in [180–187]. Despite the aforementioned advantages, PMSMs have a high acquisition cost due to the rare metals used in permanent magnets. Many vehicle manufacturers use PMSMs in their EVs, e.g., Toyota Prius, Ford Focus Electric, Nissan Leaf, BMW i3, Chevrolet Bolt EV [164,188,189]. Another type of electric machine that has been gaining popularity in recent years in EV applications is the three-phase induction machine. Compared to the previously presented electric

Energies 2020, 13, 6343 20 of 61

2.3. Power Electronics for Traction Systems Over the past few years, the technological advances in the field of power electronics have created new opportunities for the use of various types of electric machine for automotive applications, where characteristics such as high starting torque, high power density and high efficiency have always been regarded as fundamental. The dc machine was, at the beginning of the 20th century, the most used electric machine for applications related to traction systems due to its ability to produce high starting torque and ease of speed or torque control [164]. However, the presence of brushes and mechanical commutators requires recurrent maintenance and consequently increases the costs of this solution. As an alternative to these machines, brushless dc (BLDC) machines appeared, which are characterized by their high starting torque, high efficiency and absence of mechanical commutators [165]. In addition to these characteristics, they are especially suitable for applications that require high power density and, therefore, are recommended for electric mobility applications [166,167]. Regarding BLDC machine types, these can be divided into two categories [168,169]: out-runner type electric BLDC machine and in-runner type BLDC machine. An example of an EV that uses a BLDC machine is the Honda-e [170]. Similar to the BLDC machines, permanent magnet synchronous machines (PMSMs) have been around for several decades for numerous applications, as presented in [171–178]. These types of machine, also known in the literature as brushless ac and permanent magnet ac motors, uses permanently magnetized magnets to create the magnetic flux, thus avoiding the need for a dc source, as well as the respective slip rings and brushes. This increases the reliability as well as the power density of the electric machine, making viable its application in EVs [165,179]. Although they have existed for several decades, PMSMs started to gain more popularity during the last decade due to the emergence of new metals used in the manufacture of permanent magnets, as well as due to the innovative construction alternatives for manufacturing this machine as demonstrated in [180–187]. Despite the aforementioned advantages, PMSMs have a high acquisition cost due to the rare metals used in permanent magnets. Many vehicle manufacturers use PMSMs in their EVs, e.g., Toyota Prius, Ford Focus Electric, Nissan Leaf, BMW i3, Chevrolet Bolt EV [164,188,189]. Another type of electric machine that has been gaining popularity in recent years in EV applications is the three-phase induction machine. Compared to the previously presented electric machines, the three-phase induction machine, in particular the induction machine with a squirrel-cage rotor, has the advantage of not having to provide energy for rotating parts. In addition, they do not have permanent magnets such as PMSM and BLDC machines, which means that they are more affordable. However, the implementation of permanent magnets in the induction machine is also possible, as investigated in [190–192], to improve some characteristics such as power factor, efficiency and torque. Nevertheless, the induction machine has the disadvantage of not differentiating the magnetization of the winding power supply itself, which, in EV applications, translates into a greater amount of energy required from the batteries [165,179,193]. Despite this disadvantage, its high-performance capability is already proven through some models already available on the market, such as Tesla’s EVs Roadster, Model S and Model X, Mahindra Reva e2o, Honda Fit EV, which use this type of machine for their propulsion [164]. More recently, boosted by the advances in power electronics and digital control systems, reluctance machines have emerged, presenting as main feature a robust and simpler construction, when compared with the electric machines previously discussed [194]. The rotor of these electric machines consists of a piece of rolled steel with no windings or permanent magnets. This characteristic makes it possible to reduce the rotor’s inertia and, consequently, increase its acceleration, contributing to applications where high speeds of rotation are required. In addition, as the heat produced is mainly confined to the stator, it becomes easier to cool this type of electric machine. These machines combine the efficiency of synchronous machines, since they have no slip, with the absence of permanent magnets as seen in induction machines. Regarding reluctance, they can branch out into stepper motors, synchronous reluctance machines and switched reluctance machines [195,196]. There are also hybrid reluctance permanent magnet machines, which combines the performance of PMSM and reluctance Energies 2020, 13, 6343 21 of 61 machines, allowing an additional degree of freedom for the control [197–200]. These electric machines are a recent, but promising technology in the field of electric mobility [201–210]. In [211] an analysis and a comparison among PMSMs, switched reluctance machines, and induction machines are presented, focusing on EV applications, as well as other types of applications, such as aircraft, ships, and trains. The selection of an electric machine for the traction system of an EV falls not only on the quality/price criterion but also, and increasingly, on an acceptable mechanical and electromagnetic performance [212,213]. Any EV makes use of an electric machine that needs to be controlled according to the driver’s preference. The control of an electric machine can be either applied to the rotational speed or to the torque, in either case with the reference variable being controlled by the driver. The speed control is related to cruise control systems, where the driver defines the reference speed of the EV that will result in a given rotational speed for the electric machine. On the other hand, the torque control consists of the most common approach in vehicles in general, with the accelerator pedal of the vehicle being used to establish a reference torque for the electric machine. Nonetheless, in both cases, the electric machine needs to be controlled, where it is of utmost importance to measure and control the voltage, current, and frequency variables. Therefore, power electronics converters are indispensable for driving the electric machines, since they are able to produce the required voltage, current, and frequency in each instant, guaranteeing the proper operation of the electric machine. The power electronics converters for EV traction systems, as well as for other applications, can be classified in accordance with the passive elements present in the dc link. These can be classified as voltage source converters, where the energy storage element in the dc link is a capacitor, storing energy in the form of an electric field, or current source inverters, where the energy storage element in the dc link is an inductor, storing energy in the form of a magnetic field. The topologies of power electronics converters are also related to the type of electric machine that needs to be driven (for instance, switched reluctance machines use a different power converter topology than those of PMSMs or induction machines). The majority of applications in the field of power electronics is supplied with voltage sources.

ThisEnergies type 2020 of, 13 converter, x FOR PEER is REVIEW generally more eﬃcient than its current source counterpart. Additionally,22 of 63 it has capacitors in the dc link, which are less bulky and heavy than inductors for similar power levels. TheThe capacitance capacitance value value of theseof these capacitors capacitors must must be highbe high enough enough to smoothto smooth the ripplethe ripple currents currents generated bygenerated the switching by the of switching the power of the semiconductors, power semiconduc andtors, the and root the mean root squaremean square (rms) (rms) value value of the of dc link currentthe dc can link vary current between can vary 50% between and 80% 50% with and respect 80% with to the respect electric to machinethe electric nominal machine current nominal [212 ]. currentIn order [212]. to drive dc electric machines, generally, it is used the power converter present in Figure 18a. In order to drive dc electric machines, generally, it is used the power converter present in Figure This converter allows the operation of the electric machine in four quadrants, i.e., allowing the rotation 18a. This converter allows the operation of the electric machine in four quadrants, i.e., allowing the of the machine in both directions with the possibility of performing regenerative braking also in both rotation of the machine in both directions with the possibility of performing regenerative braking directions.also in both The directions. power converter The power comprises converter four comp fully-controlledrises four fully-controlled power semiconductors power semiconductors and a capacitor bankand in a thecapacitor dc link. bank Besides in the adjustingdc link. Besides the polarity adjusting of thethe voltage,polarity of and the consequently voltage, and consequently the direction of the currentthe direction applied of to the the current electric applied machine to the stator electric winding, machine this stator converter winding, also this allows converter the also controllability allows of boththevariables. controllability of both variables.

S1 S3 S1 S3 S5 Three-Phase dc Electric ac Electric Machine Machine vdc C1 vdc C1

S2 S4 S2 S4 S6

(a) (b)

FigureFigure 18. 18.Voltage Voltage sourcesource power converter converter for for the the control control of: ( of:a) dc (a )electric dc electric machines; machines; (b) three-phase (b) three-phase acac electric electric machines. machines.

The three-phase voltage source converter shown in Figure 18b comprises six semiconductors and a capacitor bank in the dc link. Being a three-phase voltage source converter, it allows the production of three phase-to-phase voltages, where their amplitude, frequency and phase can be controlled. Hence, this converter is used in the vast majority of drive systems for three-phase ac electric machines. It should be noted that, despite being also used in an ac electric machine, the power electronics converters that drive the switched reluctance machines are different from the previously presented converter, mainly due to the fact that the torque produced by this type of electric machine is independent of the current direction. The topology presented in Figure 19 is the most flexible for the four-quadrant operation of the switched reluctance machine. The main benefit of this converter is the independent control of each phase. On the other hand, it presents the disadvantage of using two switching devices and two diodes in each phase of the machine. The power electronics converter presented in Figure 20 is denominated by a split-capacitor converter. This converter needs only one switching device and one diode per phase, however, it requires a split dc link voltage. This implies that the maximum voltage that can be applied to the electric machine stator windings is only half of the total dc link voltage. Furthermore, this converter is only applicable to switched reluctance machines with an even number of phases [214].

S1 D2 S3 D4 S5 D6 S7 D8

Switched Reluctance Machine vdc C L1 L2 L3 L4

D1 D3 D5 D7 S2 S4 S6 S8

Figure 19. Classical power converter for switched reluctance machines.

Energies 2020, 13, x FOR PEER REVIEW 22 of 63

The capacitance value of these capacitors must be high enough to smooth the ripple currents generated by the switching of the power semiconductors, and the root mean square (rms) value of the dc link current can vary between 50% and 80% with respect to the electric machine nominal current [212]. In order to drive dc electric machines, generally, it is used the power converter present in Figure 18a. This converter allows the operation of the electric machine in four quadrants, i.e., allowing the rotation of the machine in both directions with the possibility of performing regenerative braking also in both directions. The power converter comprises four fully-controlled power semiconductors and a capacitor bank in the dc link. Besides adjusting the polarity of the voltage, and consequently the direction of the current applied to the electric machine stator winding, this converter also allows the controllability of both variables.

S1 S3 S1 S3 S5 Three-Phase dc Electric ac Electric Machine Machine vdc C1 vdc C1

S2 S4 S2 S4 S6

(a) (b)

Figure 18. Voltage source power converter for the control of: (a) dc electric machines; (b) three-phase Energiesac electric2020, 13 ,machines. 6343 22 of 61

The three-phase voltage source converter shown in Figure 18b comprises six semiconductors and aThe capacitor three-phase bank voltagein the sourcedc link. converter Being a shownthree-phase in Figure voltage 18b comprises source converter, six semiconductors it allows the and productiona capacitor bankof three in the phase-to-phase dc link. Being voltages, a three-phase where voltage their source amplitude, converter, frequency it allows and the phase production can be of controlled.three phase-to-phase Hence, this voltages, converter where is used their in amplitude, the vast majority frequency of and drive phase systems can be for controlled. three-phase Hence, ac electricthis converter machines. is used in the vast majority of drive systems for three-phase ac electric machines. ItIt should should be be noted noted that, that, despite despite being being also also used used in in an an ac ac electric electric machine, machine, the the power power electronics electronics convertersconverters that that drive drive the the switched switched reluctance reluctance mach machinesines are are different different from from the the previously previously presented presented converter,converter, mainlymainly duedue to to the the fact fact that that the torque the torque produced produced by this typeby this of electric type of machine electric is independentmachine is independentof the current of direction. the current The direction. topology The presented topology in Figurepresented 19 is in the Figure most 19 flexible is the formost the flexible four-quadrant for the four-quadrantoperation of the operation switched of reluctancethe switched machine. reluctance The machine. main benefit The ofmain this benefit converter of this is theconverter independent is the independentcontrol of each control phase. of On each the phase. other hand,On the it presentsother hand, the disadvantageit presents the of disadvantage using two switching of using devices two switchingand two diodes devices in and each two phase diodes of the in machine. each phase The of power the machine. electronics The converter power presentedelectronics in converter Figure 20 presentedis denominated in Figure by a 20 split-capacitor is denominated converter. by a split-c Thisapacitor converter converter. needs only This one converter switching needs device only and one one switchingdiode per device phase, and however, one diode it requires per phase, a split however, dc link voltage. it requires This a impliessplit dc thatlink thevoltage. maximum This implies voltage thatthat the can maximum be applied voltage to the that electric can machinebe applied stator to the windings electric machine is only half stator of thewindings total dc is linkonly voltage.half of theFurthermore, total dc link this voltage. converter Furthermore, is only applicable this converter to switched is reluctanceonly applicable machines to withswitched an even reluctance number machinesof phases with [214 ].an even number of phases [214].

S1 D2 S3 D4 S5 D6 S7 D8

Switched Reluctance Machine vdc C L1 L2 L3 L4

D1 D3 D5 D7 S2 S4 S6 S8

Energies 2020, 13, x FORFigureFigure PEER 19. 19.REVIEW ClassicalClassical power power converter converter for for switched switched reluctance reluctance machines. machines. 23 of 63

S1 D2 S3 D4 vdc C1 Switched L1 L3 Reluctance Machine

L2 L4

vdc C2 D1 D3 S2 S4

Figure 20. Split-capacitor power converter for switched reluctance machines.machines.

InIn low-speed applications,applications, wherewhere aa properproper currentcurrent controlcontrol isis desirable forfor all the operating power rangerange ofof thethe electricelectric machine,machine, thethe classicalclassical powerpower converterconverter usedused forfor switchedswitched reluctancereluctance electricelectric machines cancan be be simpliﬁed, simplified, obtaining obtaining the powerthe power converter converter presented presented in Figure in 21 Figure. This power21. This converter power isconverter known asis known a Miller as converter a Miller converter due to its due creator to its andcreator it is and also it namedis also named as a c-dump as a c-dump converter converter [214]. The[214]. main The advantage main advantage of this converterof this converter is the use is the of n use+ 1 of switching n + 1 switching devices devices and n + and1 diode n + 1 for dioden phases for n ofphases the electric of the electric machine, machin contrarilye, contrarily to the previous to the previous converter, converter, which uses which 2n usesswitching 2n switching devices devices and 2n diodesand 2n perdiodes phase. per On phase. the other On the hand, other the hand, main drawbackthe main drawback of this topology of this is topology related to is high-speed related to operation,high-speed since operation, it only usessince one it on switchingly uses one devices switching to control devices the common to control point the ofcommon the electric point machine of the phaseselectric [machine215]. phases [215].

S1 D2 D3 D4 D5

Switched Reluctance Machine vdc C L1 L2 L3 L4

D1 S2 S3 S4 S5

Figure 21. Miller power converter for switched reluctance machines.

As previously referred to, current source converters use an inductor in the dc link instead of a capacitor for storing energy, making the converter operating as a current source. In Figure 22 is shown a three-phase current source converter. The main advantages of this type of power converters, when used in EVs, are: (i) inherent short-circuit protection; (ii) the produced ac currents have lower high-frequency ripple; (iii) it is not necessary to connect antiparallel diodes to the fully-controlled switching devices; (iv) it is possible to obtain higher output ac voltages than the input dc voltage. However, this type of power converter also presents disadvantages compared to voltage source converters: higher volume and weight; higher risk of over-voltages and voltage spikes, as well as the necessity of using switching devices capable of blocking bipolar voltages—the reverse-blocking insulated gate bipolar transistor (RB-IGBT) is an example of such power semiconductor, which has the disadvantage of having higher conduction and switching losses than regular IGBTs, besides being more expensive. Alternatively to the use of RB-IGBTs, a diode can be placed in series with each IGBT, which also increases the overall losses and cost of the power converter [212,216,217].

Energies 2020, 13, x FOR PEER REVIEW 23 of 63

S1 D2 S3 D4 vdc C1 Switched L1 L3 Reluctance Machine

L2 L4

vdc C2 D1 D3 S2 S4

Figure 20. Split-capacitor power converter for switched reluctance machines.

In low-speed applications, where a proper current control is desirable for all the operating power range of the electric machine, the classical power converter used for switched reluctance electric machines can be simplified, obtaining the power converter presented in Figure 21. This power converter is known as a Miller converter due to its creator and it is also named as a c-dump converter [214]. The main advantage of this converter is the use of n + 1 switching devices and n + 1 diode for n phases of the electric machine, contrarily to the previous converter, which uses 2n switching devices and 2n diodes per phase. On the other hand, the main drawback of this topology is related to high-speedEnergies 2020, 13operation,, 6343 since it only uses one switching devices to control the common point of23 ofthe 61 electric machine phases [215].

S1 D2 D3 D4 D5

Switched Reluctance Machine vdc C L1 L2 L3 L4

D1 S2 S3 S4 S5

FigureFigure 21. 21. MillerMiller power power converter converter for for swit switchedched reluctance reluctance machines. machines.

AsAs previously previously referred referred to, to, current current source source converters converters use use an an inductor inductor in in the the dc dc link link instead instead of of a a capacitorcapacitor for storingstoring energy,energy, making making the the converter converter operating operating as a currentas a current source. source. In Figure In Figure22 is shown 22 is a shownthree-phase a three-phase current source current converter. source converter. The main The advantages main advantages of this type of of this power type converters, of power converters, when used whenin EVs, used are: in (i) EVs, inherent are: short-circuit(i) inherent short-circuit protection; (ii) protection; the produced (ii) th ace currentsproduced have ac lowercurrents high-frequency have lower high-frequencyripple; (iii) it is notripple; necessary (iii) it is to not connect necessary antiparallel to connect diodes antiparallel to the fully-controlled diodes to the switching fully-controlled devices; switching(iv) it is possible devices; to (iv) obtain it is higher possible output to obtain ac voltages higher than output the inputac voltages dc voltage. than the However, input dc this voltage. type of However,power converter this type also of presents power disadvantagesconverter also comparedpresents disadvantages to voltage source compared converters: to voltage higher volumesource converters:and weight; higher higher volume risk of over-voltagesand weight; higher and voltage risk of spikes,over-voltages as well asand the voltage necessity spikes, of using as well switching as the necessitydevices capable of using of blockingswitching bipolar devices voltages—the capable of blocking reverse-blocking bipolar insulatedvoltages—the gate bipolarreverse-blocking transistor insulated(RB-IGBT) gate is an bipolar example transistor of such (RB-IGBT) power semiconductor, is an example which of such has power the disadvantage semiconductor, of having which higher has theconduction disadvantage and switchingof having higher losses thanconduction regular and IGBTs, switching besides losses being than more regular expensive. IGBTs, Alternatively besides being to morethe use expensive. of RB-IGBTs, Alternatively a diode can to the be use placed of RB-IGBTs in series with, a diode each can IGBT, be placed which in also series increases with each the overallIGBT, whichlosses also and increases cost of the the power overall converter losses and [212 cost,216 ,of217 the]. power converter [212,216,217]. Energies 2020, 13, x FOR PEER REVIEW 24 of 63

S1 S3 S5 Switched Reluctance L Machine

S2 S4 S6

C1 C2 C3

FigureFigure 22. 22. CurrentCurrent source source power power converter converter for for the the cont controlrol of of three-phase three-phase ac electric machines.

AsAs mentioned,mentioned, EVs EVs are are related related to ato new a andnew revolutionary and revolutionary type of vehicles,type of ve however,hicles, thehowever, conversion the conversionof conventional of conventional vehicles, based vehicles, on ICEs, intobased EVs on can IC beEs, an into attractive EVs can and low-costbe an attractive solution forand a transitorylow-cost solutionperiod. This for a process transitory is a challengeperiod. This that process has become is a challenge popular in that the lasthas fewbecome years. popular Some universities in the last few and years.research Some centers universities have been and developing research conversions centers have of ICEbeen vehicles developing to EVs. conversions Examples of of these ICE conversionsvehicles to EVs.include Examples an Opel of Kadett these byconversions the University include of Zagreban Opel [218 Kadett], a VW by the Lupo University by the Eindhoven of Zagreb University [218], a VW of LupoTechnology by the [219 Eindhoven], a Honda University Civic by CREATE of Technology Lab of the [219], University a Honda of Carnegie Civic by Mellon CREATE [220 ],Lab a Porsche of the University914 by Massachusetts of Carnegie InstituteMellon [2 of20], Technology a Porsche [914221 ],by a Massachusetts Volvo 460 GLE Institute by the Polytechnic of Technology Institute [221], ofa VolvoViseu [460222 GLE], and by a the Jaguar Polytechnic E-Type byInstitute Jaguar of itself Viseu [223 [222],]. The and group a Jaguar of energy E-Type and by Jaguar power itself electronics [223]. The(GEPE) group of theof Universityenergy and of power Minho electronics has developed (GEPE) a prototype of the University of a plug-in of EVMinho that ishas called developed CEPIUM a prototype(Carro Elétrico of a Plug-In plug-in da EV Universidade that is called do Minho CEPIUM), which (Carro can Elétrico be seen Plug-In in Figure da 23Universidade. CEPIUM isdo a Minho vehicle), whichwhere thecan ICEbe seen was replacedin Figure by 23. an CEPIUM electric machine. is a vehicle An axialwhere ﬂux the PMSM ICE was is used replaced due to by technological an electric machine.advantages, An namelyaxial flux high PMSM eﬃciency, is used low due maintenance to technological costs, advantages, and reduced namely size forhigh the efficiency, same power low maintenancewhen compared costs, with and other reduced types size of for electric the same machine. power Everything when compared necessary with for other the types conversion of electric was machine. Everything necessary for the conversion was developed by the research group, including the power electronics circuits for the traction system and for the EVBC with G2V and V2G features [224,225].

(a) (b) (c)

Figure 23. Developed prototype of a plug-in EV (CEPIUM—Carro Elétrico Plug-In da Universidade do Minho): (a) EV prototype; (b) powertrain; (c) power converter detail.

Recently, several companies around the world have been specializing in the conversion of ICE vehicles to EVs. These companies have focused on the development of conversion kits for specific vehicle models and universal conversion kits that can be applied to any vehicle model. Examples of these companies include: ZEEV, from Portugal; Retrofuture, from France; EV Europe, from the Netherlands; EV West, from the United States of America; and Freedom Won, from South Africa. Moreover, this approach allows for a more sustainable transition from ICE vehicles to EVs, since it is based on the partial reutilization of parts, instead of the total replacement of conventional ICE vehicles, contributing to reduce the ecological footprint. The conversion of ICE vehicles to EVs gained some attention of the market and may represent a feasible solution in that transition.

2.4. Unified Systems for Traction and Battery Charging Typically, an EV comprises two main power systems, one regarding the traction system and the other regarding the EVBC. By analyzing both systems, considerable similarities can be found in both power electronics topologies, specifically, in the converter that drives the electric machine responsible

Energies 2020, 13, x FOR PEER REVIEW 24 of 63

S1 S3 S5 Switched Reluctance L Machine

S2 S4 S6

C1 C2 C3

Figure 22. Current source power converter for the control of three-phase ac electric machines.

As mentioned, EVs are related to a new and revolutionary type of vehicles, however, the conversion of conventional vehicles, based on ICEs, into EVs can be an attractive and low-cost solution for a transitory period. This process is a challenge that has become popular in the last few years. Some universities and research centers have been developing conversions of ICE vehicles to EVs. Examples of these conversions include an Opel Kadett by the University of Zagreb [218], a VW Lupo by the Eindhoven University of Technology [219], a Honda Civic by CREATE Lab of the University of Carnegie Mellon [220], a Porsche 914 by Massachusetts Institute of Technology [221], a Volvo 460 GLE by the Polytechnic Institute of Viseu [222], and a Jaguar E-Type by Jaguar itself [223]. The group of energy and power electronics (GEPE) of the University of Minho has developed a prototype of a plug-in EV that is called CEPIUM (Carro Elétrico Plug-In da Universidade do Minho), which can be seen in Figure 23. CEPIUM is a vehicle where the ICE was replaced by an electric Energiesmachine.2020, 13 An, 6343 axial flux PMSM is used due to technological advantages, namely high efficiency, low24 of 61 maintenance costs, and reduced size for the same power when compared with other types of electric machine. Everything necessary for the conversion was developed by the research group, including developedthe power by electronics the research circuits group, for including the traction the sy powerstem and electronics for the EVBC circuits with for G2V the and traction V2G features system and for the[224,225]. EVBC with G2V and V2G features [224,225].

(a) (b) (c)

FigureFigure 23. 23.Developed Developed prototype prototype of of a aplug-in plug-in EV (CEPIUM— CarroCarro Elétrico Elétrico Plug-In Plug-In da Universidade da Universidade do do MinhoMinho): (a):) ( EVa) EV prototype; prototype; (b ()b powertrain;) powertrain; ((cc)) power converter converter detail. detail.

Recently,Recently, several several companies companies around around thethe world have have been been specializing specializing in the in theconversion conversion of ICE of ICE vehiclesvehicles to EVs.to EVs. These These companies companies havehave focusedfocused on on the the development development of ofconversion conversion kits kitsfor specific for specific vehicle models and universal conversion kits that can be applied to any vehicle model. Examples of vehicle models and universal conversion kits that can be applied to any vehicle model. Examples of these companies include: ZEEV, from Portugal; Retrofuture, from France; EV Europe, from the these companies include: ZEEV, from Portugal; Retrofuture, from France; EV Europe, from the Netherlands; EV West, from the United States of America; and Freedom Won, from South Africa. Netherlands;Moreover, EVthis West,approach from allows the for United a more States sustainable of America; transition and from Freedom ICE vehicles Won, to from EVs, since South it Africa.is Moreover,based on this the approach partial reutilization allows for aof more parts, sustainable instead of transitionthe total replacement from ICE vehicles of conventional to EVs, sinceICE it is basedvehicles, on the contributi partial reutilizationng to reduce ofthe parts, ecological instead footprint. of the totalThe conversion replacement of ICE of conventionalvehicles to EVs ICE gained vehicles, contributingsome attention to reduce of the themarket ecological and may footprint. represent Thea feasible conversion solution of inICE that vehiclestransition. to EVs gained some attention of the market and may represent a feasible solution in that transition. 2.4. Unified Systems for Traction and Battery Charging 2.4. Unified Systems for Traction and Battery Charging Typically, an EV comprises two main power systems, one regarding the traction system and the otherTypically, regarding an EV the comprises EVBC. By analyzing two main both power systems, systems, considerable one regarding similarities the can traction be found system in both and the otherpower regarding electronics the EVBC. topologies, By analyzing specifically, both in the systems, converter considerable that drives the similarities electric machine can be responsible found in both power electronics topologies, specifically, in the converter that drives the electric machine responsible for the EV traction and in the converter responsible for the EVBC. Additionally, it should be noted that these two systems never operate simultaneously, thereby many topologies and control algorithms that aim to unify both power converters into a single one have been investigated targeting to reduce the volume, weight, and total cost of the system. The first unified power converter was proposed by a partnership between the USA Department of Energy and NASA [226,227], followed by the patented proposals from Rippel and Cocconi in 1992 [228]. Since then, numerous topologies have been studied and proposed, where some exemplificative cases can be found in [229–231]. The diversity of topologies is large, including solutions based on split-phase winding machines [232,233], power converters based on voltage source [234] and current source structures [235], and with or without galvanic isolation [236–239]. A simple topology that allows the unification of both power converters is shown in Figure 24. The main feature of this topology is the possibility to be applied to any type of three-phase ac electric machine, with or without access to all winding terminals, since the electric machine is disconnected from the power converter during the battery charging operation. In order to prevent harmonic current injection into the power grid and operate with a unitary power factor, it is necessary to add filters in series. These filters comprise three inductors with a high current rating, therefore being heavy and bulky, which is very disadvantageous considering their application in EVs [234,240]. Dusmez and Khaligh [241] proposed a topology similar to that previously presented, with the main difference being the fact that only two high current rating inductors are used instead of three. In this topology, the third inductor is attained with the series connection of two windings of the electric machine. As with the previous topology, this one allows fast battery charging operation through a three-phase power grid. Since the power converters are bidirectional, this topology allows the Energies 2020, 13, x FOR PEER REVIEW 25 of 63 for the EV traction and in the converter responsible for the EVBC. Additionally, it should be noted that these two systems never operate simultaneously, thereby many topologies and control algorithms that aim to unify both power converters into a single one have been investigated targeting to reduce the volume, weight, and total cost of the system. The first unified power converter was proposed by a partnership between the USA Department of Energy and NASA [226,227], followed by the patented proposals from Rippel and Cocconi in 1992 [228]. Since then, numerous topologies have been studied and proposed, where some exemplificative cases can be found in [229–231]. The diversity of topologies is large, including solutions based on split-phase winding machines [232,233], power converters based on voltage source [234] and current source structures [235], and with or without galvanic isolation [236–239]. A simple topology that allows the unification of both power converters is shown in Figure 24. The main feature of this topology is the possibility to be applied to any type of three-phase ac electric Energiesmachine,2020 with, 13, 6343 or without access to all winding terminals, since the electric machine is disconnected25 of 61 from the power converter during the battery charging operation. In order to prevent harmonic current injection into the power grid and operate with a unitary power factor, it is necessary to add operation with extra functionalities, such as returning back to the power grid part of the energy stored filters in series. These filters comprise three inductors with a high current rating, therefore being in the EV battery. heavy and bulky, which is very disadvantageous considering their application in EVs [234,240].

S1 S3 S5 S7 KM1 Three-Phase ac Electric Machine C1 vdc

L EV 4 battery C2 L1 L2 L3 S2 S4 S6 S8

Smart Grid

Figure 24. 24. UnifiedUniﬁed power power converter converter topology topology based based on the on disconnection the disconnection of the ofthree-phase the three-phase ac electric ac machine.electric machine.

DusmezIn [237] isand presented Khaligh a [241] topology proposed of a unified a topology system sim forilar fast to battery that previously charging presented, using a transformer with the mainthat is difference external to being the EV, the i.e.,fact it that is located only two in thehigh charging current station.rating inductors This topology are used is similar instead to of the three. two presentedIn this topology, above, the where third the inductor electric machineis attained is with disconnected the series from connection the power of two converter windings through of the a electriccontactor. machine. The transformer As with isthe located previous between topology, the power this one converter allows and fast the battery power charging grid. This operation topology throughallows for a three-phase galvanic isolation power and grid. the Since adjustment the power of theconverters dc link voltage.are bidirectional, Accordingly, this topology the EV can allows only thebe charging operation in with a specific extra locationfunctionalities, [237]. such as returning back to the power grid part of the energy storedIn in 1994, the EV AC battery. Propulsion Inc. patented the topology which is in use in BMW Mini E electric car [234In [237],241,242 is presented]. This topology a topology allows of a bidirectional unified system power for fast flow, battery thus allowing charging additional using a transformer operation thatmodes, is external such as V2Gto the and EV, V2H. i.e., However,it is located it doesin the not charging allow fast station. battery This charging topology to be is performed,similar to the since two it presentedis not possible above, to establish where the a connection electric machine with a three-phaseis disconnected power from grid. the Moreover, power converter it allows fastthrough battery a contactor.chargingEnergies to The 2020 be transformer, performed, 13, x FOR PEER sinceis REVIEW located it is possiblebetween to the establish power converter connection and with the a power three-phase grid. This power 26topology of grid.63 allowsFigure for galvanic 25 shows isolation a three-phase and the topology adjustment using of the the electric dc link machine voltage. windings Accordingly, as coupling the EV inductors can only withbe charging the powerFigure in a grid 25specific shows [243 ].location a It shouldthree-phase [237]. be referred topology that using a special the electric type of machine ac electric windings machine as mustcoupling be used in orderIninductors 1994, to accomplish AC with Propulsion the power this topology, Inc.grid patented[243]. i.e., It anshould the open-end topology be referred winding which that a electricis special in use machine,type in BMW of ac since electricMini it E ismachine electric necessary car [234,241,242].to havemust access be used This to thein topology order six terminals to accomplish allows of bidirectional the this stator topology, windings. power i.e., an flow, Thisopen-end topologythus winding allowing presents electric additional a machine, limitation operation since in the it is necessary to have access to the six terminals of the stator windings. This topology presents a modes,battery chargingsuch as V2G mode. and More V2H. specifically, However, ifit andoes asynchronous not allow fast machine battery is used,charging the connectionto be performed, of the limitation in the battery charging mode. More specifically, if an asynchronous machine is used, the sincestator it windings is not possible to the powerto establish grid willa connection create a rotating with a three-phase magnetic field, power which grid. in Moreover, turn will result it allows in a connection of the stator windings to the power grid will create a rotating magnetic field, which in fast battery charging to be performed, since it is possible to establish connection with a three-phase torqueturn that will will result force in thea torque electric that machine will force to the rotate. electric machine to rotate. power grid.

S1 S3 S5 KM1 Three-Phase Smart dc-dc EV Grid ac Electric Machine battery C1 vdc

S2 S4 S6

Figure 25. Unified power converter topology based on open-end winding ac electric machines (with Figure 25. Unified power converter topology based on open-end winding ac electric machines (with access access to all winding’s terminals). to all winding’s terminals). Rambabu Surada et al. [244] proposed the topology that can be used with any type of three-phase Rambabu Surada et al. [244] proposed the topology that can be used with any type of three-phase ac electric machine for the traction system. This topology can operate in five operation modes: (i) ac electricboost-type machine converter, for the adjusting traction the system. dc link voltag Thise topology from the canbattery operate voltage; in (ii) ﬁve buck-type operation dc–dc modes: (i) boost-typeconverter, converter, charging the adjusting battery du thering dc regenerative link voltage braking from theoperatio batteryn; (iii) voltage; plug-in (ii) system, buck-type charging dc–dc the EV battery from the power grid; (iv) independent single-phase power converter, in case of power grid fault; and (v) connection to the power grid, delivering back to the power grid some part of the energy stored in the EV battery. This topology does not allow fast battery charging. In [239] is presented a topology that uses the midpoints of the machine stator windings to connect with the power grid. It has the advantage of being used as a fast battery charger. However, depending on the type of electric machine, the rotor can still rotate, making it necessary to implement a braking system for the electric machine when the battery charging operation is in process. The need to access the midpoints of the electric machine windings is a disadvantage of this topology, since it increases the complexity of the electric machine [229,239,245–247]. Figure 26 presents a topology of a unified system based on the connection of the power grid in the intermediate points of the split-phase stator windings [229,239,245–246]. Once again, the stator windings of the ac electric machine are used as coupling filters. The topology is based on two three-phase power converters that are connected to the winding terminals of each phase of the electric machine, making it possible to increase the voltage applied to the electric machine in traction mode [239]. The main advantage of this topology is the fact that it discards a contactor for interfacing the power grid. As in the previous topology, it is required the access to the intermediate points of the split-phase stator windings. There are other types of electric machine that can be used in this configuration, such as ac electric machines with two stators [232,239,248].

Energies 2020, 13, 6343 26 of 61 converter, charging the battery during regenerative braking operation; (iii) plug-in system, charging the EV battery from the power grid; (iv) independent single-phase power converter, in case of power grid fault; and (v) connection to the power grid, delivering back to the power grid some part of the energy stored in the EV battery. This topology does not allow fast battery charging. In [239] is presented a topology that uses the midpoints of the machine stator windings to connect with the power grid. It has the advantage of being used as a fast battery charger. However, depending on the type of electric machine, the rotor can still rotate, making it necessary to implement a braking system for the electric machine when the battery charging operation is in process. The need to access the midpoints of the electric machine windings is a disadvantage of this topology, since it increases the complexity of the electric machine [229,239,245–247]. Figure 26 presents a topology of a unified system based on the connection of the power grid in the intermediate points of the split-phase stator windings [229,239,245,246]. Once again, the stator windings of the ac electric machine are used as coupling filters. The topology is based on two three-phase power converters that are connected to the winding terminals of each phase of the electric machine, making it possible to increase the voltage applied to the electric machine in traction mode [239]. The main advantage of this topology is the fact that it discards a contactor for interfacing the power grid. As in the previous topology, it is required the access to the intermediate points of the split-phase stator windings. There are other types of electric machine that can be used in this configuration, such as ac electric machines with two stators [232,239,248]. Energies 2020, 13, x FOR PEER REVIEW 27 of 63

S1 S3 S5 S7 S9 S11 Three-Phase ac Electric Machine C1 vdc

S2 S4 S6 S8 S10 S12

Smart Grid

FigureFigure 26. 26.Unified Unified power power converter converter topologytopology based based on on an an open-end open-end winding winding permanent permanent magnet magnet synchronous machine with split windings and two power converters. synchronous machine with split windings and two power converters. In [249] is presented a topology that uses the neutral point of the ac electric machine in order to In [249] is presented a topology that uses the neutral point of the ac electric machine in order to connect to the power grid. It is not necessary to implement a braking system for the electric machine connect to the power grid. It is not necessary to implement a braking system for the electric machine during the battery charging operation because the power flow is unidirectional and it does not duringproduce the battery torque charging[249]. Omar operation Hegazy becauseet al. proposed the power a bidirectional flow is unidirectional single-phase andunified it does topology not produce in torque[238]. [249 This]. Omarpower Hegazyconverter etpresents al. proposed four operation a bidirectional modes: (i) single-phaseelectric machine unified driving, topology transferring in [ 238]. Thisenergy power from converter the dc presentslink to the four electric operation machine; modes: (ii) three-phase (i) electric active machine rectifier, driving, transferring transferring energy energy fromfrom the dcthe linkelectric to themachine electric to machine;the dc link; (ii) (iii) three-phase battery charging active from rectifier, a single transferring-phase power energy grid; from(iv) the electricreturning machine back to tothe a single-phase dc link; (iii) power battery grid charging part of the from energy a single-phase stored in the batteries. power grid; Ivan Subotic (iv) returning et backal. to [230] a single-phase proposed a topology power grid that partuses ofa five-phase the energy ac storedelectric inmachine. the batteries. A five-phase Ivan traction Subotic system et al. [230] proposedpresents a topology a good compromise that uses a five-phasebetween two ac electricrequirem machine.ents that Aare five-phase usually opposed, traction systemnamely presentsfault a goodtolerance compromise and system between complexity. two requirements This topology that needs are a usuallybidirectional opposed, dc–dc converter namely fault to interface tolerance the and battery and the dc link. Saeid Haghbin et al. presented a topology based on a split-phase PMSM with system complexity. This topology needs a bidirectional dc–dc converter to interface the battery and two power converters, namely one per each set of windings. The EVBC is bidirectional and non- the dc link. Saeid Haghbin et al. presented a topology based on a split-phase PMSM with two power isolated, and it does not allow fast battery charging operation [233]. converters,In [231] namely is presented one per a each unified set ofsystem windings. for a sw Theitched EVBC reluctance is bidirectional machine. The and power non-isolated, converter and it doesand not the allow winding fast batteryinductances charging of the operation electric machine [233]. combined behave as an interleaved boost-type dc–dcIn [231 converter.] is presented Other aproposals unified regarding system for unified a switched systems reluctance for this type machine. of electric The machine power can converter be andfound the winding in [250–254]. inductances of the electric machine combined behave as an interleaved boost-type Figure 27 shows the topology of a unified system based on a current source converter. This topology allows charging the battery from regenerative braking without the need for inverting the dc link current [235]. In this solution, either the ac electric machine (traction mode) or the power grid (battery charging mode) is connected to the power converter at each time, depending on the KM1 position. The dc link inductor is interfaced with the EV battery through an asymmetrical h-bridge converter, allowing the dc link current to be adjusted.

Three-Phase ac Electric S S S Machine KM1 1 3 5 S 7 D1 EV battery L

D1 S8 Smart Grid S2 S4 S6

C1 C2 C3

Figure 27. Unified power converter topology based on current source converter.

Energies 2020, 13, x FOR PEER REVIEW 27 of 63

S1 S3 S5 S7 S9 S11 Three-Phase ac Electric Machine C1 vdc

S2 S4 S6 S8 S10 S12

Smart Grid

Figure 26. Unified power converter topology based on an open-end winding permanent magnet synchronous machine with split windings and two power converters.

In [249] is presented a topology that uses the neutral point of the ac electric machine in order to connect to the power grid. It is not necessary to implement a braking system for the electric machine during the battery charging operation because the power flow is unidirectional and it does not produce torque [249]. Omar Hegazy et al. proposed a bidirectional single-phase unified topology in [238]. This power converter presents four operation modes: (i) electric machine driving, transferring energy from the dc link to the electric machine; (ii) three-phase active rectifier, transferring energy from the electric machine to the dc link; (iii) battery charging from a single-phase power grid; (iv) returning back to a single-phase power grid part of the energy stored in the batteries. Ivan Subotic et al. [230] proposed a topology that uses a five-phase ac electric machine. A five-phase traction system presents a good compromise between two requirements that are usually opposed, namely fault tolerance and system complexity. This topology needs a bidirectional dc–dc converter to interface the battery and the dc link. Saeid Haghbin et al. presented a topology based on a split-phase PMSM with two power converters, namely one per each set of windings. The EVBC is bidirectional and non- isolated,Energies 2020 and, 13 ,it 6343 does not allow fast battery charging operation [233]. 27 of 61 In [231] is presented a unified system for a switched reluctance machine. The power converter and the winding inductances of the electric machine combined behave as an interleaved boost-type dc–dcdc–dc converter. converter. Other Other proposals proposals regarding regarding unified uniﬁed sy systemsstems for for this this type type of of electric machine can can be foundfound in in [250–254]. [250–254]. FigureFigure 2727 shows shows the the topology topology of aof uniﬁed a unified system system based based on a current on a current source converter.source converter. This topology This topologyallows charging allows charging the battery the from battery regenerative from regenera brakingtive braking without without the need the for need inverting for inverting the dc linkthe dccurrent link current [235]. [235]. In this In solution,this solution, either either the the ac electricac electric machine machine (traction (traction mode) mode) or orthe the powerpower grid grid (battery(battery charging charging mode) mode) is is connected connected to tothe the powe powerr converter converter at each at each time, time, depending depending on the on KM1 the position.KM1 position. The dc Thelink dcinductor link inductor is interfaced is interfaced with thewith EV battery the EV through battery an through asymmetrical an asymmetrical h-bridge converter,h-bridge converter, allowing allowingthe dc link the current dc link to current be adjusted. to be adjusted.

Three-Phase ac Electric S S S Machine KM1 1 3 5 S 7 D1 EV battery L

D1 S8 Smart Grid S2 S4 S6

C1 C2 C3

FigureFigure 27. 27. UnifiedUniﬁed power power converter converter topology topology based based on on current current source source converter. converter.

2.5. Summary

Regarding EV battery charging systems (EVBC), Table3 presents a comparison based on the topologies presented in item 2.1, namely, on-board and off-board EVBC. With the objective of establishing a comparison in terms of main features, it was considered the multilevel characteristic (i.e., establishing that the topology is considered as multilevel when the voltage levels are higher than 3), the interleaved characteristic (i.e., with more than two parallel paths for current), isolation (i.e., mainly considering the dc–dc power converters, since it is not usual to have isolation on the ac-dc power converters), and bidirectional power flow (i.e., the possibility of the EVBC to operate in G2V/V2G mode).

Table 3. Comparison of the main features of the power electronics converters for on-board and oﬀ-board EV battery charging systems (EVBC).

Multilevel Interleaved Galvanic Bidirectional Topology Topology Isolation Operation Figure3a No No No No Figure3b No No No Yes ac–dc Figure3c No Yes No Yes Figure3d Yes No No Yes on-board Figure4a No No No No Figure4b No No No Yes dc–dc Figure4c No No Yes No Figure4d No No Yes Yes Figure6a No No No No Figure6b No No No Yes ac–dc Figure6c No Yes No Yes Figure6d No No No Yes oﬀ-board Figure7a No Yes No No Figure7b Yes Yes No Yes dc–dc Figure7c No No Yes Yes Figure7d No No Yes Yes Energies 2020, 13, 6343 28 of 61

Table4 shows a comparison among the uniﬁed systems for traction and battery charging topologies presented in item 2.4. The comparison takes in consideration metrics such as the ability to perform fast battery charging, capability of bidirectional power ﬂow, galvanic isolation, the necessity for external inductors, and the need for special electric machines with access to additional winding terminals. It is considered that all the presented topologies allow slow battery charging. Moreover, the analyzed three-phase topologies also allow a single-phase connection.

Table 4. Comparison of uniﬁed systems for traction and battery-charging topologies.

Fast Battery Bidirectional Galvanic External Special Charging Operation Isolation Inductors Machine S. Haghbin, 2013 [234] Yes Yes No Yes No S. Dusmez, 2012 [241] Yes Yes No Yes No S. Haghbin, 2013 [237] Yes Yes Yes No No A. G. Cocconi, 1994 [242] Yes Yes No Yes Yes M. Zaja, 2014 [243] Yes Yes No No Yes R. Surada, 2010 [244] No Yes No Yes No L. D. Sousa, 2010 [239] Yes Yes No No Yes L. Wang, 2012 [249] No No No No Yes O. Hegazy, 2011 [238] No Yes No Yes No I. Subotic, 2016 [230] Yes Yes No No Yes S. Haghbin, 2012 [233] No Yes No No Yes H. C. Chang, 2009 [231] No Yes No No Yes G. J. Su, 2011 [235] Yes Yes No No No

3. Power Electronics for Railway Systems This item presents a review of power electronics for railway systems, showing the importance of power electronics converters in the area of electriﬁed railways, in which these converters can be installed outside the train for the purpose of power quality improvement, or to interface between the high voltage three-phase power grid and the medium voltage single-phase traction power grid. On the other hand, power electronics converters can also be installed inside the train for traction and regenerative braking applications. Additionally, special power electronics applications in railway systems are also presented in this item. Taking into consideration the existing opportunities in the railway industry, not only in the development of the electric train itself, but also on the aforementioned topics, there is a strong investment in technological development for electriﬁed railway systems. Subsequently, various solutions based on power electronics converters are also presented in this item.

3.1. Power Quality Compensators Power quality deterioration has direct adverse impacts either on the power grid, including the electric locomotive itself, or on the signaling and communication system between trains. Power quality deterioration will result in higher operating costs of electriﬁed trains, as well as in polluting the three-phase ac power grids with high total harmonic distortion and negative sequence components [255]. In order to overcome the power quality deterioration, power compensators based on passive components is a technology used in the last decades, and even nowadays. However, these passive compensators are not able to follow the fast-dynamic changes of non-linear loads, which is the case of the traction load (electric locomotive). Therefore, there is a need for dynamic compensators that follow the non-linear dynamic changes, and power electronics compensators serve this purpose [256–259]. In this context, a possible solution is the use of a static synchronous compensator (STATCOM), which consists of a three-phase voltage source converter, coupling power transformer and ﬁlter inductors installed on the three-phase ac power grid. This type of solution allows reactive power exchange between the Energies 2020, 13, 6343 29 of 61

STATCOM and the three-phase ac power grid. For example, in Japan, five sets of 34 MVA to 60 MVA STATCOM devices are available in the Tokaido Shinkansen high-speed railway, which links Tokyo and Shin-Osaka stations [260]. Figure 28a depicts the STATCOM device connected in parallel with the three-phase ac power grid. This parallel connection signifies that the STATCOM can be connected or disconnected without disturbing the operation of the traction substation. Consequently, the robustness and readiness of the railway substation will not be affected by the STATCOM operation [261]. Moreover, this device does not require an internal power supply, meaning that the STATCOM has a neutral contribution in terms of active power. That is, except by the negligible STATCOM internal power losses, STATCOM will not consume or provide active power to the three-phase ac power grid, and its main objective is solely to exchange reactive power with the three-phase ac power grid in order to improve power quality. Consequently, the STATCOM exchanges the instantaneous reactive power among the phases of the three-phase ac power grid, thus acting as a source of reactive power [262]. Energies 2020, 13, x FOR PEER REVIEW 30 of 63

3~ac 3~ac Power High Voltage Power High Voltage Grid Grid Coupling V/V Transformer Transformer Single-phase TPS Medium Voltage STATCOM V/V Transformer

Single-phase TPS Medium Voltage Coupling Transformer Ly RPC Lx

(a) (b)

FigureFigure 28. Power 28. Power electronics electronics compensators compensators in ac railwayin ac railway electrification: electrification: (a) static (a) synchronousstatic synchronous compensator compensator (STATCOM); (b) rail power conditioner (RPC). (STATCOM); (b) rail power conditioner (RPC). Another possibility is to consider a power converter connected to the single-phase traction Another possibility is to consider a power converter connected to the single-phase traction power power system (TPS), where the rail power conditioner (RPC) is used for the purpose of power quality systemimprovement (TPS), where in the the ac railelectrified power railways. conditioner The RPC (RPC) structure is used contains forthe two purpose single-phase of powerback-to- quality improvementback converters in the with ac electrified a single dc railways. link in between The RPC and two structure coupling contains transformers, two single-phase as shown in back-to-backFigure converters28b [263]. with The a single STATCOM dc link device in between is a three-phase and two couplingconverter, transformers,whereas the RPC as shownis composed in Figure of two 28 b [263]. The STATCOMback-to-back device single-phase is a three-phase converters. converter,Each conv whereaserter is connected the RPC isto composeda load section oftwo through back-to-back a single-phasebuck-type converters. coupling transformer. Each converter The first is proposal connected of a to 60 a kV, load 20 sectionMVA RPC through for Tohoku a buck-type Shinkansen coupling transformer.railway in The Japan first was proposal implemented of a 60 in kV, 2004 20 [264]. MVA The RPC implemented for Tohoku RPC Shinkansen at that time railway was based in Japan on was multilevel converters and a Scott power transformer. Another project was implemented in March implemented in 2004 [264]. The implemented RPC at that time was based on multilevel converters 2009, where RPC systems with gate commutated thyristors were installed in four substations of the and aTokaido Scott power Shinkansen transformer. in Japan: Another Shimizu project substati wason, implemented Shin Kikugawa in Marchsubstation, 2009, Shin where Biwajima RPC systems withsubstation, gate commutated and Ritto substation. thyristors Detailed were installed information in fouron that substations project can be of found the Tokaido in [265]. Shinkansen in Japan: ShimizuNew power substation, electronics Shin compensators Kikugawa substation,have the advantage Shin Biwajima of scalability substation, and modularity and Ritto in which substation. Detailedmodular information multilevel on converter that project (MMC) can is adopted be found to inreplace [265 ].the conventional two-level converter. The NewMMC power advantages electronics are quite compensators numerous, such have as the the scalability advantage to different of scalability power and and modularity voltage levels, in which modularreliability multilevel to guarantee converter continuous (MMC) operation is adopted even if to few replace submodules the conventional stop working, two-level and reduced converter. grid-connected filter size, due to the good quality of the synthesized waveforms [266]. Figure 29 The MMC advantages are quite numerous, such as the scalability to different power and voltage levels, shows the RPC based on the MMC system. When compared to the conventional RPC system reliabilitypresented to guarantee in Figure 28b, continuous buck-type coupling operation transf evenormers if few used submodules to interface between stop working, the RPC and and the reduced grid-connectedsingle-phase filter TPS size,can be due dispensed to the good with [267]. quality of the synthesized waveforms [266]. Figure 29 shows the RPC based on the MMC system. When compared to the conventional RPC system presented in 3~ac Figure 28b, buck-type coupling transformers usedPower to interfaceHigh Voltage between the RPC and the single-phase TPS can be dispensed with [267]. Grid V/V Transformer Single-phase TPS Medium Voltage

Lyu Cdc Lzu Lxu Lyl Lzl Lxl

Single submodule

Figure 29. Rail power conditioner (RPC) based on modular multilevel converter (MMC).

Energies 2020, 13, x FOR PEER REVIEW 30 of 63

3~ac 3~ac Power High Voltage Power High Voltage Grid Grid Coupling V/V Transformer Transformer Single-phase TPS Medium Voltage STATCOM V/V Transformer

Single-phase TPS Medium Voltage Coupling Transformer Ly RPC Lx

(a) (b)

Figure 28. Power electronics compensators in ac railway electrification: (a) static synchronous compensator (STATCOM); (b) rail power conditioner (RPC).

Another possibility is to consider a power converter connected to the single-phase traction power system (TPS), where the rail power conditioner (RPC) is used for the purpose of power quality improvement in the ac electrified railways. The RPC structure contains two single-phase back-to- back converters with a single dc link in between and two coupling transformers, as shown in Figure 28b [263]. The STATCOM device is a three-phase converter, whereas the RPC is composed of two back-to-back single-phase converters. Each converter is connected to a load section through a buck-type coupling transformer. The first proposal of a 60 kV, 20 MVA RPC for Tohoku Shinkansen railway in Japan was implemented in 2004 [264]. The implemented RPC at that time was based on multilevel converters and a Scott power transformer. Another project was implemented in March 2009, where RPC systems with gate commutated thyristors were installed in four substations of the Tokaido Shinkansen in Japan: Shimizu substation, Shin Kikugawa substation, Shin Biwajima substation, and Ritto substation. Detailed information on that project can be found in [265]. New power electronics compensators have the advantage of scalability and modularity in which modular multilevel converter (MMC) is adopted to replace the conventional two-level converter. The MMC advantages are quite numerous, such as the scalability to different power and voltage levels, reliability to guarantee continuous operation even if few submodules stop working, and reduced grid-connected filter size, due to the good quality of the synthesized waveforms [266]. Figure 29 shows the RPC based on the MMC system. When compared to the conventional RPC system Energies 2020presented, 13, 6343 in Figure 28b, buck-type coupling transformers used to interface between the RPC and the 30 of 61 single-phase TPS can be dispensed with [267].

3~ac Power High Voltage Grid

V/V Transformer Single-phase TPS Medium Voltage

Lyu Cdc Lzu Lxu Lyl Lzl Lxl

Single submodule

FigureFigure 29. Rail 29. powerRail power conditioner conditioner (RPC) (RPC) based based on modular modular multilevel multilevel converter converter (MMC). (MMC).

3.2. Power Management Converters Energies 2020, 13, x FOR PEER REVIEW 31 of 63 Power electronics converters in electrified railways are not only related to the application of power quality3.2. Power improvement, Management Converters but also to power management applications between traction substations, or betweenPower the three-phase electronics acconverters power in grid electrified and the railwa single-phaseys are not TPS.only Inrelated that to regard, the application this item of presents two relevantpower powerquality electronicsimprovement, solutions. but also The to firstpowe isr anmanagement interface converterapplications between between thetraction three-phase ac powersubstations, grid and or thebetween single-phase the three-ph TPS,ase ac as power shown grid inand Figure the single-phase 30[ 256]. TPS. This In that interface regard, is this useful to performitem frequency presents conversiontwo relevant power when electronics the single-phase solutions. TPS The frequencyfirst is an interf is diaceff convertererent from between the three-phase the ac powerthree-phase grid frequency. ac power For grid instance, and the single-phase this interface TPS, converter as shown in can Figure be an 30 appropriate [256]. This interface solution is to link useful to perform frequency conversion when the single-phase TPS frequency is different from the the 50 Hz three-phase ac power grid with the 16.7 Hz single-phase TPS [258]. On the other hand, three-phase ac power grid frequency. For instance, this interface converter can be an appropriate this powersolution converter to link the presents 50 Hz three-phase the single-phase ac power grid traction with the load 16.7 as Hz a single-phase balanced three-phase TPS [258]. On load the to the power grid.other hand, Moreover, this power all theconverter reactive presents power the required single-phase by traction the traction load as load a balanced can be three-phase provided by the interfaceload converter, to the power which grid. isMoreover, essential all tothe obtain reactive a po unitarywer required power by factorthe traction for theload three-phasecan be provided ac power grid. Moreover,by the interface the energy converter, resulting which fromis essential regenerative to obtain brakinga unitarycan power be storedfactor for in the batteries. three-phase This ac interface converterpower prevents grid. Moreover, harmonic the energy content resulting generated from regene by therative locomotives braking can passing be stored from in batteries. the single-phase This interface converter prevents harmonic content generated by the locomotives passing from the TPS to thesingle-phase three-phase TPS to ac the power three-phase grid. ac The power main grid. disadvantage The main disadvantage of this converterof this converter is that is that it should it be rated toshould endure be rated the full to endure load power.the full load In some power. references, In some references, this solution this solu istion called is called a static a static frequency converterfrequency [256,258 converter]. [256,258].

3~ac Power High Voltage Grid

P Coupling Transformer P

Q Interface Single-phase TPS Converter Medium Voltage

Figure 30.FigureInterface 30. Interface converter converter between between the high-voltage the high-vol three-phasetage three-phase power gridpower and grid the medium-voltageand the medium-voltage single-phase traction power system (TPS). single-phase traction power system (TPS). Another interface converter between two traction substations is presented in Figure 31. This Anotherconverter interface is used to interface converter between between two phases two (with traction different substations out-of-phase is angles) presented for the purpose in Figure 31. This converterof power istransmit used from to interface two substations between instead two of phases only one (with substation diﬀerent [263]. out-of-phaseIn other terms, angles)it is for the purposepossible of to power shift the transmit active power from from two one substations substation insteadto another. of Additionally, only one substation reactive power [263 can]. In other terms, itbe is provided possible by to the shift interface the active converter. power This from is usef oneul substation to extract a to higher another. amount Additionally, of power during reactive the power can be providedpeak load demand, by the interface avoiding converter.overloading Thisconditions is useful of traction to extract substations a higher and amount allowing of a powergreater during flow of locomotives by increasing the power capacity of the overhead catenary. An interface converter located between two traction substations, which is commercialized by Hitachi, Japan, is presented in [268]. The system started the operation in 2015 and is designed to shift active power from one substation to another, including the energy resulting from the regenerative braking of the trains. It is worth mentioning that this interface converter between two traction substations is only able to transmit power between traction substations and has no ability to accomplish power quality improvement [263].

Energies 2020, 13, 6343 31 of 61 the peak load demand, avoiding overloading conditions of traction substations and allowing a greater ﬂow of locomotives by increasing the power capacity of the overhead catenary. An interface converter located between two traction substations, which is commercialized by Hitachi, Japan, is presented in [268]. The system started the operation in 2015 and is designed to shift active power from one substation to another, including the energy resulting from the regenerative braking of the trains. It is worth mentioning that this interface converter between two traction substations is only able to transmit power betweenEnergies 2020 traction, 13, x FOR substations PEER REVIEW and has no ability to accomplish power quality improvement32 of 63 [263]. Energies 2020, 13, x FOR PEER REVIEW 32 of 63 Energies 2020, 13, x FOR PEER REVIEW 32 of 63 3~ac 3~ac 3~ac 3~ac Power High Voltage Power Power High Voltage Power Grid3~ac 1 Grid3~ac 2 PowerGrid 1 High Voltage PowerGrid 2 Grid 1 V/V Transformer Grid 2V/V Transformer V/V Transformer V/V Transformer V/V Transformer Single-phase TPS V/V Transformer Single-phase TPS Medium Voltage Neutral Section Single-phaseMedium Voltage TPS Neutral Section Medium Voltage Neutral Section

Q1 Interface Q1 Interface Q2 Converter Q2 L1 Converter L2 Q1 L1 Interface L2 Q2 P1 P2 P1 L1 Converter L2 P2

P1 P2 Figure 31. Interface converter between two traction substations. FigureFigure 31. Interface31. Interface converter converter betweenbetween two two traction traction substations. substations. Figure 31. Interface converter between two traction substations. 3.3. Traction3.3. Traction and Regenerative and Regenerative Braking Braking Converters Converters 3.3. TractionPower andelectronics Regenerative converters Braking areConverters essential to drive traction machines for the rail transport Power electronicsPower electronics converters converters are essentialare essential to drive to drive traction traction machines machines for for the the rail rail transport transport system, system,Power including electronics the high-speedconverters acare electrified essential railto waysdrive and traction the light machines rail powered for the by rail dc transportoverhead includingsystem, the high-speed including the ac high-speed electriﬁed ac railways electrified and rail theways light and rail the poweredlight rail powered by dc overhead by dc overhead catenary line. system,catenary including line. The theconfigurations high-speed ofac theelectrified TPS with rail regenerativeways and the braking light rail are powered shown in by Figures dc overhead 32 and The conﬁgurations of the TPS with regenerative braking are shown in Figures 32 and 33 for the ac and catenary33 for the line. ac andThe theconfigurations dc overhead of catenary the TPS lines,with regenerativerespectively. braking In the primary are shown converters in Figures ac-dc 32 andand the dc overhead33dc–ac for theit is ac catenary important and the lines,dc to overhead have respectively. three-phase catenary In lines,ac the cu rrents,respectively. primary thus converters powering In the primary th ac-dce traction converters and dc–acmachines. ac-dc it is andThe important to havedc–acprimary three-phase it isconverters important ac currents, are to important have thusthree-phase to powering perform ac thecu therrents, power traction thus conversion powering machines. and th toe Theusetraction the primary powermachines. resulting converters The are from the regenerative braking of the three-phase traction electric machines. In light railway importantprimaryfrom to the perform converters regenerative the are power importantbraking conversion ofto performthe three-phas and the power toe usetraction conversion the powerelectric and resultingmachines. to use the from Inpower light the resulting railway regenerative applications, such as suburban and regional trains, the dc link voltage is selected in the range of 1500 brakingfromapplications, of the the three-phase regenerative such as suburban traction braking and electricof regionalthe three-phas machines. trains, thee Intraction dclight link voltageelectric railway ismachines. selected applications, in In the light range such railway of as 1500 suburban applications,V dc, whereas such in high-speed as suburban ac and railway regional the dctrains, link thevoltage dc link is selected voltage isin selecteda range betweenin the range 1500 of V 1500 and and regional trains, the dc link voltage is selected in the range of 1500 V dc, whereas in high-speed ac V3600 dc, Vwhereas [263]. in high-speed ac railway the dc link voltage is selected in a range between 1500 V and railway3600 the V dc [263]. link voltage is selected in a range between 1500 V and 3600 V [263]. TPS Single-phase ac (25 kV, 50 Hz) TPS Single-phase ac (25 kV, 50 Hz) Pantograph TPS Single-phase ac (25 kV, 50 Hz) Pantograph 3~ac Traction 3~ac Traction Pantograph motors 3~acmotors Traction motors Three-phase Three-phase supply Single-phase supply Single-phase Three-phase transformer transformer supply Single-phase Regenerative Regenerative transformer braking dc supply or braking Auxuliary dc supply or Regenerative Auxuliary batteries Running rail converter batteries Running rail braking converter dc supply or Auxuliary batteries Running rail converter Figure 32. Traction power supply system configuration with regenerative braking: Ac overhead Figure 32. Traction power supply system configuration with regenerative braking: Ac overhead catenary line. Figure 32.FigurecatenaryTraction 32. line. Traction power supplypower supply system system configuration configuration with regenerative with regenerative braking: braking: Ac overhead Ac overhead catenary line. catenary line. TPS Single-phase dc (1500 V) TPS Single-phase dc (1500 V) TPS Single-phase dc (1500 V) 3~ac Traction 3~ac Traction motors 3~acmotors Traction motors Three-phase Three-phase supply Three-phasesupply Regenerative supply Regenerative braking braking dc supply or Regenerative dc supply or Running railbraking batteries Running rail dc batteriessupply or

Running rail batteries Figure 33. Traction power supply system configuration with regenerative braking: dc overhead Figure 33. Traction power supply system configuration with regenerative braking: dc overhead Figure 33.catenaryTraction line. power supply system configuration with regenerative braking: dc overhead catenary line. Figurecatenary 33. line. Traction power supply system configuration with regenerative braking: dc overhead catenary line.

Energies 2020, 13, 6343 32 of 61

The auxiliary converters are used to supply the train auxiliary loads. There is a dc–ac auxiliary Energies 2020, 13, x FOR PEER REVIEW 33 of 63 converterEnergies that 2020 is linked, 13, x FOR to PEER the REVIEW main dc link voltage, and its output is connected to the main transformer33 of 63 for auxiliaryTheThe loads. auxiliaryauxiliary The outputconvertersconverters of theareare used dc–acused toto auxiliary supplysupply thethe converter traintrain auxiliaryauxiliary is a three-phase loads.loads. ThereThere 380isis aa dc–ac Vdc–ac supply, auxiliaryauxiliary which is used toconverterconverter feed, for thatthat instance, isis linkedlinked the toto on-boardthethe mainmain dcdc fluorescent linklink voltvoltage,age, lights, andand coolingitsits outputoutput fans, isis connectedconnected electric outlets,toto thethe mainmain and air conditioningtransformertransformer [269]. forfor The auxiliaryauxiliary traction loads.loads. system TheThe is outputoutput configured ofof thethe dc–ac withdc–ac externalauxiliaryauxiliary batteriesconverterconverter connectedisis aa three-phasethree-phase to an 380380 auxiliary VV dc–ac powersupply,supply, converter. whichwhich isis usedused When toto feed,feed, the train forfor instance,instance, is passing thethe throughonon-board-board afluorescentfluorescent neutral section, lights,lights, coolingcooling conventional fans,fans, electricelectric ac trains may suffoutlets,outlets,er shutdowns andand airair dueconditioningconditioning to instantaneous [269].[269]. TheThe catenary tractiontraction line systemsystem power isis configured interruptions.configured withwith The externalexternal solution batteriesbatteries proposed connectedconnected toto anan auxiliaryauxiliary dc–acdc–ac powerpower converter.converter. WhenWhen thethe traintrain isis passingpassing throughthrough aa neutralneutral section,section, in [269] can useEnergies the 2020 energyEnergies, 13, x FOR 2020 storedPEER, 13, x REVIEWFOR PEER in batteriesREVIEW to avoid shutdowns of the auxiliary33 of 63 power33 of 63 supply. conventionalconventional acac trainstrains maymay suffersuffer shutdownsshutdowns duedue toto instantaneousinstantaneous catenarycatenary lineline powerpower interruptions.interruptions. The batteries are charged by the regenerative braking power or by the power from the catenary line. TheThe solutionsolutionThe proposedproposed auxiliaryThe auxiliary convertersinin [269][269] converters canarecan used useuse toare the thesupply used energyenergy to the supply train storedstored auxiliarythe train inin auxiliarybatteriesloads.batteries There loads. toto is aavoidThereavoid dc–ac is auxiliaryshutdowns shutdownsa dc–ac auxiliary ofof thethe When theauxiliaryauxiliary batteriesconverter powerpower are convertersupply. supply.that installed is linked ThThthatee batteries is directlybatteriesto linked the main to areare nearthe dc chargedcharged mainlink the voltdc main bybyage,link thethe andvolt dc regeneraregenera age,its link, outputand theytive tiveits is output brakingbrakingconnected can beis connected powerpower usedto the ortomainor to byby power the thethe main powerpower the train transformertransformer for auxiliary for auxiliaryloads. The loads. output The of output the dc–ac of the auxiliary dc–ac auxiliaryconverter converter is a three-phase is a three-phase 380 V 380 V when passingfromfrom thethe through catenarycatenary a line.line. neutral WhenWhen section. thethe batteriesbatteries areare instalinstalledled directlydirectly nearnear thethe mainmain dcdc link,link, theythey cancan bebe supply, whichsupply, is whichused to is feed,used forto feed,instance, for instance,the on-board the onfluorescent-board fluorescent lights, cooling lights, fans, cooling electric fans, electric used to power the train when passing through a neutral section. Tableused4 topresents poweroutlets, theand theoutlets, train air power conditioningand when air electronics paconditioningssing [269]. through The [269]. technologies traction aThe neutral systemtraction section. used issystem configured in is theconfigured with propulsion external with externalbatteries system batteries of electric locomotives.TableTable Usually,connected 44 presentspresentsconnected a to propulsion an auxiliary thethe to power poweran auxiliarydc–ac system electronics electronicspower dc–ac forconverter. power dc technologiestechnologies traction converter. When the When power train usedused isthe passing supply traininin thethe is through passing propulsionpropulsion is simpler a through neutral system systemasection, and neutral slightly ofsection,of electricelectric cheaper conventional ac trains may suffer shutdowns due to instantaneous catenary line power interruptions. than thelocomotives.locomotives. one forconventional an Usually, acUsually, traction ac trains aa propulsionpropulsion powermay suffer supply, shutdowns systemsystem since dueforfor to dcdc the instantaneous tractiontraction costs of power powercatenary power supply linesupply transformers power isis interruptions. simplersimpler in andand the slightlyslightly dc traction The solutionThe proposedsolution proposed in [269] canin [269] use thecan energy use the stored energy in storedbatteries in batteriesto avoid toshutdowns avoid shutdowns of the of the cheapercheaper thanthan thethe oneone forfor anan acac tractiontraction powerpower supply,supply, sincesince thethe costscosts ofof powerpower transformerstransformers inin thethe dcdc power supply canauxiliary be auxiliarypower saved. supply. power On Th supply. thee batteries other The arebatteries hand, charged areonly by charged the oneregenera by the stage tiveregenera braking oftive power power braking or conversion powerby the poweror by the is power required in tractiontraction powerpower supplyfromsupply the catenarycancan bebe line.saved.saved. When OnOn the the thebatteries otheothe arerr hand, hand,installed onlyonly directly oneone near stagestage the main ofof powerpower dc link, conversionconversionthey can be isis the dc traction powerfrom the catenary with dc line. electric When the machines batteries are (dc–dcinstalled directly converter), near the e.g.,main tramdc link, lines,they can which be makes this requiredrequired used inin thethe to power dcdcused tractiontraction tothe power train powerpower whenthe train pa withwith ssingwhen dc dcthrough pa electricelectricssing athrough neutral machinesmachines asection. neutral (dc–dc(dc–dc section. converter),converter), e.g.,e.g., tramtram lines,lines, whichwhich solutionmakesmakes the cheapest thisthis solutionsolutionTable among 4 presents theTablethe cheapestcheapest the4 thepresents otherpower amonamon the electronics propulsion powergg thethe electronics otherothertechnologies systems propulsionpropulsion technologies used [270 in systemssystemsthe used]. propulsion in the [270].[270]. propulsion system of system electric of electric locomotives.locomotives. Usually, Usually,a propulsion a propulsion system for system dc traction for dc powertraction supply power is supply simpler is andsimpler slightly and slightly As shownAsAs shownshown in Table inin Table5Table, power 5,5, powerpower electronics electronicselectronics converters converteconvertersrs are areare essential essentialessential to to performperform perform powerpower power conversionconversion conversion cheaper thancheaper the onethan for the an one ac tractionfor an ac power traction supply, power since supply, the sincecosts ofthe power costs oftransformers power transformers in the dc in the dc with different frequencies and amplitudes. For instance, when dc traction machines are utilized, dc– with diwithfferent different frequenciestraction frequencies powertraction supply andpower and amplitudes.can supply amplitudes.be saved. can be On saved. Forthe othe instanOn instance, rthe hand, ce,othe whenonlyr hand, when one dc onlystage traction dc one of traction powerstage machines of conversion power machines are conversion isutilized, are is utilized,dc– dc–dc convertersdcdc convertersconvertersrequired are areare crucialinrequired thecrucialcrucial dc tractionin to the toto adjust dc adjustadjust power traction the withthethe power dc dcdc electric voltagewithvoltagvoltag dc machines eelectrice values. values. machines (dc–dc OnOn On converter), the (dc–dcthe the otherother otherconverter), e.g., side,side, tram side, e.g., dc–aclines,dc–ac dc–actram which convertersconverters lines, converters which areare are makes this solution the cheapest among the other propulsion systems [270]. indispensableindispensableindispensable tomakes drive thistoto drive drivesolution the ac thethe the electric ac accheapest electricelectric machinesamon machinesmachinesg the other by by bypropulsion changing changingchanging systems the the [270]. voltagevoltage voltage frequencyfrequency frequency andand and amplitude,amplitude, amplitude, As shownAs in shown Table 5,in powerTable 5,electronics power electronics converte rsconverte are essentialrs are essentialto perform to powerperform conversion power conversion knownknown as as variable variable voltage voltage variable variable frequency frequency drive drive [270,271]. [270,271]. Future Future converter converter technologies technologies are are likely likely known as variablewith voltagedifferentwith differentfrequencies variable frequencies and frequency amplitudes. and amplitudes. driveFor instan [ 270Force, instanwhen,271 ].ce,dc Futuretractionwhen dc machines traction converter machines are utilized, technologies are dc–utilized, dc– are likely to utilize SiC devices for more efficient traction systems, besides the benefits of higher temperature to utilizeto utilize SiC devicesdc SiC converters devicesdc for converters moreare for crucial more eareffi to cientcrucialefficient adjust tractiontothe adjusttraction dc voltag the systems, dcsye values. stems,voltag eOn besidesvalues.besides the other On the thethe side, benefits other benefits dc–ac side, convertersof dc–ac ofhigher higher converters are temperature temperature are operationoperationindispensable capabilitycapabilityindispensable to andand drive thethe theto drivepossibilityacpossibility electric the ac machines electric ofof manufacturingmanufacturing machines by changing by changing the smallervoltagesmaller the frequency voltage andand lighter lighterfrequency and amplitude, powerpower and amplitude, convertersconverters operation capability andknown the as possibility variable voltage of variable manufacturing frequency drive smaller [270,271]. and Future lighter converter power technologies converters are likely without withoutwithout knowntransformers.transformers. as variable However,voltageHowever, variable therethere frequency areare drivestillstill [270,271].issuissueses to Futureto bebe convertersolvedsolved technologiesinin termsterms ofareof likelybothboth costscosts andand to utilize toSiC utilize devices SiC for devices more forefficient more tractionefficient sy tractionstems, besidessystems, the besides benefits the of benefits higher oftemperature higher temperature transformers.technology,technology, However, providedprovided there thatthat SiC SiC are devicesdevices still issuesstill still havehave to poorpoor be solvedglobal global easeease in termsofof procurement procurement of both and and costs highhigh and costscosts technology, [271].[271]. operationoperation capability capability and the possibilityand the possibility of manufacturing of manufacturing smaller andsmaller lighter and power lighter converters power converters provided that SiCwithout devices transformers.without still transformers. have However, poor However, there global are there still ease issuare ofesstill procurementto issube essolved to be in solved terms and inof high termsboth costscostsof both and [271 costs ]. and technology,TableTabletechnology, provided 5.5. PowerPower provided that electronicselectronics SiC devices that SiC forfor still devices tractiontraction have stillpoor machinmachin have global poore eease applicationsapplications global of procurement ease of in inprocurement railwayrailway and high systems.systems. andcosts high [271]. costs [271]. Table 5. Power electronics for traction machine applications in railway systems. Table 5. Three-PhasePower Three-PhaseTable electronics 5. Power electronics acforac traction ElectricElectric for machin traction MachinesMachinese applications machin e applications in railway in dcsystems. dc railway ElectricElectric systems. MachinesMachines

Transformer Three-Phase Three-Phase Three-Phase ac ac Electric Electric acMachines Machines Electric Machines Transformer dc Electric dc dcElectric Machines Electric Machines Machines Transformer dcdc Transformer dcdc acac acac Transformer Transformeracac dcdc Transformer dc dc Transformer dc dc Traction power ac ac Traction power ac ac ac ac dc dc (15Traction kV or powerTraction25 kV ac) powerTraction power (15 kV or 25 kV ac) (15 kV or (1525 kVkV ac)or 25 kV ac) (15 kV or 25 kV ac) ac-dc—dc–ac converter ac-dc—dc–acac-dc—dc–ac converterconverter ac-dc—dc–acac-dc—dc–acac-dc—dc–ac converter converter converter ac-dc—dc–acac-dc—dc–acac-dc—dc–ac converter converter converter MediumMediumMedium VoltageVoltage VoltageMedium Voltage (MV)/Low(MV)/Low Voltage (MV)/Low (MV)/Low Voltage Voltage Voltage (LV) Voltage (LV) (LV)power(LV) power(LV) powerpower transformer power transformer transformertransformer transformer is essential is essentialis isisessential essentialessential

Pantograph Pantograph PantographPantographPantograph PantographPantographPantograph dc dc dc dcdcdc ac ac dc dcdc dc Traction powerTraction power dc dc acac dcdc dcdc TractionTraction power powerpower(750 V–3000 V dc) (750 V–3000 V dc) (750(750 V–3000V–3000V–3000 V VV dc) dc)dc) dc–dc—dc–acdc–dc—dc–ac converter converter dc–dc converterdc–dc converter No need for MV/LV power transformer dc–dc—dc–acdc–dc—dc–acNo converter need converterconverter for MV/LV power transformerdc–dcdc–dc dc–dc converter converterconverter NoNo need need for MVMV/LV/LV power power transformer transformer 3.4. Special3.4. Power Special Electronics Power Electronics Applications Applications inNo Electrified need in Electrified Railwaysfor MV/LV Railways power transformer Power electronicsPower electronics converters converters for special for applications special applications in electrified in electrified railways arerailways quite arenumerous. quite numerous. 3.4. Special Power Electronics Applications in Electrified Railways 3.4. Special3.4. Special PowerMost Power Electronics of theMost Electronics new of railway the Applications new A lines railwaypplications are equippedlines in arein Electrified Electrifiedequipped with energy with Railways Railways storage energy systems, storage suchsystems, as batteries, such as batteries,which are which are essential essentialfor continuous for continuous electrical electricalpower feeding power when feeding passing when through passing non-electrifiedthrough non-electrified sections. Onsections. On PowerPower electronicselectronics convertersconverters forfor specialspecial applicationsapplications inin electrifiedelectrified railwaysrailways areare quitequite numerous.numerous. Power electronicsthe otherthe hand, converters other energy hand, storage energy for specialsystems storage aresystems applications importan are important for enabling in electrifiedt for enablingthe recovery railwaysthe recoveryof energy areof from energy quite from numerous. Most of the new railway lines are equipped with energy storage systems, such as batteries, which are Most ofMost the newof theregenerative railway new regenerativerailway braking lines linesare [272].braking are equipped To equipped [272].achieve To with theachieve with highes energy enthetergy profits,highes storagestorage t powerprofits, systems, systems,electronics power electronicssuch converters such as asbatteries, converters batteries,enable whichenable which are are essentialessential on-board forfor continuouscontinuous on-boardor even off-board or electricalelectrical even off-board storage powerpower applications. storage feedingfeeding applications. In whenwhen both cases, passingInpassing both the cases, power throughthrough the converter power non-electrifiednon-electrified converter has to draw has the to sections.sections. draw the OnOn essential for continuousmaximum electrical possible power power feeding over time whenfrom renewable passing energy through sources. non-electrified In this context, energy sections. storage On the thethe otherothermaximum hand,hand, energy energypossible powerstoragestorage over systemssystems time from are arerenewable importanimportan energytt for forsources. enablingenabling In this context,thethe recoveryrecovery energy storage ofof energyenergy fromfrom other hand, energy storage systems are important for enabling the recovery of energy from regenerative regenerativeregenerative brakingbraking [272].[272]. ToTo achieveachieve thethe higheshighestt profits,profits, powerpower electronicselectronics convertersconverters enableenable brakingon-boardon-board [272]. To oror achieve eveneven off-boardoff-board the highest storagestorage profits, applications.applications. power InIn electronicsbothboth cases,cases, thethe converters powerpower converterconverter enable hashas on-board toto drawdraw orthethe even off-boardmaximummaximum storage possiblepossible applications. powerpower over Inover both timetime cases, fromfrom renewable therenewable power energyenergy converter sources.sources. has InIn to thisthis draw context,context, the maximum energyenergy storagestorage possible power over time from renewable energy sources. In this context, energy storage system applications

Energies 2020, 13, 6343 33 of 61 for railway transportation have been presented in [273]. Moreover, the possibility of a photovoltaic power supply path from the station roof to the train has been presented in [274]. In the last few decades, magnetic levitation technology has universally emerged as a sustainable and viable alternative to conventional on-wheel rail technologies. However, the development of this technology has been related wholly to that of power electronics since they have an integrated levitation and propulsion system. In this type of application, power electronics converters are used to regulate the magnet current, known as suspension choppers [275]. Hyperloop and Maglev technologies are both based on the train’s levitation. A detailed explanation of these two technologies is introduced in [276]. Despite its lower electrical efficiency and other technical difficulties, IWPT technology (Section 2.2) has also been explored in the electrification of railways [277], as a possible alternative for contactless catenary-free systems [278]. Power rectifiers, resonant inverters and efficient magnetic coupling systems are required for the transmission of power using high-frequency currents. In that regard, a high-speed prototype that was developed in South Korea, with 1 MW WPT, is presented in [279].

3.5. Summary The railway market requires traction power systems that are more energy-efficient and respond to diverse customer needs. In this section, some of the power electronics converters that can be used for railway applications were shown. These power converters contribute to solving some power quality phenomena in the three-phase power grid. In that regard, RPC and STATCOM solutions were presented to overcome three-phase current imbalances, reactive power, and harmonic contents. On the other hand, power management converters could help to increase the loading capacity of traction substations. Additionally, power electronics solutions for regenerative braking and traction machine drive in ac and dc traction machines were also presented. Applications of power electronics converters for fixed applications (power electronics converters installed in the traction substations) and moving applications (power electronics converters installed in the electric locomotives) were presented, considering future trends for superior performance and increased resilience of railway systems. Table6 summarizes the main capabilities of the power electronics converters presented in this section for the purpose of railway fixed applications.

Table 6. Capabilities of power electronics converters used in railway ﬁxed applications (in substations).

Frequency Harmonics Reactive Power Power Factor Overloading Power between Conversion Filtering Compensation Correction Capability Two Substations STATCOM No Yes Yes Yes Yes No (Figure 28a) RPC No Yes Yes Yes Yes No (Figure 28b) Between Power Yes Yes Interface Yes Yes No Yes Grids (Figure 30) (inherent) (inherent) Converter Between Traction Substations No No No No No Yes (Figure 31)

4. Power Electronics for Other Types of Electric Vehicle Besides its terrestrial variant, the transportation sector includes also other types of travelling, such as maritime and aerial. In fact, the electrification process that is being carried out in road vehicles is also noticeable in other transportation sectors, mainly due to the same factors that motivate the electrification of road vehicles, i.e., regarding both environmental and economic concerns. Therefore, this section introduces a review of power electronics for other types of EV, not only including maritime and aerial means of transportation, but also considering other types of terrestrial vehicle besides the vastly used automobile. Accordingly, this section includes ships, aircraft, trucks, buses, pallet trucks and forklifts, scooters, bikes, and small EVs. It should be noted that the power electronics structure used in the traction system of these types of EVs is very similar to that of road vehicles, i.e., the same Energies 2020, 13, 6343 34 of 61 topologies of power converters are used, differing essentially in the power rating, which can be higher, for instance, in the case of electric ships and trucks, or lower, such as the case of electric bicycles and scooters.

4.1. Electric Ships Besides terrestrial locomotion, maritime transportation is also undergoing an electrification process. In fact, since the 19th century electric power systems have been installed on ships [280]. Concerning the powertrain level, the utilization of electrical propulsion systems on ships offer several advantages over diesel propulsion engines and gas turbines, such as superior dynamics, i.e., start, stop and speed variations, less fuel consumption, less mechanical vibration (and, therefore, a higher comfort for the passengers) and a higher flexibility for adding electric machines to the powertrain [281]. Moreover, an electric ship can be seen as an islanded microgrid [282,283]. In [284], the utilization of fuel cells is investigated in the electric propulsion of ships, aiming to reduce the emission of greenhouse gases, volatile organic compounds and particulate matter, which are highly related to the exhaust of non-electric ships. In [285] is studied a design of electrical distribution systems for ships and oil platforms, taking into consideration control systems and protections and a zonal distribution to improve modularity, being considered both ac and dc systems. In [286] is proposed a hybrid energy storage system based on a battery and a superconducting magnetic energy storage element in order to improve the stability of dc power systems of fully electric ships, being used a two quadrant buck-boost dc–dc converter for the battery and an asymmetrical h-bridge dc–dc converter for the superconducting magnetic energy storage element. In [287] is presented a conventional structure of an on-board ship power system (Figure 34), having been studied the power flow in the electric power system with energy storageEnergies and 2020 energy, 13, x FOR management PEER REVIEW systems, comprising ac, dc and hybrid ac-dc power grid architectures.36 of 63

Main Power Grid

D1 D3 D5 S1 S3 S5 Diesel Wound Rotor Generator Induction Induction Machine Machine C1 vdc

D2 D4 D6 S2 S4 S6

Diesel Wound Rotor Generator Induction D7 D9 D11 S7 S9 S11 Machine Induction Machine C2 vdc

D8 D10 D12 S8 S10 S12

isolation Auxiliary Power Grid

Electrical Appliances

FigureFigure 34. Typical34. Typical structure structure of of an an electric electric shipship powerpower system system based based on on two two induction induction machines. machines.

4.2. Electric Aircraft Aerial transportation has also been a target of research towards electrification, not only in terms of powertrains but also in subsystems that traditionally use hydraulic, mechanical and pneumatic power, aiming to reduce emissions, fuel consumption and operating and maintenance costs. This paradigm has been referred to as the “more electric aircraft” [288,289]. In [290] is presented a starter generator system for electric aircraft based on a three-phase three-level neutral point clamped dc–ac converter, with a higher power density being achieved compared to conventional starter generator systems without using power electronics converters. In [291] is presented the design of an integrated modular machine drive for electric aircraft based on a five-phase PMSM machine and a SiC-based five-phase five-leg dc–ac traction converter (Figure 35), and [292] studied a design method for a cooling system to be used on electric aircraft powertrains. In [293] is analyzed the regenerative braking operation of an electric taxiing system, i.e., when an aircraft moves by its own on the ground instead of in the air to move, for instance, between the runway and the hangar. In [294] is studied a fault management design for electric aircraft, while in [295] are analyzed the effects of fault currents in aircraft electric power systems both at the electrical and thermal levels. An optical sensing method for detecting partial discharges in higher voltage (above 1 kV) electric aircraft is proposed in [296], and [297] investigated the conduction of electromagnetic interference in more electric aircraft microgrids.

Energies 2020, 13, 6343 35 of 61

4.2. Electric Aircraft Aerial transportation has also been a target of research towards electrification, not only in terms of powertrains but also in subsystems that traditionally use hydraulic, mechanical and pneumatic power, aiming to reduce emissions, fuel consumption and operating and maintenance costs. This paradigm has been referred to as the “more electric aircraft” [288,289]. In [290] is presented a starter generator system for electric aircraft based on a three-phase three-level neutral point clamped dc–ac converter, with a higher power density being achieved compared to conventional starter generator systems without using power electronics converters. In [291] is presented the design of an integrated modular machine drive for electric aircraft based on a five-phase PMSM machine and a SiC-based five-phase five-leg dc–ac traction converter (Figure35), and [ 292] studied a design method for a cooling system to be used on electric aircraft powertrains. In [293] is analyzed the regenerative braking operation of an electric taxiing system, i.e., when an aircraft moves by its own on the ground instead of in the air to move, for instance, between the runway and the hangar. In [294] is studied a fault management design for electric aircraft, while in [295] are analyzed the effects of fault currents in aircraft electric power systems both at the electrical and thermal levels. An optical sensing method for detecting partial discharges in higher voltage (above 1 kV) electric aircraft is proposed in [296], and [297] investigated the conductionEnergies 2020, 13 of, x electromagnetic FOR PEER REVIEW interference in more electric aircraft microgrids. 37 of 63

S1 S3 S5 S7 S9 Left Propeller PMSM dc-link C1 vdc

S2 S4 S6 S8 S10 Gas Wound Field ac-dc Propeller Synchronous converter Machine

S11 S13 S15 S17 S19 Right Propeller PMSM C2 vdc

S12 S14 S16 S18 S20

dc-ac converter Auxiliary Power Grid

Electrical Appliances

FigureFigure 35. Typical35. Typical structure structure of of an an electric electric aircraft aircraft powerpower system system based based on on two two five-phase ﬁve-phase permanent permanent magnetmagnet synchronous synchronous machines machines (PMSMs). (PMSMs).

4.3. Electric4.3. Electric Trucks Trucks ElectricElectric trucks trucks consist consist of of aa terrestrial means means of transportation, of transportation, despite despite not having not the having same thewell- same well-deﬁneddefined and and planned planned routes routes as as other other means means of oftransp transportationortation such such as buses, as buses, depending depending on the on type the type of truck.of truck. For For freight freight transportation transportation road road trucks, trucks, whose whose routesroutes can be be planned, planned, Siemens Siemens has has developed developed a a solution based on pantographs and hybrid trucks, which is called eHighway [298]. Pilot projects solution based on pantographs and hybrid trucks, which is called eHighway [298]. Pilot projects have have already been implemented on public highways in Sweden and Germany, with Siemens referring already been implemented on public highways in Sweden and Germany, with Siemens referring to this to this technology as the most cost-effective solution regarding electric trucks despite the required investment. In [299] is proposed a conceptual model for enhancing the eHighway system, aiming to endow it with autonomous driving. In [300] is proposed an energy management strategy for hybrid trucks, aiming to maximize the batteries lifetime while also reducing emissions. Besides freight trucks, the electrification of trucks is also researched regarding mining applications. In [301] is presented a model of a trolley assist system for mining trucks, which allows the machines of the trucks to be supplied by engaging them on the trolley lines. This system is based on three-phase three-leg active front end ac-dc converters and helps to increase the drive speed on uphill trajectories while economizing fuel, besides allowing to perform regenerative braking on downhill trajectories. In [302] is studied the stress of mechanical and electrical components in electric mining trucks, both for uphill and downhill operations. In [303] is proposed an energy-saving control strategy regarding the repetitive start-up operation of mining trucks, since their normal operation consists of short distance travels and continuous start and stop, which is highly energy consuming. At the powertrain level, in [304] is proposed a modular three-port traction converter based on four winding switched reluctance machines for plug-in hybrid electric trucks. This converter is based on two h-bridges connected in a cascaded way, i.e., connected to each other by the ac side, and the dc negative rail is common to both h-bridges. The traction converter can be connected to a battery, a generator and also to the power grid, i.e., the proposed system is a unified system. Moreover, the operation of the

Energies 2020, 13, 6343 36 of 61 technology as the most cost-effective solution regarding electric trucks despite the required investment. In [299] is proposed a conceptual model for enhancing the eHighway system, aiming to endow it with autonomous driving. In [300] is proposed an energy management strategy for hybrid trucks, aiming to maximize the batteries lifetime while also reducing emissions. Besides freight trucks, the electrification of trucks is also researched regarding mining applications. In [301] is presented a model of a trolley assist system for mining trucks, which allows the machines of the trucks to be supplied by engaging them on the trolley lines. This system is based on three-phase three-leg active front end ac-dc converters and helps to increase the drive speed on uphill trajectories while economizing fuel, besides allowing to perform regenerative braking on downhill trajectories. In [302] is studied the stress of mechanical and electrical components in electric mining trucks, both for uphill and downhill operations. In [303] is proposed an energy-saving control strategy regarding the repetitive start-up operation of mining trucks, since their normal operation consists of short distance travels and continuous start and stop, which is highly energy consuming. At the powertrain level, in [304] is proposed a modular three-port traction converter based on four winding switched reluctance machines for plug-in hybrid electric trucks. This converter is based on two h-bridges connected in a cascaded way, i.e., connected to each other by the ac side, and the dc negative rail is common to both h-bridges. The traction converter can be connected to a battery, a generator and also to the power grid, i.e., the proposed system is a Energies 2020, 13, x FOR PEER REVIEW 38 of 63 unified system. Moreover, the operation of the converter allows the machine stator windings to be connectedconverter eitherallows in the series machine or parallel. stator Figurewindings 36 presents to be connected a traction either system in for series an electric or parallel. truck Figure based on36 twopresents three-phase a traction ac system electric for machines. an electric truck based on two three-phase ac electric machines.

D1 D3 D5 S1 S3 S5 Internal Synchronous Three-Phase Combustion Machine ac Electric Engine Machine C vdc

D2 D4 D6 S2 S4 S6

S7 S9 S11 Three-Phase ac Electric Machine

S8 S10 S12

Figure 36. Traction systemsystem forfor anan electricelectric trucktruck basedbased onon twotwo three-phasethree-phase acac electricelectric machines.machines.

4.4.4.4. Electric Buses TheThe electrificationelectrification ofof busesbuses is oneone ofof thethe mostmost promisingpromising migrations towards electric mobility, sincesince busesbuses representrepresent a a type type of of public public transportation transportation with with well-defined well-defined and and planned planned routes, routes, easing easing the batterythe battery charging charging necessity necessity and scheduling. and scheduling. Electric Electric buses are buses already are beingalready used being in cities used worldwide in cities andworldwide their adoption and their is continuouslyadoption is continuously growing [305 grow,306],ing with [305,306], many scheduling with many approaches scheduling andapproaches models beingand models proposed being in proposed the literature in the in literature order to in maximize order to maximize energy e ffienergyciency efficiency [307–309 [307–309].]. A case A study case regardingstudy regarding the electrification the electrification of a bus of depot a bus in depot London in isLondon presented is presented in [310], with in [310], reported with savings reported in bothsavings financial in both (£1.7 financial million (£1.7 over million a 14 year over lifetime) a 14 year and lifetime) environmental and environmental (48,200 tons (48,200 of CO tons2 between of CO2 2019between and 2050)2019 and aspects, 2050) when aspects, compared when compared to traditional to tr dieseladitional buses. diesel In [311 buses.] is presentedIn [311] is a presented scheduling a approachscheduling for approach a single depotfor a single comprising depot a comprising mixture of electrica mixture and of diesel electric buses, and aimingdiesel buses, to minimize aiming the to operatingminimize costthe andoperating carbon cost emissions and carbon using constraintsemissions ofusing time constraints between travels of time and between driving rangetravels of and the driving range of the electric buses. At the powertrain level, in [312] is studied the design of an energy management system for a hybrid electric bus and in [313] is studied the energy management system of a dual-machine fully electric bus. Due to their fixed routes, electric buses are especially predisposed to the implementation of IWPT functionalities. As mentioned previously in this paper (Section 2.2.7), a system termed OLEV was proposed by KAIST in South Korea, which consists of an electric bus that is supplied wirelessly through a coil embedded in the road [314]. This reduces the need for using large battery banks, which is advantageous since batteries are one of the most expensive parts of EVs and the main reason for them to have a lower driving range compared to ICE vehicles. In [315] is presented an electric powertrain for electric buses based on a hybrid energy storage system. Besides the battery bank, the proposed solution also includes a super capacitor bank to store the energy collected upon the regenerative braking system (Figure 37).

Energies 2020, 13, 6343 37 of 61 electric buses. At the powertrain level, in [312] is studied the design of an energy management system for a hybrid electric bus and in [313] is studied the energy management system of a dual-machine fully electric bus. Due to their ﬁxed routes, electric buses are especially predisposed to the implementation of IWPT functionalities. As mentioned previously in this paper (Section 2.2.7), a system termed OLEV was proposed by KAIST in South Korea, which consists of an electric bus that is supplied wirelessly through a coil embedded in the road [314]. This reduces the need for using large battery banks, which is advantageous since batteries are one of the most expensive parts of EVs and the main reason for them to have a lower driving range compared to ICE vehicles. In [315] is presented an electric powertrain for electric buses based on a hybrid energy storage system. Besides the battery bank, the proposed solution also includes a super capacitor bank to store the energy collected upon the regenerative braking system (Figure 37). Energies 2020, 13, x FOR PEER REVIEW 39 of 63

S1 S3 S5 Three-Phase Battery ac Electric Bank Machine C vdc

S2 S4 S6

S7 S9 S11 Super dc-dc Three-Phase Capacitor converterdc-dc ac Electric Bank Machine

S8 S10 S12

FigureFigure 37. Traction system for an electric bus based on two three-phase ac electricelectric machines and super capacitorscapacitors forfor energyenergy recovery.recovery.

4.5.4.5. Electric Pallet Trucks andand ForkliftsForklifts EVsEVs cancan alsoalso bebe usedused forfor industrialindustrial applications.applications. For instance, in [[316]316] are proposed twotwo unifiedunified systemssystems forfor industrialindustrial EVs,EVs, namelynamely aa palletpallet trucktruck andand aa forklift, both encompassing galvanicgalvanic isolationisolation betweenbetween thethe powerpower gridgrid andand thethe batterybattery (Figure(Figure 38 38).). TheThe pallet trucktruck isis powered byby aa 1.51.5 kW dc electric machine,machine, usingusing anan isolatedisolated cuk dc–dc converter, whilewhile thethe forkliftforklift isis poweredpowered byby aa 66 kW wound rotor inductioninduction machine,machine, with with the the rotor rotor windings windings being being connected connected to the powerto the gridpower and grid the statorand the windings stator connectedwindings connected to the traction to the/charging traction/charging front-end powerfront-en converter.d power converter. Also, regarding Also, regarding forklifts, inforklifts, [317,318 in] is[317,318] studied is the studied design the and design implementation and implementation of switched of switched reluctance reluctance machines machines for their for traction their traction system. Thesystem. energy The e ffienergyciency efficiency of electric of forklifts electric isforklifts studied is instudied [319] forin induction[319] for induction machines machines and in [320 and] for in PMSM,[320] for with PMSM, the with energy the e ffienergyciency efficiency of both types of both of ty electricpes of machineselectric machines being compared being compared in [321]. in At [321]. the energyAt the energy storage storage level, in level, [322] in is presented[322] is presented the design the and design control and ofcontrol a phase-shift of a phase-shift full-bridge full-bridge isolated dc–dcisolated converter dc–dc converter for the on-board for the on-board EVBC of an EVBC electric of an forklift. electric Hybrid forklift. energy Hybrid storage energy systems storage for systems electric forkliftsfor electric are forklifts presented are in presented [323] based in on[323] batteries based andon batteries ultracapacitors, and ultracapacitors, in [324] based in on [324] batteries based and on fuelbatteries cells, and infuel [325 cells,] based and on in fuel [325] cells based and ultracapacitors,on fuel cells and using ultracapacitors, no standard batteries. using no An standard electric energybatteries. recovery An electric system energy for an electro-hydraulicrecovery system forkliftfor an iselectro-hydraulic studied in [326]. Besidesforklift full-electricis studied in forklifts, [326]. inBesides the literature full-electric can alsoforklifts, be found in the research literature regarding can also hybrid be found forklifts, research with regarding the hybridization hybrid offorklifts, diesel forkliftswith the being hybridization studied in of [327 diesel] and forklifts simulation being and studied experimental in [327] validation and simulation of a hybrid and forkliftexperimental being presentedvalidation inof [a328 hybrid]. forklift being presented in [328].

S1 S3 S5 KM1 Lra Lsa Smart Grid Lrb Lsb Battery Bank C vdc Lrc Lsc

Wound Rotor Induction Machine S2 S4 S6

Figure 38. Unified system for traction and battery charging of an electric forklift.

4.6. Electric Motorcycles/Scooters In developing countries, one of the most used means of transportation is the motorcycle, not only because it has affordable purchase cost but because it allows for more flexible mobility, with the electric approach being a promising solution [329,330]. The replacement of ICE motorcycles by

Energies 2020, 13, x FOR PEER REVIEW 39 of 63

S1 S3 S5 Three-Phase Battery ac Electric Bank Machine C vdc

S2 S4 S6

S7 S9 S11 Super dc-dc Three-Phase Capacitor converterdc-dc ac Electric Bank Machine

S8 S10 S12

Figure 37. Traction system for an electric bus based on two three-phase ac electric machines and super capacitors for energy recovery.

4.5. Electric Pallet Trucks and Forklifts EVs can also be used for industrial applications. For instance, in [316] are proposed two unified systems for industrial EVs, namely a pallet truck and a forklift, both encompassing galvanic isolation between the power grid and the battery (Figure 38). The pallet truck is powered by a 1.5 kW dc electric machine, using an isolated cuk dc–dc converter, while the forklift is powered by a 6 kW wound rotor induction machine, with the rotor windings being connected to the power grid and the stator windings connected to the traction/charging front-end power converter. Also, regarding forklifts, in [317,318] is studied the design and implementation of switched reluctance machines for their traction system. The energy efficiency of electric forklifts is studied in [319] for induction machines and in [320] for PMSM, with the energy efficiency of both types of electric machines being compared in [321]. At the energy storage level, in [322] is presented the design and control of a phase-shift full-bridge isolated dc–dc converter for the on-board EVBC of an electric forklift. Hybrid energy storage systems for electric forklifts are presented in [323] based on batteries and ultracapacitors, in [324] based on batteries and fuel cells, and in [325] based on fuel cells and ultracapacitors, using no standard batteries. An electric energy recovery system for an electro-hydraulic forklift is studied in [326]. Besides full-electric forklifts, in the literature can also be found research regarding hybrid forklifts, Energieswith the2020 hybridization, 13, 6343 of diesel forklifts being studied in [327] and simulation and experimental38 of 61 validation of a hybrid forklift being presented in [328].

S1 S3 S5 KM1 Lra Lsa Smart Grid Lrb Lsb Battery Bank C vdc Lrc Lsc

Wound Rotor Induction Machine S2 S4 S6

FigureFigure 38.38. UniﬁedUnified system for traction andand batterybattery chargingcharging ofof anan electricelectric forklift.forklift.

4.6.4.6. Electric MotorcyclesMotorcycles/Scooters/Scooters InIn developingdeveloping countries, countries, one one of of the the most most used used means means of transportationof transportation is the is motorcycle,the motorcycle, not only not becauseonly because it has it ahasffordable affordable purchase purchase cost cost but but because because it allowsit allows for for more more flexible flexible mobility, mobility, with with thethe electric approach being a promising solution [329,330]. The replacement of ICE motorcycles by electric electricEnergies approach 2020, 13, x FOR being PEER a REVIEW promising solution [329,330]. The replacement of ICE motorcycles40 of 63 by motorcycles and consequent environmental and energy savings, in different and specific parts of the world,electric is referredmotorcycles to in and several consequent works inenvironmenta the literature,l and as demonstratedenergy savings, in in [331 different–334]. Theand impactspecific of electricparts motorcycles of the world, and is referred scooters to in in the several distribution works in network the literature, is addressed as demonstrated in [330]. In in [335 [331–334].], a prototype The of aimpact motorcycle of electric with motorcycles electric propulsion and scooters is designed in the distribution and developed, network where is addressed a 7.4 kW in [330]. BLDC In machine [335], is drivena prototype by a three-phaseof a motorcycle voltage with sourceelectric inverter. propulsion In [is336 designed] is presented and developed, a comprehensive where a design7.4 kW of a controllerBLDC machine for a BLDC is driven machine, by a providing three-phase an electromagneticvoltage source inverter. brake and In kinetic[336] is energy presented recovery. a In [comprehensive337] is presented design a solution of a controller for measuring for a BLDC the machine, rotor position providing of the an electromagnetic BLDC machine brake in order and to enablekinetic the energy implementation recovery. In of[337] vector is presented control algorithmsa solution for such measuring as field-oriented the rotor position control. of Inthe [ 338BLDC,339 ], technologiesmachine in for order electric to enable motorcycle the implementation traction systems of vector are proposed, control algorithms where the such BLDC as machinefield-oriented type is used.control. In [340 In], [338,339], a 2003 Honda technologies CBR600RR for electric motorcycle motorcycle was converted traction systems to electric, are where proposed, a battery where bank the of 155BLDC V at 40machine Ah has type been is designedused. In [340], and a is 2003 used Hond an aca electricCBR600RR machine motorcycle that allows was converted a prototype to electric, with an estimatedwhere a maximumbattery bank power of 155 of V 60 at HP. 40 Ah In [has341 ],been an electricdesigned superbike and is used race an demonstrates ac electric machine that electric that allows a prototype with an estimated maximum power of 60 HP. In [341], an electric superbike race motorcycles can race at high speeds on demanding tracks. demonstrates that electric motorcycles can race at high speeds on demanding tracks. One means of transportation that has also been gaining interest in urban areas is the electric One means of transportation that has also been gaining interest in urban areas is the electric scooter, which is framed in the group of light EVs. This has been implemented in several countries as scooter, which is framed in the group of light EVs. This has been implemented in several countries a vehicle-sharing system, aiming to alleviate the traffic mostly caused by cars [342]. The majority of as a vehicle-sharing system, aiming to alleviate the traffic mostly caused by cars [342]. The majority theseof these electric electric scooters scooters are are not not human-assisted, human-assisted, i.e., i.e., the the power power appliedapplied to perform the the motion motion comes comes entirelyentirely from from the the electric electric machine. machine. In In [ 343[343]] isis proposedproposed a kick assisted assisted electric electric scooter, scooter, which which is isa type a type of electricof electric human-powered human-powered hybrid hybrid vehicle. vehicle. The mainThe dimainfference difference between between an assisted an andassisted a non-assisted and a electricnon-assisted scooter iselectric very similarscooter tois thevery di ffsimilarerence betweento the difference electric motorcyclesbetween electric and bicycles.motorcycles In [ 344and] is proposedbicycles. an In electric [344] is scooter proposed with an G2V, electric V2G, V2Hscooter and with energy-harvesting G2V, V2G, V2H functionalities and energy-harvesting (Figure 39 ), andfunctionalities in [345] is proposed (Figure a39), hybrid and energyin [345] storage is propos systemed a forhybrid an electric energy scooterstorage based system on for ultracapacitors an electric andscooter IWPT, based reaching on ultracapacitors an overall effi ciencyand IWPT, of 86.4%. reaching an overall efficiency of 86.4%.

L1 D KM1

S2 S4 S6

L3 PV Panel L2 PM DC Smart Machine Grid S1 KM2 C vdc L4

S3 S5 S7

FigureFigure 39. 39.Uniﬁed Unified converter converter topology topology for for an an electric electric scooter scooter with with G2V, G2V, vehicle-to-home vehicle-to-home (V2H), (V2H), V2G V2G and energy-harvestingand energy-harvesting functionalities. functionalities.

4.7. Electric Bicycles The bicycle plays an important role in today’s society. It is an environmentally friendly vehicle that is economically accessible to a wider range of the population, representing a good means of transportation for short distances, such as in urban areas. Bicycles contribute to reducing energy consumption, air pollution, city traffic and, in addition, their regular use improves the health of the cyclist. On the other hand, it does not undergo taxes, parking costs, driving license or high maintenance service costs in comparison with ICE vehicles. However, on long journeys, the bicycle has disadvantages in terms of exhaustion of the cyclist. In [346,347] is presented an effort control system for cycling, which contributes to promote the user’s mobility and physical health. In view of the disadvantages of traditional bicycles, the development of electric bicycles has begun, generally known as e-bicycles or e-bikes. In some designs the bicycle can be propelled solely by its electric powertrain, in others, electric assistance is added to pedaling, making it possible for less-fit individuals to ride on steep terrains and for longer distances, thus extending the usefulness of the bicycle [348,349]. The first electric bicycle was patented in 1895 by Ogden Bolton Jr. It used a six-pole dc hub machine mounted in the rear wheel and had no gears, consuming up to 100 A from a 10 V battery

Energies 2020, 13, 6343 39 of 61

4.7. Electric Bicycles The bicycle plays an important role in today’s society. It is an environmentally friendly vehicle that is economically accessible to a wider range of the population, representing a good means of transportation for short distances, such as in urban areas. Bicycles contribute to reducing energy consumption, air pollution, city traffic and, in addition, their regular use improves the health of the cyclist. On the other hand, it does not undergo taxes, parking costs, driving license or high maintenance service costs in comparison with ICE vehicles. However, on long journeys, the bicycle has disadvantages in terms of exhaustion of the cyclist. In [346,347] is presented an effort control system for cycling, which contributes to promote the user’s mobility and physical health. In view of the disadvantages of traditional bicycles, the development of electric bicycles has begun, generally known as e-bicycles or e-bikes. In some designs the bicycle can be propelled solely by its electric powertrain, in others, electric assistance is added to pedaling, making it possible for less-fit individuals to ride on steep terrains and for longer distances, thus extending the

Energiesusefulness 2020, of13, thex FOR bicycle PEER REVIEW [348,349 ]. 41 of 63 The ﬁrst electric bicycle was patented in 1895 by Ogden Bolton Jr. It used a six-pole dc hub machine [350].mounted Over in the the years, rear wheel new designs and had for no electric gears, consuming bicycles have up toemerged. 100 A from However, a 10 V batterythe greatest [350]. evolution Over the occurredyears, new in designs the 1990s, for electric due to bicycles the technological have emerged. progress However, of electric the greatest machines, evolution power occurred electronics in the 1990s,controllers due toand the sensors, technological as well progress as batteries. of electric The evolution machines, of power battery electronics technologies controllers allowed and for sensors,greater energyas well storage as batteries. capacity, The evolutionreliability ofand battery robustness, technologies enormously allowed contributing for greater to energy the popularization storage capacity, of thereliability electric and bicycles robustness, [351]. enormously contributing to the popularization of the electric bicycles [351]. Nowadays, electric bicycles are are equipped with BLDC machines (Figure 40),40), but there are also electric bicycles with brushed dc electric machinesmachines and switched reluctance machinesmachines [[352–354].352–354].

S1 S3 S5 BLDC Battery Machine C vdc

S2 S4 S6

Figure 40. TractionTraction system for an electric bicycle based on a brushless brushless direct current (BLDC) machine.

Normally, the machines for electric bicycles ar aree named according to the place where they are connected: hub-drive hub-drive or or mid-drive mid-drive machines. machines. The The hub-drive hub-drive machine machine is is placed placed in in the the front front or rear wheel, more specificallyspecifically on the wheel hub.hub. The mi mid-drived-drive machine is placed directly between the pedals, what ensures a low and mid-placed center ofof gravitygravity [[355,356].355,356]. Recently, control systems were developed to adjust automatically the effort effort control of of the the cyclist, cyclist, based on the use of an electric electric bicycle bicycle and and a a smartp smartphone.hone. In In accordance accordance with with the the Directive Directive 2002/24/EC 2002/24/EC of the the European European Parliament Parliament and and of of the the council council of of18 18March March 2002 2002 relating relating to the to type-approval the type-approval of two of ortwo three-wheel or three-wheel electric electric machine machine vehicles, vehicles, bicycl bicycleses with with pedal pedal assistance assistance are are equipped equipped with with an auxiliary electric machine whose maximum continuous power is is 0.25 0.25 kW. kW. The controller will reduce progressively thethe powerpower and and cut cut it it o ffoffif theif the speed speed reaches reaches 25 km25 /km/h,h, or if or the if cyclistthe cyclist stops stops pedaling pedaling [356]. [356].These These systems systems allow allow people people with diwithfferent different physical phys conditioningical conditioning to ride to together ride together in the in same the track,same wheretrack, where each member each member can choose can choose the adequate the adequa electricte electric machine machine assistance assistance level [ 357level–359 [357–359].]. In the current market, the diversity diversity for for electric electric bi bicyclescycles is is enormous, enormous, existing existing mountain mountain bicycles, bicycles, folding bicycles, bicycles, cargo cargo bicycles bicycles and and urban urban bicycles, bicycles, where where the the cyclist cyclist has has a great a great choice choice for forbuying. buying. In mountainIn mountain bicycles, bicycles, a large a large number number of ofcompanies companies already already produce produce electric electric bicycles. bicycles. The The RENOWATT RENOWATT 4.1 model of Coluer company is an example of a mountain bicycle. It has full suspension and uses an electric machinemachine toto provide provide assistance assistance to to the the cyclist cyclis whent when he rides he rides up to up a speed to a speed of 25 km of/ h.25 Thiskm/h. electric This electric machine has a nominal power of 250 W, a maximum torque of 70 Nm and a nominal voltage of 36 V, adding 2.88 kg to the weight of the bicycle. It is powered with a lithium-ion battery with a nominal capacity of 504 Wh (36 V, 14 Ah) [360]. However, many people prefer to ride around the city or just as a mode of transport to reach work. In this case, the type of bicycle mentioned above is not the most suitable for this type of cyclist. The urban bicycle type is more adequate, with one example being the ELOPS 900 E. It has a BLDC machine in the rear wheel with a power of 250 W and a maximum torque of 30 Nm to assist the cyclist. The electric machine is powered by a battery with a capacity of 418 Wh (36 V, 11.6 Ah) and a range of 40 km to 70 km, depending on variables such as the weight of the cyclist, the selected assistance mode, and the slope of the terrain.

Energies 2020, 13, 6343 40 of 61 machine has a nominal power of 250 W, a maximum torque of 70 Nm and a nominal voltage of 36 V, adding 2.88 kg to the weight of the bicycle. It is powered with a lithium-ion battery with a nominal capacity of 504 Wh (36 V, 14 Ah) [360]. However, many people prefer to ride around the city or just as a mode of transport to reach work. In this case, the type of bicycle mentioned above is not the most suitable for this type of cyclist. The urban bicycle type is more adequate, with one example being the ELOPS 900 E. It has a BLDC machine in the rear wheel with a power of 250 W and a maximum torque of 30 Nm to assist the cyclist. The electric machine is powered by a battery with a capacity of 418 Wh (36 V, 11.6 Ah) and a range of 40 km to 70 km, depending on variables such as the weight of the cyclist, the selected assistance mode, and the slope of the terrain.

4.8. Other Small Electric Vehicles Besides the previously mentioned types of EV, there are also some smaller EVs, i.e., with lower operating power. In the literature can be found power electronics solutions regarding the energy storage systems of small EVs, such as a system based on batteries and ultracapacitors [361], a two-stage battery charger with a power decoupling cell [362], based on a modified bridgeless boost-type front-end power factor correction converter and a phase-shift full-bridge isolated dc–dc converter, and a multi-input single-output dc–dc converter for small aerial vehicles [363], capable of interfacing batteries, ultracapacitors and fuel cells. The types of small EVs analyzed in this section are golf carts, Segways, Tuk-Tuks, step scooters and wheelchairs. Similar to road vehicles, the first golf carts to appear were electric, with gasoline-powered golf carts appearing later. However, contrary to what happened with road vehicles, the electric variant has always been more popular due to gasoline rationalization in the 1950s in consequence of World War II, when golf carts were experiencing their early growth, besides offering more safety due to slower speeds than the gasoline-powered golf carts and also due to lower noise and no emission of pollutants. In [364–367] are presented implementations of solar-powered electric golf carts, which, besides the battery, are comprised by photovoltaic solar panels in their rooftop, with a buck-type dc–dc converter being used to interface the photovoltaic solar panels with the battery. In [368] is studied a hybrid energy storage system for a golf cart, being composed by batteries, fuel cells and ultracapacitors, with only one dc–dc converter being used, namely a simple boost-type topology. In [369] is presented the design of an induction machine especially conceived for use in the traction system of a golf cart. The Segway is a type of EV designed for a single person, being a self-balancing low-power two-wheel EV. It was invented in 2001 by Dean Kamen [370]. In this type of EV, the employed electric machines are based on in-wheel machines, since the vehicle itself is only physically composed of two wheels intermediated by a platform, as well as a shaft in order for the user to travel safely. In [371] is presented the design of a Segway from the point of view of mechanics, fabrication, powertrain and electronics, and [372] analyzes in detail the mathematical model and control system of this type of EV. The design and development of Segway prototypes can be studied in [373–375]. The TukTuk is a three-wheeled motorized transport with a cabin, dedicated to the transport of passengers or goods, widely used in developing countries. In view of environmental concerns, maintenance and cost of use, this means of transport has an electrical homolog which can be found in the literature by e-Tuk-Tuk, Easy Bike, bicitaxis, rickshaws, among others [376–378]. Usually, these EVs contain a BLDC machine with a power rate of around 1.1 kW [379]. In [380] a 1 kW battery charger is proposed to charge the electric Tuk-Tuk batteries from the power grid. The electric wheelchair is also a type of small EV, and it can be indispensable equipment for senior and disabled people. A review in terms of speed, traction, suspension and stability control for electric wheelchairs is reported in [381]. In [382] is presented the conversion of a mechanical wheelchair to an electric one, and in [383] is presented an all-terrain wheelchair, which is capable of navigating through obstacles, such as stairs and ramps. An electric power assisted wheelchair is presented in [384] with the purpose of reducing battery utilization. At the traction level, a torque distribution algorithm for the Energies 2020, 13, 6343 41 of 61 two wheels of an electric wheelchair is presented in [385], a differential drive based on BLDC machines is presented in [386], as shown in Figure 41, and the design and implementation of a spherical induction machine for electric wheelchairs is presented in [387]. The regenerative braking operation concerning electric wheelchairs is studied in [388,389], based on boost-type and buck-boost unidirectional dc–dc converters, respectively. The design and implementation of a solar power integration in an electric wheelchair, with a solar photovoltaic panel acting as a roof for the wheelchair, is presented in [390], and the battery charging operation via IWPT for electric wheelchairs is studied in [391], with the primary circuitry being based on a full-bridge inverter and the secondary based on a diode full-bridge

Energiesac-dc converter2020, 13, x FOR and PEER a buck-type REVIEW dc–dc converter. 43 of 63

S1 S3 S5 S7 S9 S11 Left Wheel Right Wheel BLDC BLDC Battery Machine Machine Bank C1 vdc

S2 S4 S6 S8 S10 S12

FigureFigure 41. 41. TractionTraction system system for for an an electric electric wheelchair wheelchair based based on on two two BLDC BLDC machines. machines.

5.5. Conclusions Conclusions AroundAround the the world, world, environmental environmental issues issues are are perc perceivedeived as as a a priority priority topic, topic, with with the the transport transport sectorsector going going through through a technological revolution towardstowards sustainability.sustainability. Specifically, Specifically, electric electric mobility mobility is isglobally globally identified identified as as the the main main alternative alternative to to transport transport based based on on non-renewable non-renewable polluting polluting fuels, fuels, and and it itis is being being increasingly increasingly supported supported by by the the variousvarious modelsmodels ofof electric vehicles (EVs) (EVs) currently currently produced produced byby the the automotive automotive industry. industry. Regardless Regardless of of the the ve vehiclehicle model model or or type, type, the the common common basis basis to to support support electricelectric mobility mobility is is power power electronics. electronics. Throughout Throughout this this article, article, several several power power electronics electronics technologies technologies forfor electric electric mobility mobility are are presented. presented. RegardingRegarding EV EV battery battery chargers chargers (EVBC), (EVBC), both both wire wiredd and and wireless wireless power power transfer transfer technologies technologies are are addressed.addressed. A A review review of of power power electronics electronics converte convertersrs that that can can be be used used for for on-board on-board and and off-board off-board EVBCEVBC is is presented, presented, covering covering the the main main topologies topologies that that can can be be used used in in the the front-end front-end and and back-end back-end powerpower stages. stages. Some Some developed developed laboratory laboratory prototyp prototypeses are are also also presented, presented, including including on-board on-board and and off-boardoff-board EVBC, EVBC, along along with with illustrative illustrative experimental experimental results results obtained obtained for for the the main main operation operation modes. modes. AsAs demonstrated, demonstrated, both both on-board on-board and and off-board off-board EVBC EVBC are are fundamental fundamental in in supporting supporting the the integration integration ofof EVs EVs into into the the power power grid, grid, presenting presenting innovative innovative features features far far beyond beyond the the simple simple charging charging process, process, e.g.,e.g., bidirectional bidirectional systems systems allow allow the the exchange exchange of of active active power power with with the the grid, grid, and and also also other other features, features, suchsuch as as the the operation operation in in isolated isolated mode mode or or the the possibility possibility of of contributing contributing to to improving improving the the electrical electrical powerpower grid grid quality. quality. In In this this context, context, for for selecting selecting the the more more interesting interesting power power electronics electronics topologies topologies for on-boardfor on-board or off-board or off-board EVBC, EVBC, the the key key features features that that should should be be considered considered are: are: the the multilevel multilevel and and interleavedinterleaved characteristic characteristic for for reducing reducing the the coupling coupling passive passive filters; filters; The The galvanic galvanic isolation isolation between between the EVthe battery EV battery and andthe thepower power grid, grid, guaranteed guaranteed by by th thee dc–dc dc–dc back-end back-end converter; converter; The The four-quadrant four-quadrant operationoperation to ensure thatthat thethe bidirectional bidirectional power power flow flow is possibleis possible (allowing (allowing operation operation in G2V, in G2V, V2G, V2G, V2H, V2H,and other and other modes). modes). Still regarding Still regarding EVBC, EVBC, some some solutions solutions based based on dc–dc on dc–dc converters converters to provide to provide power powerto the auxiliaryto the auxiliary battery battery from thefrom EV the battery EV battery are also are presented. also presented. Regarding Regarding traction traction systems, systems, a brief a briefpresentation presentation of electric of electric machines machines used for used the for EV propulsion,the EV propulsion, as well asas the well electric as the machines electric machines that equip thatcommercially equip commercially available EVs, available was made. EVs, was From made. these From electric these machines, electric permanent machines, magnet permanent synchronous magnet synchronousmachines (PMSMs) machines and (PMSMs) three-phase and induction three-phase machines induction dominate machines the applications dominate the in EVs,applications while other in EVs,electric while machine other technologieselectric machine are emerging technologies alternatives are emerging (e.g., reluctance alternatives machines (e.g., andreluctance hybrid machines reluctance andpermanent hybrid magnetreluctance machines). permanent Depending magnet on machines). the electric machineDepending used, on di fftheerent electric drives machine and controllers used, differentmay be necessary.drives and Drive controllers systems may consist be necessary. of solutions Drive based systems on power consist electronics of solutions that based are capableon power of electronics that are capable of acting on the voltage, current, and frequency supplied to the electric machine windings. The unification of both the power converters of the traction system and the EVBC is also presented. This unification is possible because, except in the case of dynamic inductive wireless power transfer, these two systems are not required to operate simultaneously. The unified system allows a reduction in volume, weight and total costs when compared with the traditional solutions (traction and battery charging as separated systems), also allowing additional operation modes that comply with the paradigm of smart grids. Regarding the power level of unified systems for EVs, increasing the number of phases or the number of windings of electric machines (i.e., the use of open- end split winding electric machines) leads to an increase in the number of power semiconductor legs employed in the power converters, permitting an operation similar to an interleaved configuration, which is one of the main solutions to reduce the current stress and increase the power level of a power electronics system.

Energies 2020, 13, 6343 42 of 61 acting on the voltage, current, and frequency supplied to the electric machine windings. The unification of both the power converters of the traction system and the EVBC is also presented. This unification is possible because, except in the case of dynamic inductive wireless power transfer, these two systems are not required to operate simultaneously. The unified system allows a reduction in volume, weight and total costs when compared with the traditional solutions (traction and battery charging as separated systems), also allowing additional operation modes that comply with the paradigm of smart grids. Regarding the power level of unified systems for EVs, increasing the number of phases or the number of windings of electric machines (i.e., the use of open-end split winding electric machines) leads to an increase in the number of power semiconductor legs employed in the power converters, permitting an operation similar to an interleaved configuration, which is one of the main solutions to reduce the current stress and increase the power level of a power electronics system. Additionally, other types of vehicle, such as railway vehicles, ships, aircraft, trucks, buses, forklifts, scooters and bicycles are also targets for electrification, and were also addressed in this paper. For instance, power electronics converters in electric railways are indispensable, and comprise a significant part of the railway electrification system, with the objective of improving power quality, efficiency and resilience, both for fixed applications (in traction substations) and for moving scenarios (in electric locomotives). These converters can be installed at the medium-voltage level, outside or inside the locomotive, for different applications. In that regard, power quality compensators, power management converters and power converters for traction and regenerative braking have been addressed in this paper. Public electric transportation, such as electric buses and railway vehicles, together with electric trucks, are reported in the literature to be more economical and to reduce environmental damage, when compared to their traditional internal combustion engine counterparts. Lightweight personal EVs, such as electric bicycles and scooters, on the other hand, are becoming increasingly popular for individual use in short-distance traveling, and being envisioned as a viable sustainable complementary model in urban areas. Besides terrestrial transportation, high-power vehicles such as aircraft and ships, traditionally non-electric, are now being also electrified. These EVs, in addition to the expected contribution in the overall reduction in polluting fuel consumption, usually exhibit superior dynamic performance than their fueled counterparts, i.e., faster start, stop and speed variation, sharing the same technical principles with the lighter EVs. Following this observed tendency of mass electrification of all sorts of vehicle, power electronics is an indispensable common support technology, and an essential constituent of all electrification systems and infrastructures. This is even more relevant considering emerging technologies of semiconductors, new magnetic materials, multipurpose control platforms, and others, which improve the performance of the power electronics systems, and make possible new scenarios for power electronics applications, such as the autonomous vehicles. It is probably too early to predict how far this all-electric mobility revolution will take us, but the crucial role of power electronics advances in this transformation is fully ensured and underway.

Author Contributions: J.L.A., L.A.L.C., D.P., T.J.C.S., L.M., M.T., V.M.: writing—original draft preparation, literature review, analysis and evaluation. J.L.A., L.A.L.C., D.P., T.J.C.S., L.M., M.T., V.M.: conceptualization, methodology, and writing—review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This work has been supported by FCT – Fundação para a Ciência e Tecnologia with-in the Project Scope: UID/CEC/00319/2020. This work has been supported by the FCT Project DAIPESEV PTDC/EEI-EEE/30382/2017, and by the FCT Project newERA4GRIDs PTDC/EEI-EEE/30283/2017. Tiago Sousa is supported by the doctoral scholarship SFRH/BD/134353/2017 granted by FCT. Conﬂicts of Interest: The authors declare no conﬂict of interest. Energies 2020, 13, 6343 43 of 61

Abbreviations and Acronyms

BLDC Brushless dc CC-CV Constant Current—Constant Voltage CO2 Carbon Dioxide DIWPT Dynamic Inductive Wireless Power Transfer EV Electric Vehicle EVBC EV Battery Charger G2V Grid-to-Vehicle GA Ground Assembly ICE Internal Combustion Engine ICNIRP International Commission on Non-Ionizing Radiation Protection IGBT Insulated Gate Bipolar Transistor IMD Implanted Medical Device IWPT Inductive Wireless Power Transfer LV Low-Voltage MOSFET Metal Oxide Semiconductor Field Eﬀect Transistor MMC Modular Multilevel Converter MV Medium-Voltage OLEV On-Line Electric Vehicle PMSM Permanent Magnet Synchronous Machines PP Parallel-Parallel PS Parallel-Series PWM Pulse-Width Modulation RB-IGBT Reverse-Blocking Insulated Gate Bipolar Transistor RPC Rail Power Conditioner SAE Society of Automotive Engineers SiC Silicon Carbide SP Series-Parallel SS Series-Serial STATCOM Static Synchronous Compensator TPS Traction Power System UPS Uninterruptible Power Supply V2H Vehicle-to-Home V2G Vehicle-to-Grid VA Vehicle Assembly WPT Wireless Power Transfer

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