Electric Vehicles Charging Stations' Architectures, Criteria, Power

electronics

Review Electric Vehicles Charging Stations’ Architectures, Criteria, Power Converters, and Control Strategies in Microgrids

Dominic Savio Abraham 1 , Rajesh Verma 2, Lakshmikhandan Kanagaraj 3, Sundar Rajan Giri Thulasi Raman 4 , Narayanamoorthi Rajamanickam 1 , Bharatiraja Chokkalingam 1,* , Kamalesh Marimuthu Sekar 5 and Lucian Mihet-Popa 6

1 Department of Electrical and Electronics Engineering, SRM Institute of Science and Technology, Chennai 603203, India; [email protected] (D.S.A.); [email protected] (N.R.) 2 Department of Electrical Engineering Department, King Khalid University, Abha 62529, Saudi Arabia; [email protected] 3 Department of Electrical and Electronics Engineering, Adhiparasakthi College of Engineering, Kalavai 632506, India; [email protected] 4 Department of Electrical and Electronics Engineering, Sathyabama Institute of Science and Technology, Chennai 600119, India; [email protected] 5 Department of Electrical and Electronics Engineering, Kongu Engineering College, Tamilnadu 638060, India; [email protected] 6 Faculty of Engineering, Østfold University College, Kobberslagerstredet 5, 1671 Fredrikstad, Norway; [email protected] * Correspondence: [email protected] Abstract: The usage of electric vehicles (EV) has been increasing over the last few years due to a rise in

Citation: Savio Abraham, D.; Verma, fossil fuel prices and the rate of increasing carbon dioxide (CO2) emissions. EV-charging stations are R.; Kanagaraj, L.; Giri Thulasi Raman, powered by existing utility power grid systems, increasing the stress on the utility grid and the load S.R.; Rajamanickam, N.; demand at the distribution side. DC grid-based EV charging is more efficient than AC distribution Chokkalingam, B.; Marimuthu Sekar, because of its higher reliability, power conversion efficiency, simple interfacing with renewable energy K.; Mihet-Popa, L. Electric Vehicles sources (RESs), and integration of energy storage units (ESU). RES-generated power storage in local Charging Stations’ Architectures, ESU is an alternative solution for managing the utility grid demand. In addition, to maintain the EV Criteria, Power Converters, and charging demand at the microgrid levels, energy management and control strategies must carefully Control Strategies in Microgrids. power the EV battery charging unit. In addition, charging stations require dedicated converter Electronics 2021, 10, 1895. https:// doi.org/10.3390/electronics10161895 topologies, control strategies, and need to follow set levels and standards. Based on EV, ESU, and RES accessibility, different types of microgrid architecture and control strategies are used to ensure Academic Editor: Taha Selim Ustun optimum operation at the EV-charging point. Based on the above said merits, this review paper presents different RES-connected architecture and control strategies used in EV-charging stations. It Received: 26 June 2021 highlights the importance of different charging station architectures with current power converter Accepted: 30 July 2021 topologies proposed in the literature. In addition, a comparison of microgrid-based charging station Published: 6 August 2021 architecture with its energy management, control strategies, and charging converter controls are also presented. The different levels and types of charging stations used for EV charging, in addition to Publisher’s Note: MDPI stays neutral controls and connectors used, are also discussed. An experiment-based energy management strategy with regard to jurisdictional claims in was developed to control power flow among the available sources and charging terminals for the published maps and institutional affil- effective utilization of generated renewable power. The main motive of the EMS and its control is to iations. maximize the usage of RES consumption. This review also provides the challenges and opportunities in EV-charging, and parameters in selecting appropriate charging stations.

Keywords: microgrid; electric vehicle; energy management controls; renewable energy sources; Copyright: © 2021 by the authors. energy storage unit Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons 1. Introduction Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Electric vehicles are becoming popular due to their less emissions and lower fossil- 4.0/). fuel dependency [1]. The renewable energy sources used in distribution networks, in

Electronics 2021, 10, 1895. https://doi.org/10.3390/electronics10161895 https://www.mdpi.com/journal/electronics Electronics 2021, 10, x FOR PEER REVIEW 2 of 46

Electronics 2021, 10, 1895 2 of 45

Electric vehicles are becoming popular due to their less emissions and lower fossil- fuel dependency [1]. The renewable energy sources used in distribution networks, in con- connectionnection with with charging charging station station electrification electriﬁcation of of smart smart grids, grids, provide provide a choicechoice forfor highhigh powerpower conversion conversion efﬁciency efficiency and and emission emission reduction reduction [ 2[2].]. The The microgrid microgrid consists consists of of a a groupgroup ofof distributeddistributed energyenergy sourcessources andand energyenergy storagestorage unitsunits utilizedutilized locallylocally byby differentdifferent types types ofof loadsloads andand operatedoperated inin aa grid-connected grid-connected ororislanding islandingmode mode[ 3[3].]. AA typical typical EV EV charging charging station,station, as as partpart ofof aa microgridmicrogrid infrastructure,infrastructure, is is shownshown inin FigureFigure1 .1. However, However, large large capacity capacity penetrationpenetration ofof EVEV chargingcharging pointspoints increasesincreases thethedemand demandin in charging charging infrastructure; infrastructure;this this impactimpact raisesraises the the demand demand on on the the utility utility grid grid [4]. [ To4]. mitigate To mitigate the problems the problems related related to power to powerdemand, demand, powers powers generated generated locally locally from fromthe RES the are RES integrated are integrated with withsuitable suitable power power con- converterverter topologies topologies [5]. [ 5Charging]. Charging station station faci facilitieslities are are provided provided by by EV EV manufacturers manufacturers as as a apart part of of their their charging charging infrastructure; infrastructure; for for exam example,ple, Tesla Tesla created created solar solar city cityand andNissan Nissan Leaf Leafcreated created sun sunpower power [6]. [ 6However,]. However, charging charging stations stations developed developed using using renewable energyenergy integrationintegration further further reduce reduce the the cost cost of of charging charging and and emission, emission, and and increase increase the the coordination coordina- oftion the of utility the utility grid [grid7,8]. [7,8].

Charging Station

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FigureFigure 1.1.An An EV-chargingEV-charging station station as as part part of of the the microgrid microgrid infrastructure. infrastructure.

DCDC microgridmicrogrid systemssystems areare popularpopular becausebecause ofof theirtheir simplesimple voltagevoltage regulationregulation andand real-timereal-time control,control, alsoalso usedused inin DC-powered DC-powered homeshomes andand industrial industrial applications applications [9[9–11].–11]. AA schematicschematic diagram diagram of of EV EV charging charging stations stations with with a grid-connecteda grid-connected ESU ESU is shownis shown in Figurein Figure2 . DC2. DC microgrids microgrids are are designed designed and and operated operated using using a a novel novel topology topology with with a a combination combination ofof hybridhybrid sourcessources [[12,13].12,13]. TheThe firstfirst lowlow voltagevoltage microgridmicrogrid waswas proposedproposed inin 2002,2002, andand isis currentlycurrently experiencing experiencing many many enhancement enhancement changes changes due due to distributed to distributed generation generation [14]. This[14]. lowThis voltage low voltage microgrid microgrid system system consists consists of different of different scattered scattered energy energy sources sources with different with dif- typesferent of types AC orof DCAC loads.or DC Theloads. same The development same development was seen was on seen an ACon an microgrid AC microgrid in 2004, in developed2004, developed with 10 with kW, 10 better kW, reliability, better reliability, high efficiency, high efficiency, and simple and control simple [15 control]. Similarly, [15]. severalSimilarly, DC several microgrids DC microgrids have been have developed been developed and used and for differentused for different applications, applications, such as communicationsuch as communication systems, systems, and ESU and with ESU distributed with distributed renewable renewable sources [16sources]. [16]. PV-integrated microgrids are directly connected to EV charging stations using renewable energy without ESU through an EV-PV converter [17–19]. Generally, the PV power generation is variable in nature and its regulation is made through a grid connection. It also has various advantages, such as high quality uninterrupted power supply to the load, automatically isolated from the utility grid during fault conditions, and provides power to the utility grid when deficient [20,21]. Different classifications of microgrids used for EV charging are shown in Figure3.

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Control center PV EV Aggregator Grid

ESU Electronics 2021, 10, 1895 Information 3 of 45 Electronics 2021, 10, x FOR PEER REVIEW Power 3 of 46 Parking Lot Charging Plugged in PV charging point Mobile Charging Station

Control center PV EV Aggregator Grid

ESU Information Power Electric Parking Lot Charging PEV Plugged in PV charging point Electric PEV Bike Electric PEV Mobile Charging Station Bike Bike PEV PEV Figure 2. A general schematic of a charging station.

PV-integrated microgrids are directly connected to EV charging stations using renewable energy without ESU through an EV-PV converter [17–19]. Generally, the PV power generation is variable in nature and its regulation is made through a grid connec- Electric tion.PEV It also has various advantages, such as high quality uninterruptedElectric power supplyPEV to Bike Electric PEV Bike the load, automatically isolated fromBike the utilityPEV grid duringPEV fault conditions, and provides power to the utility grid when deficient [20,21]. Different classifications of microgrids Figure 2. A general schematic of a charging station. Figureused for 2. A EV general charging schematic are shown of a charging in Figure station. 3. PV-integrated microgrids are directly connected to EV charging stations using renewable energy withoutMicrogrid ESU through an EV-PV converter [17–19]. Generally, the PV power generation is variable in nature and its regulation is made through a grid connection. It also has various advantages, such as high quality uninterrupted power supply to the load, automatically isolated from the utility grid during fault conditions, and provides Modes power toBus the Type utility grid when deficientSources [20,21]. Different classificationsUsage of microgrids used for EV charging are shown in Figure 3.

Microgrid Isolated Grid AC DC Hybrid Renewable Non- Residential Commercial Connected renewable

FigureFigure 3. Types of microgrids. microgrids. Modes Bus Type Sources Usage ChargingCharging atat workplaces through through an an installed installed PV PV in inthe the building’s building’s rooftop rooftop and and park- park- inging lot lot reduces reduces landland andand initial cost cost investment investment [22]. [22]. According According to toa national a national household household survey,survey, 90% 90% of of vehiclesvehicles are parked for for 5 5 to to 6 6 hours hours in in a aparking parking lot, lot, so soworkplace workplace charging charging supportssupports thethe vehiclevehicle in grid (V2G) (V2G) charging charging [23]. [23]. The The charging charging stations stations in indifferent different places places Isolated Grid AC DC Hybrid Non- areare shown shown inin FigureFigure4 4.. RegardlessRegardlessRenewable of the power source, source, automobile automobileResidential makers makers areCommercial are required required Connected to achieve extremely high reliability standards.renewable Furthermore, the enormous energy capac- to achieve extremely high reliability standards. Furthermore, the enormous energy ca- ity and potentially volatile nature of some battery technologies pose a serious safety haz- pacity and potentially volatile nature of some battery technologies pose a serious safety ard. Reliability,Figure availability, 3. Types and of microgrids.maintainability are main concerns in charging stations, hazard. Reliability, availability, and maintainability are main concerns in charging stations, restricting large-scale commercial utilization of these vehicles. The EV is reliant on grid restricting large-scale commercial utilization of these vehicles. The EV is reliant on grid powerCharging and the at charging workplaces system's through dependability an installed [24]. PV Thus, in the a grid-connected building’s rooftop charging and sta-park- power and the charging system’s dependability [24]. Thus, a grid-connected charging ingtion lot reliability reduces landmodel and was initial created. cost It investmentsought to investigate [22]. According the reliability, to a national availability, household and station reliability model was created. It sought to investigate the reliability, availability, and survey,maintainability 90% of vehicles issues facedare parked by EV forcharging 5 to 6 hoursstations; in studya parking how lot, fault so events workplace are logically charging supportsmaintainabilityrelated tothe one vehicle another, issues in gridfacedhow (V2G)aby PEV's EV charging reliability charging [23]. stations;is influenced The charging study by howthese stations fault fault events inevents, differentare and logically howplaces arerelatedproper shown tomanagement onein Figure another, 4. strategies Regardless how a PEV’s can of improve reliability the power a ve is hicle'ssource, inﬂuenced availability automobile by these [25]; faultmakers and events, to are look required and into how toproper theachieve impact management extremely of a charging high strategies relistationability can on improve PEVstandards. availability. a vehicle’s Furthermore, A availabilitymodified the probabilisticenormous [25]; and toenergy index look intocapac-was the ityimpact and potentially of a charging volatile station nature on PEV of some availability. battery Atechnologies modiﬁed probabilistic pose a serious index safety was haz- also ard.proposed Reliability, to evaluate availability, the power and supply’s maintainability reliability. are An main IC was concerns designed in charging to be controlled stations, by restrictingan external large-scale BMS control commercial unit via a utilization serial peripheral of these interface vehicles. (SPI), The which EV is alsoreliant allowed on grid for powerthe retrieval and the of charging acquired system's data [26 dependability]. EV batteries [24]. are charged Thus, a throughgrid-connected conductive charging coupling, sta- wireless charging, or replaced using battery swapping technique. Wireless charging in the tion reliability model was created. It sought to investigate the reliability, availability, and U.K. to test roads that charge electric cars as they drive is shown in Figure5[27,28]. maintainability issues faced by EV charging stations; study how fault events are logically related to one another, how a PEV's reliability is influenced by these fault events, and how proper management strategies can improve a vehicle's availability [25]; and to look into the impact of a charging station on PEV availability. A modified probabilistic index was

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alsoalso proposedproposed toto evaluateevaluate thethe powerpower supply'ssupply's reliability.reliability. AnAn ICIC waswas designeddesigned toto bebe con-con- trolledtrolled byby anan externalexternal BMSBMS controlcontrol unitunit viavia aa serialserial peripheralperipheral interfaceinterface (SPI),(SPI), whichwhich alsoalso allowedallowed forfor thethe retrievalretrieval ofof acquiredacquired datadata [26].[26]. EVEV batteriesbatteries areare chargedcharged throughthrough conduc-conduc- Electronics 2021, 10, 1895 tivetive coupling,coupling, wirelesswireless charging,charging, oror replacedreplaced usingusing batterybattery swappingswapping technique.technique. WirelessWireless4 of 45 chargingcharging inin thethe U.K.U.K. toto testtest roadsroads thatthat chargecharge electricelectric carscars asas theythey drivedrive isis shownshown inin FigureFigure 55 [27,28].[27,28].

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FigureFigureFigure 4. 4.4.Charging ChargingCharging station stationstation in inin different differentdifferent locations. locationslocations

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WirelessWirelessWireless charging chargingcharging of ofof an anan EV EVEV is isis done donedone by byby either eithereither inductive inductiveinductive or oror capacitive capacitivecapacitive coupling. coupling.coupling. In InIn the thethe conductiveconductiveconductive coupling couplingcoupling type, type,type, an anan electrical electricalelectrical outlet outletoutlet plug plugplug is used isis usedused to charge toto chargecharge the EVs thethe [EVsEVs29]. [29]. Here,[29]. Here,Here, two separatetwotwo separateseparate coils are coilscoils used areare for usedused power forfor powerpower transfer—one transfer—onetransfer—one coil is placed coilcoil isis placed insideplaced the insideinside vehicle thethe andvehiclevehicle acts andand as theactsacts receiving asas thethe receivingreceiving coil, while coil,coil, the whilewhile other thethe one otherother is placed oneone isis on placedplaced the parking onon thethe parkingparking slot to transfer slotslot toto transfer thetransfer power. thethe Inpower.power. capacitive InIn capacitivecapacitive charging, charging,charging, four capacitive fourfour capacitivecapacitive plates are platesplates used areare for usedused charging forfor chargingcharging [30]. The [30].[30]. cost TheThe of thecostcost chargingofof thethe chargingcharging can be reduced cancan bebe reducedreduced by developing byby developingdeveloping a level-based aa level-basedlevel-based charging station.chargingcharging The station.station. time taken TheThe timetime for chargingtakentaken forfor is chargingcharging reduced is throughis reducedreduced battery throughthrough swapping batterybattery technology swappingswapping [ technology22technology,31,32]. The [22,31,32].[22,31,32]. pros and TheThe cons prospros of theandand types conscons of ofof charging thethe typestypes are ofof chargingcharging presented areare in presente Tablepresente1. Thedd inin different TableTable 1.1. The batteryThe differentdifferent swapping batterybattery stations swappingswapping are shownstationsstations in areare Figure shownshown6[ 33 inin]. FigureFigure EV chargers 66 [33].[33]. areEVEV classiﬁedchargerschargers are intoare classifiedclassified two types: intointo off-board twotwo types:types: charger off-boardoff-board and on-boardchargercharger andand charger. on-boardon-board The charger charger.charger. located TheThe chargercharger inside the locatedlocated EV is insideinside called the anthe on-board EVEV isis calledcalled charger anan on-boardon-board and the chargerchargercharger located andand thethe outside chargercharger the locatedlocated EV is outside calledoutside an thethe off-board EVEV isis calledcalled charger anan [off-boardoff-board34,35]. A blockchargercharger diagram [34,35].[34,35]. of AA theblockblock types diagramdiagram of chargers ofof thethe is typestypes given ofof in chch Figureargersargers7 is.is givengiven inin FigureFigure 7.7. The primary use of an on-board charger is for low power application, and an off- board charger is used for high power DC fast charging [35]. In an on-board charger, EVs are charged from AC sources; the main issues are with power limitation and charging time [36]. Off-board chargers offer fast charging and vehicle-to-grid charging. The beneﬁts and issues of the chargers are presented in Table2. Energy sources are important in charging stations and most energy generation depends on fossil fuel technology. Hence, a charging station with renewable energy source (RES) requires a large and suitable area for installation [13,37,38]. Different combinations of sources are used in EV charging stations. Commonly, photovoltaic (PV) and wind energy are used as RES to integrate with microgrids. Therefore, an RES is a suitable replacement for conventional sources [39], as it also reduces degradation of the environment [40,41]. Electronics 2021, 10, 1895 5 of 45

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Table 1. Pros and cons of different charging systems.

Charging System Pros Cons Suitable for slow and fast charging Need of standard connectors and cable Conductive charging High-efﬁcient charging Requires complex charging infrastructure Multiple taping possible Coil type needs to be standardized No problems in standardization of connectors Cost of coil increases vehicle price Wireless charging Dynamic charging can be implemented Complexity increases on the location of the transmitter Charging can be done in all climate conditions Losses are more in wireless charging Battery replacement done in less than a minute Standardization required for battery size and type ElectronicsBattery swapping2021, 10, x FOR PEERWith REVIEW the help of swapping, distance travel Charging station should be able to manage a larger number of batteries5 of 46

isFigure increased. 6. Battery swapping station [33]. User responsible for battery maintenance

Table 1. Pros and cons of different charging systems.

Charging System Pros Cons Suitable for slow and fast charging Need of standard connectors and cable Conductive charging High-efficient charging Requires complex charging infrastructure Multiple taping possible No problems in standardization of con- Coil type needs to be standardized nectors Cost of coil increases vehicle price Wireless Dynamic charging can be implemented Complexity increases on the location of the transmit- charging Charging can be done in all climate ter conditions Losses are more in wireless charging Battery replacement done in less than a Standardization required for battery size and type minute Charging station should be able to manage a larger Battery swapping With the help of swapping, distance number of batteries travel is increased. User responsible for battery maintenance Figure 6. Battery swapping station [33]. Figure 6. Battery swapping station [33].

Table 1. Pros and cons of different charging systems. Local Load Charging System Pros Cons Inlet DC Suitable for slow and fast charging Motor EV Need of standardAC connectors and cable Conductive charging High-efficientCoupler charging On-board RequiresDC complex charging infrastructure 1or 3 Multiple taping possible charger DC Differential Phase Protection No problems in standardization of con- Coil type needs to be standardized AC Unit BMS nectors Cost of coil increases vehicle price Wireless Supply Dynamic charging can be implemented ComplexityBattery Pack increases on the location of the transmit- charging Charging can be done in all climate Electric Vehicle ter ElectronicsCharging 2021, 10, x FOR Point PEER (EVSE) REVIEW 6 of 46 conditions Losses are more in wireless charging Battery replacement done in less than a Standardization required for battery size and type (a) minute Charging station should be able to manage a larger Battery swapping With the help of swapping, distance number of batteries Local Load travel is increased. Inlet User responsible for battery maintenance DC Motor EV AC Coupler 1or 3 DC Differential Phase Protection AC DC DC AC Unit DC DC BMS Supply Local Load

Battery Pack Inlet DC Motor EV AC Charging Point (EVSE) Electric Vehicle Coupler On-board DC 1or 3 (b) charger DC Differential FigurePhase 7. EVFigure charger: 7. EVProtection ( acharger:) On-board (a) On-board charger charger arrangement arrangement of of supply supply equipment, (b) ( Off-boardb) Off-board charger charger arrangement arrangement of sup- of supply ply equipment. BMS equipment.AC Unit Supply The primary use of an on-board charger is for low power application, and an off- board charger is used for high power DC fast chargingBattery [35]. Pack In an on-board charger, EVs are charged from AC sources; the main issues are with power limitation and charging time [36]. Off-board chargers offer fast chargingElectric and vehicle-to-grid Vehicle charging. The benefits Charging Point (EVSE) and issues of the chargers are presented in Table 2. Energy sources are important in charging stations and most energy generation depends on fossil fuel technology. Hence, a charging station with renewable energy source (RES) requires a large and suitable area for installation [13,37,38]. Different(a) combinations of sources are used in EV charging stations. Commonly, photovoltaic (PV) and wind energy are used as RES to integrate with microgrids. Therefore, an RES is a suitable replacement for conventional sources [39], as it also reduces degradation of the environment [40,41].

Table 2. Challenges and benefits of different types of charging.

Charger Benefits and Uses Challenges Type Slow charging, less power transfer at a Charge possible at any location time On-board with an electric outlet Difficult to implement vehicle-to any- Simple BMS can be used thing charging Weight of charger added to the EV Used in higher power rating (kW) Battery heating issue Off-board Fast charging Difficult to allocate places Does not add to vehicle weight Cost of charging is high In solar power integration, the PV panels are connected in parallel and series combinations. The Wind Energy Conversion System (WECS) consists of blades, gearbox, and generator. Energy production is based on wind speed in a particular area. The installation of WECS with moderate and high wind conditions requires high maintenance costs [42]. The housing/enclosure of a charging station/socket protects the electrical and electronic circuitry against climate conditions and intrusion of objects. For surge protection against lightning strikes and transient over-voltages, a type 2 device according to SS-EN 61643-11 must be installed [43]. A galvanic isolation between the mains and the vehicle is required to avoid unwanted common mode currents. When an emergency stop is needed, the entire load current is interrupted, and all live conductors, including the neutral con- ductor, are disconnected using an emergency stop button. The electrical and electronic

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Table 2. Challenges and beneﬁts of different types of charging.

Charger Type Benefits and Uses Challenges Charge possible at any location with an Slow charging, less power transfer at a time On-board electric outlet Difficult to implement vehicle-to anything charging Simple BMS can be used Weight of charger added to the EV Used in higher power rating (kW) Battery heating issue Off-board Fast charging Difficult to allocate places Does not add to vehicle weight Cost of charging is high

In solar power integration, the PV panels are connected in parallel and series combinations. The Wind Energy Conversion System (WECS) consists of blades, gearbox, and generator. Energy production is based on wind speed in a particular area. The installation of WECS with moderate and high wind conditions requires high maintenance costs [42]. The housing/enclosure of a charging station/socket protects the electrical and electronic circuitry against climate conditions and intrusion of objects. For surge protection against lightning strikes and transient over-voltages, a type 2 device according to SS-EN 61643-11 must be installed [43]. A galvanic isolation between the mains and the vehicle is required to avoid unwanted common mode currents. When an emergency stop is needed, the entire load current is interrupted, and all live conductors, including the neutral con- ductor, are disconnected using an emergency stop button. The electrical and electronic circuitry in a charging station/socket must be protected from external mechanical impacts by the enclosure [44]. The enclosure of a charging station/socket located outside must meet the requirements of IP code 43, as deﬁned by SS-EN 60529. The IP code 43 indicates that housing protects the electrical and electronic circuitry from intrusion by objects larger than 1 mm in diameter as well as water spray [45]. It is thus clear that charging stations that use different sources need to be studied in detail. Detailed reviews on charging station architecture, standards, converters, and energy management control strategies are few. The main premises of this review paper are: 1. The impacts of selection of charging stations, sizing of the charging systems, and selection of sources. The different sources connected should ensure a suitable charging system, made with different technologies like on-board and off-board chargers. Suitable charging station architecture and selection of sources will reduce costs, di- mensions, weights, and power rating, among other factors. 2. Charging stations require standards to connect charging cords, cables, and connectors. The implementation of charging stations with standards can maximize the utilization of charging stations. 3. The cost and performance of a charger depends on the semiconducting devices and its arrangement. All of this impacts one’s choice, and the size of the charging system is based on the converter topology used for conversion and gain of the corresponding converter. 4. A charging station with renewable energy reduces the demand in the existing grid system and reduces fossil fuel-based conversion of energy. In addition, the effective use of renewable sources and utilization of sources through energy management increases usage of the EV. The key contributions and structure of this paper are as follows: This review provides a detailed study of different EV charging architecture when powered by RES. In addition, the converter topology, controls, and various standards and power levels in the charging station are presented. Hence, this review could provide clear motivation for selecting charging station architecture with renewable power and energy storage units. The different charging architectures were compared based on the control strategy of the charging station and feasibility of connecting the ESU. In addition, different energy management approaches are presented to achieve controlled EV charging. Electronics 2021, 10, 1895 7 of 45

This paper is structured as follows: Section2 discusses the review of architecture of multi-point EV charging station and operating principles. Section3 studies charging station standards and levels. Sections4 and5 present EV charging connectors and power electronics converters used in EV charging tied to microgrids, respectively. Sections6 and7 present energy management in a DC microgrid-based charging station and control strategy of the charging station, respectively, as well as discusses the challenges and opportunities. Section9 concludes the paper.

2. Architecture of Multi-Point EV Charging Stations A microgrid-based charging station architecture combines energy sources and ESU localization of distributed loads, offering the capability of operating in a connected grid or in islanding mode. A charging station with renewable energy sources provides an option for charging of the EV without any power conversion losses [46]. There are different types of RES connected to the DC bus, like PV, wind, super capacitor, and fuel cell [37]. Some of the problems in microgrids include steady-state and transient voltage and frequency control [47]. The different types of charging station architectures are shown in Figure8. In addition, there are problems associated with protection and short circuit, and power quality during islanding and fault conditions of the system [48]. The photovoltaic sys- Electronics 2021, 10, x FOR PEER REVIEWtem is the main source of renewable energy in an RES-powered DC bus system, with8 of 46 a different arrangement to supply power to the local load and EV. This section discusses the different architecture of microgrids used for EV charging. Based on RES and a load connection charging station, a different topology is framed, one that requires a different connectionenergy management charging station, control a strategy different [49 topolo,50]. gy is framed, one that requires a different energy management control strategy [49,50].

RES Powered Multi-Point EV Charging Station

Grid AC Bus Multiport Multiport DC microgrid Isolated DC connected charging Hybrid AC– converter converter with direct MG RES powered station DC with DC grid with AC grid connection of microgrid DC architect Microgrid interconnecti interconnecti ESU microgrid ure on on grid

FigureFigure 8. 8. RESRES Powered Powered Multi-Point Multi-Point EV EV Charging Charging Station. Station.

2.1.2.1. Isolated Isolated DC DC Microgrid Microgrid for for EV EV Charging Charging AnAn isolated isolated DC DC microgrid microgrid is is powered powered by by renewable/non-renewable renewable/non-renewable energy energy sources sources suchsuch as as PV PV or or biofuel biofuel generators generators through through de dedicateddicated converters. converters. In In isolated isolated microgrids, microgrids, a a commoncommon DC DC bus bus is used forfor efﬁcientefficient integration integration storage storage and and renewable renewable energy energy sources sources [51]. [51].Diesel Diesel generators generators are frequentlyare frequently used used to generate to generate electricity, electricity, posing posing environmental environmental and logistical challenges. Diesel power plants emit a lot of greenhouse gases. Furthermore, and logistical challenges. Diesel power plants emit a lot of greenhouse gases. Furthermore, diesel must be transported to remote locations, posing concerns, such as leaks on islands. diesel must be transported to remote locations, posing concerns, such as leaks on islands. Renewable energy sources (RESs) have thus been implemented in many parts of the world Renewable energy sources (RESs) have thus been implemented in many parts of the world to address such challenges. The generation of renewable energy sources, however, has a lot to address such challenges. The generation of renewable energy sources, however, has a of changes and uncertainties, which might lead to problems in stability [52]. DC microgrid lot of changes and uncertainties, which might lead to problems in stability [52]. DC mi- integration with energy sources is not required for frequency and phase synchronization, crogrid integration with energy sources is not required for frequency and phase synchro- like the AC grid system. Therefore, a DC microgrid system can be used when a DC load is nization, like the AC grid system. Therefore, a DC microgrid system can be used when a connected to the grid. The only problem is that the DC bus voltage needs to be stabilized. DC load is connected to the grid. The only problem is that the DC bus voltage needs to be This type of microgrid architecture can supply load power based on the power available stabilized. This type of microgrid architecture can supply load power based on the power available at the generation side [53]. In an isolated DC microgrid system, it is easy to optimize power flows at the DC bus with minimum cost of power control signal and transmission; a structure of this microgrid is shown in Figure 9 [54]. The isolated microgrid- based charging suggests that investing in new PV generation and implementing EV charging techniques for a new fleet will result in a lower microgrid net present cost, particularly if EV penetration is high.

Isolated DC Microgrid Control Strategy The control and management of microgrids are performed based on meteorological conditions and load consumption using short-term forecasting data [55]. Optimization- based control is mostly followed in this type of microgrid. Optimization is implemented based on the predicted output by satisfying the constraints [56–59]. The DC bus voltage-based control strategy is used for load consumption and generation, and power balancing is performed by controlling energy storage and local biofuel generators [60]. The power control required for a DC isolated microgrid is taken as the reference and denoted as p*. Power balancing is done by regulating the DC bus voltage with a proportional-integral controller: ∗ ∗ ∗ 𝑝 =𝑝 −𝑝 −𝑐(𝑣 −𝑣) −𝐶 𝑑(𝑣 −𝑣)𝑑𝑡 (1)

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at the generation side [53]. In an isolated DC microgrid system, it is easy to optimize power ﬂows at the DC bus with minimum cost of power control signal and transmission; a where Ppv is the power supplied by the PV, PL is the power required by the load, v* is the structure of this microgrid is shown in Figure9[ 54]. The isolated microgrid-based charging DC bus reference voltage, CP and CI are proportional and integral gain of the PI control- suggests that investing in new PV generation and implementing EV charging techniques for ler, respectively. Then, p* defines a distributed storage and biofuel generator optimized a new ﬂeet will result in a lower microgrid net present cost, particularly if EV penetration value. The different isolated microgrids and their load control techniques are presented is high. in Table 3.

Biofuel ESU Generator

DC AC DC DC DC Boost DC BDC Converter DC BUS

DC DC DC DC DC Converter EV 1 Converter EV 2 House

FigureFigure 9. 9. IsolatedIsolated DC DC Microgrid. Microgrid. Isolated DC Microgrid Control Strategy Table 3. Isolated microgrid-based EV charging. The control and management of microgrids are performed based on meteorological Reference Sources conditions Microgrid and load Control consumption using short-term forecasting Load Control data [ 55 ]. Optimization- PI basedcontroller-based control is mostly control followed for charging in this type of microgrid. Optimization is implemented Proportional integral PWM generation-based first [61] Solar andbased MPPT-based on the predicted control outputfor PV byto DC satisfying the constraints [56–59]. quadrant-based DC to DC converter The DC busbus voltage-basedside control strategy is used for load consumption and genera- Simulationtion, and dynamic power balancing optimization is performed SDO- by controlling energy storage and local biofuel PV Dynamic wireless power transfer (DWPT) systems [62] basedgenerators sizing [ 60for]. complete The power microgrid control required for a DC isolated microgrid is taken as the Wind, Fuel cell reference and denoted as p*. Power balancing is donefor bydynamic regulating charging the DC bus voltage control with a proportional-integral controller: PV, micro tur- Rule-based algorithm with dynamic Mixed integer linear programming for the energy [63] bine load modeling for microgrid control Z t storage unit p∗ = p − p − c (v∗ − v) − C d(v∗ − v)dt (1) PV, wind and The elements that make up a microgridPV L P i Lead-acid battery-aging0 models and average [38] diesel genera- can be optimally dimensioned and where P is the power supplied by the PV, PLambient is the power temperature required and by the control load, v* is the tor pv managed DC bus reference voltage, CP and CI are proportional and integral gain of the PI controller, PV, wind, bio- respectively. Then, p* deﬁnes a distributed storage and biofuel generator optimized value. [64] mass, and die- InvasiveThe different weed isolatedoptimization microgrids algorithm and their load Backtracking control techniques search algorithm are presented control in Table 3. sel generator The isolated microgrid uses wind diesel isolated microgrids (WDIMs) combined with wind turbine generators (WTGs) and diesel generators (DGs) to supply electricity to remote consumers. The isolated sources can be operated under different modes, such as diesel-only, wind-diesel, and wind-only. In addition, it uses different short-term energy storage technologies like batteries, ultra-capacitors, and flywheels to improve WDIM power quality, stability, and reliability [65–68].

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Table 3. Isolated microgrid-based EV charging.

Reference Sources Microgrid Control Load Control Proportional integral PWM PI controller-based control for charging and [61] Solar generation-based ﬁrst quadrant-based DC to MPPT-based control for PV to DC bus side DC converter PV Simulation dynamic optimization SDO-based Dynamic wireless power transfer (DWPT) [62] Wind, Fuel cell sizing for complete microgrid control systems for dynamic charging Rule-based algorithm with dynamic load Mixed integer linear programming for the [63] PV, micro turbine modeling for microgrid control energy storage unit The elements that make up a microgrid can be Lead-acid battery-aging models and average [38] PV, wind and diesel generator optimally dimensioned and managed ambient temperature and control PV, wind, biomass, and diesel [64] Invasive weed optimization algorithm Backtracking search algorithm control generator

The isolated microgrid uses wind diesel isolated microgrids (WDIMs) combined with wind turbine generators (WTGs) and diesel generators (DGs) to supply electricity to remote consumers. The isolated sources can be operated under different modes, such as diesel- Electronics 2021, 10, x FOR PEER REVIEWonly, wind-diesel, and wind-only. In addition, it uses different short-term energy storage10 of 46 technologies like batteries, ultra-capacitors, and ﬂywheels to improve WDIM power quality, stability, and reliability [65–68].

2.2.2.2. Grid-Connected, Grid-connected, RES-PoweredRES-powered DC MG for EV Charging TheThe grid-connected grid-connected topology topology shares shares a a common common DC DC bus bus between between all all the the sources sources and and load;load; itit also allows allows the the PV PV and and the the battery battery storage storage to work to work in parallel in parallel [17]. The [17 ].EVs The charged EVs chargedfrom the from PV or the ESU PV mostly or ESU depend mostly on depend RES; the on decision RES; the depends decision on dependspower management on power management[34]. The PV [is34 connected]. The PV isto connected the DC bus to us theing DC MPPT bus usingand ESU MPPT is connected and ESU is using connected a bidi- usingrectional a bidirectional converter. The converter. main drawback The main of drawbackthis architecture of this is architecture DC-to-AC conversion is DC-to-AC for conversiongrid integration. for grid Another integration. characteristic Another is characteristic the ability of is the the energy ability storage of the energyto feed storagethe grid toor feed load the of gridthe households. or load of the This households. architecture This can architecture operate in can various operate modes in various based modes on the basedpower on available the power at availablethe charging at the station charging [69]. station This architecture [69]. This architecture can be used can for be usedcharging for chargingelectric vehicle electric batteries vehicle batteries using DC using supply, DC as supply, shown as in shown Figure in 10. Figure 10.

PV ESU

DC DC Boost DC DC Converter BDC

DC BUS

DC DC DC AC DC DC Bidirectional Converter AC/DC Grid EV 1 Converter EV 2 Converter

Figure 10. RES-connected DC MG for EV charging with ESU. Figure 10. RES-connected DC MG for EV charging with ESU. The power prediction model developed depends on an individual customer’s power requirements. During huge power demand, the power prediction model acts quickly and efficiently to respond to the charging station [65,70]. The power required for charging the ith EV is based on the SOC of the vehicle, plugged time, and charging mode:

𝑆, −𝑆×𝐶 𝑃 = (2) 𝑃𝑇

where SEVi,req is the required SOC applied by an ith customer using the human machine interface i, PTEVi is the plugged time set to adapt the matched charging mode, CEVi is the battery capacity of the ith EV. During the time of multiple EV connections, power demand is calculated from the following equation:

𝑃 =𝑃 (3) When a particular EV (ith EV) is connected to the charging point, the energy management system will measure from the human machine interface of the ith and compensate for the lack of power for EV charging. The real-time monitoring of the ESU with respect to PTEVi is:

𝑆 −𝑆,×𝐶 𝑃 = (4) ∑ 𝑃

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The power prediction model developed depends on an individual customer’s power requirements. During huge power demand, the power prediction model acts quickly and efﬁciently to respond to the charging station [65,70]. The power required for charging the ith EV is based on the SOC of the vehicle, plugged time, and charging mode: SEVi,req − SEVi × CEVi PEVi = (2) PTEVi

N PEVs = ∑ PEVi (3) i=1

When a particular EV (ith EV) is connected to the charging point, the energy management system will measure from the human machine interface of the ith and compensate for the lack of power for EV charging. The real-time monitoring of the ESU with respect to PTEVi is: S − S × C = ESU ESU,opt ESU PBSB N (4) ∑i=1 PEVi

where SESU,opt is the rate of SOC at which ESU should be off service, CESU is the ESU capacity, and SESU is the instantaneous SOC. The power prediction model measures each charging point power requirement and provides optimum power to the EV user and the charging station owner. The different control methodologies of the grid-connected, RES-powered DC microgrid control strategy are presented in Table4.

Table 4. Comparison of control methodology.

Reference Renewables Microgrid Control ESU Capacity EV Type EV Charging and Load Control Load regulation through over-voltage and [71] PV Droop control - PEV over-current regulation control Smart load management control strategy Dynamic optimal [50] PV 1000 kWh PEV based on cost estimation from generating power ﬂow control energy and losses Rule-based control of [72] PV 130 Ah PHEV PI-PWM control of charging DC LINK voltage PI controller-based charging of EV and Power management [18] PV (SMES) 6 H/300 A EV superconducting magnetic energy control Storage (SMES) Accumulated total neutral-point current Circulating currents [73] PV - EV control used to reduce voltage ripple in elimination charging output. Model predictive [74] PV, Wind 24 kWh EV Constant current and voltage loop control controller (MPC)

In the DC microgrid based system, a proper control strategy should be used to avoid the circulating current providing optimal operation. In addition, droop control architecture can be used to maintain DC bus voltage. Furthermore, advanced controls like hierarchical control architecture, fuzzy control architecture, and multi-agent-based control architecture can be used for maintaining voltage ﬂuctuations. The limited capacity of the energy storage system is necessary to overcome ﬂuctuation of DC Bus voltage and maintain power balance [72,75–78]. Electronics 2021, 10, 1895 11 of 45

2.3. RES-Powered DC Microgrid with Direct Connection of ESU The RES of PV connected using a unidirectional converter and ESU using electrochemical battery stacks are directly connected to the DC bus. A PV converter controlled through maximum power point tracker (MPPT) provides regulated supply to the utility grid [79]. The numbers of series battery cells are determined depending on the DC bus voltage. If the ESU is directly connected to the DC bus, it is required to regulate the charging voltage [80]. The direct connection of ESU will cause a circulating current problem that leads to uneven loading of those converters. To control the power ﬂow, communication between the con- Electronics 2021,verter 10, x FOR and PEER inverter REVIEW is required using a coordinated control strategy [81]. The RES-powered 12 of 46

DC microgrid with a direct connection of ESU is shown in Figure 11.

PV ESU

DC Boost DC Converter

DC BUS

DC DC DC AC DC DC

EV 1 EV 2 Converter Inverter Grid Converter Figure 11. RES-connectedFigure EV charging 11. RES-connected with ESU. EV charging with ESU.

ModesTable 5. ofComparison Operation of andcontrol Control methodology Algorithm of RES-powered DC MG with a direct connection of ESU.

Reference RenewablesThe type Microgrid of charging Control station EV architecture type/Motor depends on the EV control charging strategy and Load of the control DC link voltage control. The PV is connected to the chargingControl station’s of load depends DC link on voltage, the SOC which available at the varies based onDC the link irradiation voltage- on the PV panel. The reference DC bus voltages are chosen [82] PV PHEV vehicle battery, based on the reference level operated based on the differentbased control sun conditions from early morning to late evening. The overall in CV or CC mode control of RES-powered DC MG with a direct connection of ESU is presented in Table5. Load current requirement is measured by power con- [83] PV Rule-based control PHEV ditioner monitors Table 5. Comparison of control methodology of RES-powered DC MG with a direct connection of ESU. Based on the real-time estimation of the load availa- Fuzzy logic power- Reference Renewables[84] MicrogridPV Control EV Type/MotorPHEV bility EVwith Charging total generating and Load Control cost-based prediction con- flow controller Control of load depends on the SOCtrol available at the DC link voltage-based [82] PV PHEV vehicle battery, based on the reference level control Different AC and Custom-made supervisor control capable to effi- [85] PV and wind Supervisor control operated in CV or CC mode DC loads ciently administrate diverse energy Load current requirement is measured by power [83] PV Rule-based controlLabView control PHEVBrushed PM DC Relationship between frequency and DC voltage- [66] Wind conditioner monitors algorithm motor based control Based on the real-time estimation of the load Fuzzy logic power-ﬂow [84] PV PHEV availability with total generating cost-based controller The microgrid with direct connections of ESU is the most common type of DC MG, prediction control frequently deployed in practical industrial applications. Based on the DC bus, the ESU, Different AC and DC Custom-made supervisor control capable to [85] PV and wind Supervisor control used as electrochemicalloads battery stacks,efﬁciently is directly administrate connected. diverse However, energy the architecture creates dynamic stability, such as uncontrollable DC bus voltage, SOC, and current limi- LabView control Relationship between frequency and DC [66] Wind Brushed PM DC motor algorithm tation [86]. It is most suitable for singular DCvoltage-based bus systems, control however, creating practical problems like unregulated battery charging and inherent imperfections in bus voltage measurement.

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The microgrid with direct connections of ESU is the most common type of DC MG, frequently deployed in practical industrial applications. Based on the DC bus, the ESU, used as electrochemical battery stacks, is directly connected. However, the architecture creates dynamic stability, such as uncontrollable DC bus voltage, SOC, and current limitation [86]. It is most suitable for singular DC bus systems, however, creating practical problems like unregulated battery charging and inherent imperfections in bus voltage measurement.

2.4. AC Bus Coupled Charging Station Architecture In AC-coupled microgrid architecture, all the loads are connected to the common AC bus, as shown in Figure 12. In this, the ESU is connected to the AC bus through the AC-to-DC bidirectional converter [87]. This type of architecture gives an option for sizing of all the parts independently. The connection to the grid and local load might provide Electronics 2021, 10, x FOR PEER REVIEW 13 of 46 more flexibility to the system in terms of charge and discharge of the battery and peak time management [88]. The AC bus-coupled architecture is preferred for home-based charging due to its well-defined standards. In addition, AC technology and its products are alreadyavailable available in the market in the market [89]. However, [89]. However, considering considering renewable renewable energy energy sources, sources, solar solar inte- integrationgration DC DC bus-based bus-based systems systems are aremore more efficien efficientt due due to fewer to fewer amounts amounts of conversion of conversion loss. loss.The TheAC ACgrid-based grid-based charging charging station station requires requires power power compensation compensation to measuremeasure activeactive powerpower at at the the point point of of common common coupling coupling [ 35[35].].

PV ESU

DC Converter DC AC AC Inverter

AC BUS

AC AC DC DC Synchronizer EV 1Rectifier EV 2 Rectifier FigureFigure 12. 12.AC AC bus-coupled bus-coupled charging charging station station architecture. architecture.

ACAC Bus-Connected Bus-connected ChargingCharging andand DischargingDischarging ControlControl ToTo exchange exchange power power between between microgrids microgrids to to a a high-capacity high-capacity power power grid, grid, the the equilib- equilib- riumrium state state of of thethe microgridmicrogrid isis followed.followed. If the power of of the the ESU ESU is is greater greater than than zero, zero, it it is isconsidered considered a discharge a discharge state; state; if not, if not, it is it cons is consideredidered as the as charging the charging state. state. Different Different intelli- intelligent algorithms are used—mainly GA, AFS, and PSO [90,91]. PSO computes infor- gent algorithms are used—mainly GA, AFS, and PSO [90,91]. PSO computes information mation through the current optimal location, and GA computes information between all through the current optimal location, and GA computes information between all chromo- chromosomes. The PSO has higher ﬁtness and less computing time. A comparison of the somes. The PSO has higher fitness and less computing time. A comparison of the AC bus- AC bus-coupled charging station architecture controls are presented in Table6. coupled charging station architecture controls are presented in Table 6.

Table 6. Comparison of AC bus-coupled charging station architecture controls.

ESU Capac- Reference Renewables Microgrid Control Load Load control ity Two types of algorithm are used—stochas- A linear regression model is tic dynamic programming (SDP) algorithm [92] PV EV - used for load prediction and and greedy algorithm (benchmark algo- control rithm) The smart metering-based microgrid con- An SOC prediction-based load [93] - trol is used with Modbus on an TCP/IP EV 30 kVA control is followed. connection using the internal LAN EMS is used to optimize power generation Keeping in mind the high and use of different sources and loads to loading impacts of the EV, [94] PV EV Optimization minimize the total cost, while satisfying the typical EV-charging methods load and device constraints. were incorporated. PV and The frequency and SOC-based [64] Frequency-based control Balanced load 7.3 MWh Wind control are used to provide

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Table 6. Comparison of AC bus-coupled charging station architecture controls.

Reference Renewables Microgrid Control Load ESU Capacity Load Control Two types of algorithm are used—stochastic dynamic A linear regression model is used [92] PV programming (SDP) algorithm EV - for load prediction and control and greedy algorithm (benchmark algorithm) The smart metering-based microgrid control is used with An SOC prediction-based load [93]- EV 30 kVA Modbus on an TCP/IP connection control is followed. using the internal LAN EMS is used to optimize power Keeping in mind the high loading generation and use of different impacts of the EV, typical [94] PV sources and loads to minimize the EV Optimization EV-charging methods were total cost, while satisfying the incorporated. load and device constraints. The frequency and SOC-based Balanced control are used to provide [64] PV and Wind Frequency-based control 7.3 MWh load high-quality power to the connected loads Custom-made supervisor control Different AC [17] PV&Wind Supervisor control capable to efﬁciently administrate & DC Load the diverse energy forms

The advantage of an AC microgrid-based distribution systems is that it can distribute over distance and can be stepped up or down. Stable voltage can be obtained by controlling reactive power and real power independently. In the grid-connected mode, when the main grid experiences an abnormal or faulty condition, the AC microgrid will isolate itself to protect the load within the microgrid [87,88]. The main advantage of AC microgrid-based charging stations is that all the existing loads are operated with an AC source; it reduces conversion losses, but DC loads are the dominant load in the charging station. Due to this conversion, efﬁciency is reduced.

2.5. Hybrid AC-DC Microgrid A hybrid microgrid is the concept of combining both AC and DC microgrid distribution systems, as shown in Figure 13. Hybrid microgrids use relative merits of both AC and DC microgrids, and offers the advantages of both [95]. All DC power sources, like photovoltaic (PV) systems and fuel cell (FC), are connected to DC microgrids through DC-DC boost converters [96]. Similarly, DC loads such as electric vehicles are connected to DC microgrids through DC-DC buck converters. In addition, the AC grid is connected to the sources of wind turbine generators, small diesel generators, and utility grid systems [97]. AC grid voltages are maintained as 230 V or 400 V to connect AC loads such as AC motors. A hybrid microgrid-based charging system commonly uses an AC supply system or is otherwise connected to the RES. There are various advantages while implementing coordinated charging, which includes bus voltage regulation, frequency regulation, and island condition. In power sharing mode the utility grid absorbs energy from the DC microgrid and maintains grid balance [50,65]. Electronics 2021, 10, x FOR PEER REVIEW 14 of 46

high-quality power to the connected loads Custom-made supervisor con- PV& Different AC & trol capable to efficiently ad- [17] Supervisor control Wind DC Load ministrate the diverse energy forms The advantage of an AC microgrid-based distribution systems is that it can distribute over distance and can be stepped up or down. Stable voltage can be obtained by controlling reactive power and real power independently. In the grid-connected mode, when the main grid experiences an abnormal or faulty condition, the AC microgrid will isolate itself to protect the load within the microgrid [87,88]. The main advantage of AC microgrid- based charging stations is that all the existing loads are operated with an AC source; it reduces conversion losses, but DC loads are the dominant load in the charging station. Due to this conversion, efficiency is reduced.

2.5. Hybrid AC-DC Microgrid A hybrid microgrid is the concept of combining both AC and DC microgrid distribution systems, as shown in Figure 13. Hybrid microgrids use relative merits of both AC and DC microgrids, and offers the advantages of both [95]. All DC power sources, like photovoltaic (PV) systems and fuel cell (FC), are connected to DC microgrids through DC-DC boost converters [96]. Similarly, DC loads such as electric vehicles are connected to DC microgrids through DC-DC buck converters. In addition, the AC grid is connected to the sources of wind turbine generators, small diesel generators, and utility grid systems [97]. AC grid voltages are maintained as 230 V or 400 V to connect AC loads such as AC motors. A hybrid microgrid-based charging system commonly uses an AC supply system or is otherwise connected to the RES. There are various advantages while implementing coordinated charging, which includes bus voltage regulation, frequency regulation, and island Electronics 2021, 10, 1895 14 of 45 condition. In power sharing mode the utility grid absorbs energy from the DC microgrid and maintains grid balance [50,65].

PV ESU DC Boost DC Converter

DC BUS

DC AC Inverter AC DC Converter

AC Load

FigureFigure 13. 13.Hybrid Hybrid AC-DC AC-DC microgrid microgrid architecture. architecture.

2.5.1. AC and DC Charging Station and Its Control Control of the entire system requires the bidirectional control system for both AC and

DC bus [98]. A three-layer coordinated control algorithm was developed to control the hybrid AC-DC micro grid to coordinate the sources without overlapping each other [99]. Different layers perform distinct operations; the ﬁrst maintains DC grid voltage and provides coordinated supply to all DC charging points. The second layer controls the AC bus RMS voltage regulation and maintains the frequency from the three-phase AC grid. The third layer operates between AC and DC microgrids through an interlinking converter. Its control facilitates different power ﬂow operations, such as from vehicle to grid and PV to ESU. The control actions of the charging point are done through a localized controller, but the overall control of the charging station is managed through the main controller [66,95].

2.5.2. Frequency Droop Control The hybrid microgrid’s control and power sharing is done through frequency droop control with the measurement of active (P) and reactive power (Q), as shown in Figure 14 [100]. The frequency droop control maintains reference frequency and the corresponding voltage amplitude equations as follows: ∗ 2 fi = fi − πr , (5) ∗ ∗ fi = fi − mi X (Pi − Pi ) (6) ∗ ∗ Vi = Vri − ni X (Qi − Qi ) (7) ∗ ∗ where Vri is the reference rated voltage of the DC bus, fi is the reference rated microgrid ∗ ∗ operating frequency. Pi , Qi are the rated value of real and reactive power and Pi, Qi are the measured real and reactive powers, respectively. Here, m and n are the droop coefﬁcients derived for the maximum ratings of the load. Electronics 2021, 10, x FOR PEER REVIEW 15 of 46

2.5.1. AC and DC Charging Station and its Control Control of the entire system requires the bidirectional control system for both AC and DC bus [98]. A three-layer coordinated control algorithm was developed to control the hybrid AC-DC micro grid to coordinate the sources without overlapping each other [99]. Different layers perform distinct operations; the first maintains DC grid voltage and provides coordinated supply to all DC charging points. The second layer controls the AC bus RMS voltage regulation and maintains the frequency from the three-phase AC grid. The third layer operates between AC and DC microgrids through an interlinking converter. Its control facilitates different power flow operations, such as from vehicle to grid and PV to ESU. The control actions of the charging point are done through a localized controller, but the overall control of the charging station is managed through the main controller [66,95].

p0 p Power

Figure 14. Frequency droop control characteristics.

2.5.3. Angle Droop Control 𝑓 = 𝑓∗ −𝜋𝑟 The angle droop control used when all the DGs, are interfaced with the microgrid(5) is shown in Figure 15[ 100]. By controlling∗ the angle between∗ the voltage and current, real 𝑓 = 𝑓 −𝑚 𝑋 (𝑃 −𝑃 ) (6) and reactive power is controlled [22]. By comparing voltage magnitude, the converter’s ∗ ∗ real or reactive power is given to the𝑉 DC=𝑉 microgrid,−𝑛 𝑋 (𝑄 −𝑄 and ) control signals are derived from(7) wherethe following 𝑉∗ is the equations. reference rated voltage of the DC bus, 𝑓∗ is the reference rated mi- ∗ ∗ crogrid operating frequency. 𝑃∗α, i𝑄=∗ α arei − themi Xrated(Pi −valuePi ) of real and reactive power and(8) Electronics 2021, 10, x FOR PEER REVIEW 16 of 46 𝑃 , 𝑄 are the measured real and reactive∗ powers, respectively.∗ Here, m and n are the Vi = Vri − ni X (Qi − Qi ) (9) droop coefficients derived for the maximum ratings of the load.

2.5.3. Angle Droop Control V The angle droop control used when all the DGs are interfaced with the microgrid is shown in Figure 15 [100]. By controllingVi* the angle between the voltage and current, real and reactive power Slopeis controlled [22]. By comparing voltageSlope magnitude, the converter’s real or reactive power is given to the DC microgrid, and control signals are derived from the following equations. Power Vi

pi Pi* Power Qi Qi* Power

FigureFigure 15. 15.Angle Angle and and voltage voltage control control characteristics. characteristics.

V and αi are the measured values when DG supplies the reactive power of Q and ∗ ∗ real power of P [101]. The main drawback𝛼 =𝛼 −𝑚 of this𝑋 (𝑃 control −𝑃 ) strategy is that it requires(8) a communication channel for angle referencing.∗ ∗ 𝑉 =𝑉 −𝑛 𝑋 (𝑄 −𝑄 ) (9)

2.5.4. Communication-BasedV and αi are the measured Control values when DG supplies the reactive power of Q and real powerIn the of P power-sharing [101]. The main method, drawback the central of this controller control strategy controls is the that voltage it requires source a inverter. commu- Therenication are differentchannel for types angle of power-sharingreferencing. methods used in the centralized control method, such as central sharing, master slave, and distributed sharing [102]. The DC bus voltage is regulated2.5.4. Communication-based through a multi-loop Control control in the ﬁrst method, and the second method operates in masterIn the slave power-sharing mode [103– 105method,]. The the performance central controller of the power controls sharing the voltage depends source on the in- masterverter. control;There are transient different time types is theof power-shar main drawbacking methods in this method.used in the In centralized order to improve control reliability,method, such smart as charging central sharing, is used throughmaster slav distributede, and distributed control. sharing [102]. The DC bus voltageA comparison is regulated of through hybrid microgrida multi-loop charging control stations’ in the first architecture method, and and control the second are presentedmethod operates in Table 7in. master slave mode [103–105]. The performance of the power sharing depends on the master control; transient time is the main drawback in this method. In order to improve reliability, smart charging is used through distributed control. A comparison of hybrid microgrid charging stations’ architecture and control are presented in Table 7.

Table 7. Comparison of hybrid microgrid charging stations’ architecture and control.

Reference Renewables Microgrid Control ESU Capacity Load control [75] PV, wind Power-based, rule-based control Battery Electromagnetic transient program Coordinated rule-based control The direct torque (DTC) and direct [106] PV 250 kW with P&O MPPT power control (DPC) The fuzzy control is adopted to opti- Wavelet Transform and Fuzzy [107] PV, wind - mize the energy management control Control-based microgrid control. of EVs PV, wind, super- Pulse width modulation (PWM)- Constant voltage and constant current [108] capacitor, and based power factor correction - control method fuel cell (PFC) control Hybridization algorithm of Parti- Voltage control loop (VCL) and cur- cle Swarm Optimization (PSO) [109] PV - rent and Applied Artificial Physics control loop (CCL) (APO) In hybrid microgrid management and control strategy, the control is based on a hierarchical control structure: primary, secondary, and tertiary. Mostly three levels of control are used for primary control, including droop-based techniques, which are most suitable for scalable hybrid microgrids. They can provide high plug-and-play capabilities while ensuring adequate power-sharing of devices [100]. The next level’s techniques are

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Table 7. Comparison of hybrid microgrid charging stations’ architecture and control.

Reference Renewables Microgrid Control ESU Capacity Load Control [75] PV, wind Power-based, rule-based control Battery Electromagnetic transient program Coordinated rule-based control with The direct torque (DTC) and direct [106] PV 250 kW P&O MPPT power control (DPC) The fuzzy control is adopted to Wavelet Transform and Fuzzy [107] PV, wind - optimize the energy management Control-based microgrid control. control of EVs PV, wind, supercapacitor, Pulse width modulation (PWM)-based Constant voltage and constant current [108] - and fuel cell power factor correction (PFC) control control method Hybridization algorithm of Particle Voltage control loop (VCL) and current [109] PV Swarm Optimization (PSO) and - control loop (CCL) Applied Artiﬁcial Physics (APO)

In hybrid microgrid management and control strategy, the control is based on a hierarchical control structure: primary, secondary, and tertiary. Mostly three levels of Electronics 2021, 10, x FOR PEER REVIEW 17 of 46 control are used for primary control, including droop-based techniques, which are most suitable for scalable hybrid microgrids. They can provide high plug-and-play capabilities while ensuring adequate power-sharing of devices [100]. The next level’s techniques are distinguisheddistinguished depending depending on on whether whether they they are are centralized centralized or or decentralized. decentralized. On On evaluating evaluating studiesstudies found found in in the the literature, literature, it itwas was determin determineded that that centralized centralized strategies strategies are more are more ad- equateadequate at a at single-user a single-user low low scale. scale.

2.6.2.6. Multiport Multiport Converter Converter with with DC DC Grid Grid Interconnection Interconnection AA multiport multiport converter converter for for the the charging charging operation operation mode mode is is used used for for different different sources sources andand loads [[110].110]. BasedBased onon the the requirements, requirements, connecting connecting points points are are designed designed as bidirectional as bidirec- tionalor unidirectional or unidirectional converters converters [111]. [111]. In addition, In addition, when when there isthere more is PVmore power PV power generation, generation,the excess the powerexcess ispower fed backis fed to back the utilityto the utility grid by grid using by using the same the same multiport multiport converter. converter.Figure Figure16 shows 16 shows multiport multiport charging charging architecture architecture with DC with bus DC interconnection bus interconnection with DC/DC with DC/DCand DC/AC and DC/AC converters converters for PV for panels, PV panels, EV, and EV, the and grid. the grid.

PV Multiport EV 1 DC-DC converter ESU EV 2

DC BUS

DC AC AC/DC Converter Grid DC Load

FigureFigure 16. 16. MultiportMultiport converter converter used used for for charging charging stations stations in in a a DC DC microgrid. microgrid.

DifferentDifferent ratings ofof sourcessources and and batteries batteries are are connected connected through through a multiport a multiport converter. converter.Multiport Multiport converters converters are designed are designed with DCwith interconnection, DC interconnection, facilitating facilitating the directthe direct use useof DCof DC power power for for EV EV charging charging from from DC DC sources. sources. The The PV PV side side DC/DC DC/DC boost converter cancan be be controlled controlled with with the the MPPT MPPT control control techni techniqueque [112]. [112 Even]. Even if direct if direct DC power DC power is avail- is ableavailable at the at DC the interconnection, DC interconnection, there there is a requirement is a requirement for fast for fastcharging, charging, which which can canbe achievedbe achieved through through the converter the converter control. control. The conv Theerter converter is used is to used interface to interface EV loads, EV which loads, are unidirectional or bidirectional based on requirement. When the charging side is bidirectional, this can be operated in both buck and boost mode [113]. While charging, the converter works as a buck converter; it increases the current limit for charging and overall control; comparisons are shown in Table 8.

Table 8. Comparison of multiport converter-based charging stations’ architecture.

Type of EV Con- ESU Ca- Ref Renewables Microgrid Control Load Control nected pacity Power balance control us- This involves DC-based charging [114] PV ing the state diagram EV - with Chademo and the Com- method bined Charging Standard (CCS). DC link voltage-based [115] PV EV 2 kW PI control of charging point converter control PWM-based bidirectional Inductively coupled power trans- [29] - Battery storage 2.16 kWh converter control fer (ICPT)-based pulse charging Different operating modes man- [116] PV MPPT controller Battery storage - age the charging

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which are unidirectional or bidirectional based on requirement. When the charging side is bidirectional, this can be operated in both buck and boost mode [113]. While charging, the converter works as a buck converter; it increases the current limit for charging and overall control; comparisons are shown in Table8.

Table 8. Comparison of multiport converter-based charging stations’ architecture.

Ref. Renewables Microgrid Control Type of EV Connected ESU Capacity Load Control Electronics 2021, 10, x FOR PEERPower REVIEW balance control This involves DC-based charging18 with of 46 [114] PV using the state diagram EV - Chademo and the Combined method Charging Standard (CCS). DC link voltage-based [115] PV EV 2 kW PI control of charging point converter control Conventional current hysteresis [117] PV PWM control Battery Bank - PWM-based bidirectional Inductively coupled power transfer [29]- Battery storage 2.16 kWh control method converter control (ICPT)-based pulse charging Due to its flexibility, a multiport converter has severalDifferent advantages. operating modesIt can manage be used [116] PV MPPT controller Battery storage - with renewable-energy sources for uninterrupted power supply withoutthe charging storage, or for storage of energy-using hybrid sources in electric vehicles. However,Conventional the current power hysteresis capacities [117] PV PWM control Battery Bank - of the multiport converters are limited. control method

2.7. MultiportDue to its Converter ﬂexibility, with a multiport AC Grid converterInterconnection has several advantages. It can be used with renewable-energyThe architecture sources represented for uninterrupted in Figure 17 power uses supplya multiport without converter storage, with or forthe storageAC bus ofconnection. energy-using Integration hybrid sourcesof different in electric sources vehicles. and loads However, through thea multiport power capacities converter of leads the multiportto a reduction converters in components are limited. counts and increases in power density [118]. This type of charging station architecture is more suitable when the charging station depends more on 2.7. Multiport Converter with AC Grid Interconnection the utility grid [119]. The disadvantage of this topology is that when different multiport convertersThe architecture are connected represented to the AC in grid, Figure it 17requires uses a AC-DC multiport conversion. converter with the AC bus connection.This charging Integration station’s of different control sourcesis the same and as loads the throughDC bus-based a multiport control, converter only it differs leads toin athe reduction number in of components multiport converters counts and connect increasesed to in the power AC bus density [120]. [118 This]. Thisarchitecture type of charginguses a communication-based station architecture is control more suitable through when the thedirect charging connection station of depends a multiport more con- on theverter. utility Comparison grid [119]. of The the disadvantage architecture of of this different topology multiport is that whenconverter-based different multiport AC mi- converterscrogrid charging are connected stations tois thepresented AC grid, in itTable requires 9. AC-DC conversion.

PV Multiport EV 1 DC-DC converter ESU EV 2

Bidirectional DC-AC Inverter

AC BUS

AC Load Grid

FigureFigure 17.17. MultiportMultiport converter-basedconverter-based ACAC microgrid.microgrid.

Table 9. Comparison of the architecture of different multiport converter-based AC microgrid charging stations.

Renewa- Type of EV ESU Capac- Reference Microgrid Control Load Control bles connected ity Rule-based control with PSO-based [91] PV EV1 150 kW SOC-based charging control microgrid control The power flow to all the port is PWM-based control strategy [121] - connected through the high fre- EV1 5 kW used for EV charging by con- quency link transformer trolling gate pulse. [122] PV MPPT control - 10 kW PWM-based control strategy

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This charging station’s control is the same as the DC bus-based control, only it differs in the number of multiport converters connected to the AC bus [120]. This architecture uses a communication-based control through the direct connection of a multiport converter. Comparison of the architecture of different multiport converter-based AC microgrid charging stations is presented in Table9.

Table 9. Comparison of the architecture of different multiport converter-based AC microgrid charging stations.

Reference Renewables Microgrid Control Type of EV Connected ESU Capacity Load Control Rule-based control with [91] PV EV1 150 kW SOC-based charging control PSO-based microgrid control The power ﬂow to all the port is PWM-based control strategy used [121]- connected through the high EV1 5 kW for EV charging by controlling frequency link transformer gate pulse. [122] PV MPPT control - 10 kW PWM-based control strategy [123] PV MPPT control - 3.3 kW Current sharing algorithm. [124] PV Power ﬂow - - Rule-based control

This multiport converter is gaining interest in the research community by connecting various renewable energy sources along with storage elements to the load/grid [86,124]. A comparison of partially isolated converters was done in terms of component count, ZVS performance, power transfer efficiency, and control strategies. The general operation of the converters is also reported briefly. The different architectures presented in the literature are compared and presented in Table 10. The different criteria influencing the selection of charging stations are environment, economic, society, and technology. Under these different criteria, the sub-criteria for selecting charging stations are environmental factors, waste discharge, and type of landscape. Under the economic factor, the selection of charging stations includes construction cost, annual profit, and maintenance cost. Social factors affecting the charging stations are accessibility, population density, etc. The technological criteria for selecting charging stations are the level of charging, types of charging, number of charging terminals, and types of connectors. Different considerations to be followed to charge an EV without loading the microgrid are charging characteristics, charging time, and types of charging. The power quality of the microgrid is affected due to power regulation, reactive power compensation, load balancing, and filtering of current harmonics. In addition, the control and management of different charging stations and standards and levels are also followed.

Table 10. The comparisons of the hardware topology presented in this paper from different points of view [53,87,88,103].

Grid DC Microgrid AC Bus Multiport Hybrid Multiport Converter Isolated DC Connected with Direct Charging Converter with Microgrid Architecture AC-DC with AC Grid Microgrid RES Powered Connections of Station DC Grid Microgrid Interconnection Grid DC Microgrid ESU Architecture Interconnection Direct DC charging (with Yes Yes Yes No Yes Yes Yes no AC conversion) Direct AC usage for No Yes Yes Yes Yes Yes Yes local load Feasible low amount of conversion losses during Yes Yes Yes No No Yes Yes V2Any (Vehicle and Grid) Fast charging and Yes Yes No No Yes Yes Yes discharging of ESU Used for high power Yes Yes Yes Yes Yes No No rating Reliability Medium High High Low Low Medium Medium Scalability Medium High Medium High Low Medium Medium Stability Stable Stable Stable Stable Unstable Stable Stable Electronics 2021, 10, 1895 19 of 45

3. Charging Station Standards and Levels Charging stations are developed based on different standards and defined by the Society of Automotive Engineers (SAE), based on charging cords and chargers [125]. The International Electro Technical Commission IEC 61851 and American standard SAEJ1772 are two standards defining the communication protocol, electrical, and physical parameters [126]. EV rectification and constant voltage regulations should follow SAE J1772 [127]. In case of utility or microgrid-based charging using off-board chargers, the standards defined by the SAWJ2293 are followed. Communication requirements for integrating systems follow SAEJ 2836 [128]. The different standards followed in charging stations and their scope of a particular standard and type of charging is presented in Table 11.

Table 11. Standards of EV charging [125–128].

Standard Scope Type Year SAE J1772 Define connectors for AC charging Conductive 2010 Communication network between electric vehicle supply SAE J2293-2 equipment (EVSE) and EV; this communication follows an Conductive 2014 Enhanced Transmission Selection (ETS) network. Provides standard communication between the utility SAE J2847-1 Conductive 2010 grid and plug-in electric vehicles. This standard defines an off-board conductive DC charger Conductive and SAE J2847-2 2015 and its communication with plug-in electric vehicles. Inductive This defines a wireless power transfer of all types of SAE J2954 plug-in electric vehicles and its coil alignment Inductive 2020 methodology Defines the magnetic field-based wireless power system IEC 61980-3 Inductive 2019 and its specific requirements. Battery IEC TS 62840-1 Gives a general overview for battery swap systems 2016 swapping Defines EVSE when it is charging from the voltage range IEC 61851-1 of 1000V to 1500V AC or DC, including on-board Conductive 2017 rechargeable energy storage systems (RESS) Defines requirements for conductive connection of an IEC 61851-21-1 Conductive 2017 electric vehicle (EV) to an AC or DC supply Defines a digital communication between a DC charging IEC 61851-24 Conductive 2014 station and an EV Describes power generation and distribution of electrical Conductive IEC 60364 2017 charging station installations in buildings &Inductive IEE2030.1.1-2015 DC quick charger for use with electric vehicles Fast charging 2016 The terminology of electric vehicle charging/battery swap Battery GB/T 29317 2012 infrastructure swapping

The EV charging station follows three voltage levels; the corresponding power rating is shown in Table3. Levels 1 and 2 mostly defines on-board charging and level 3 defines off-board charging, most of the microgrid-based level 3 charging employed in the public sector. The levels of charging stations are selected based on the power level of the local grid. The different level-based charging station connectors are shown in Figure 18. Considering the voltage level, EV chargers are classified into three types, namely, DC level 1 (200–450 V, 80 A up to 36 kW), level 2 (200–450 V, 200 A up to 90 kW), and level 3 (200–600 V, 400 A up to 240 kW) is presented in Table 12[ 129]. The charging power levels of the charging station decide the charging cost, charging time, electric vehicle charging station equipment (EVSE), and usage of the utility grid. Electronics 2021, 10, 1895 20 of 45 Electronics 2021, 10, x FOR PEER REVIEW 21 of 46

AC Level 2 AC/DC Level 3 AC Level 1 Fast charger

Figure 18. Voltage type level-based EV charging. Figure 18. Voltage type level-based EV charging.

Table 12. Levels of charging stations.

Supply Voltage Current Power PEVPEV Types of Levels Supply Voltage Current Power Types of EV Connector Levels Type Range Range Output ChargingCharging Time Chargers EV Connector Type Range Range Output Chargers 120 V 16 A 1.9 kW Time AC Level 1 Single phase AC 7 h On-board J1772™/AC 120240 V V 13–1616 A A 1.93 kW AC Level 1 SingleSingle/three phase phase AC 7 h On-board J1772™/AC AC Level 2 208–240240 V V13 A–16 80 A A 3 20 kW kW 3 h On-board J1772™/AC AC Single/ three AC Level 2 three phase 208 V–240 V 80 A 20 kW 3 h On-board J1772™/AC AC Level 3 300–600 V 400 A max 120–240 kW 30 min On-board CHAdeMO AC/Comophase chargingAC three phase DC Level 1 DC 200–500 V <80 A120 40 kW– kW 22 min Off-board J1772™/AC AC Level 3 AC/Como charg- 300 V–600 V 400 A max 30 min On-board CHAdeMOCHAdeMO/DC DC Level 2 DC 200–500 V <200 A240 100 kW kW 10 min Off-board ing SAE/DC DC Level 1 DC 200 V–500 V < 80 A 40 kW 22 min Off-board J1772™/ACCHAdeMO/DC DC Level 3 DC 200–600 V <400 A 240 kW 30 min Off-board CHAdeMO/SAE/DC DC DC Level 2 DC 200 V–500 V < 200 A 100 kW 10 min Off-board SAE/DC 4. EV Charging Connectors CHAdeMO/ DC DC Level 3 DC 200 V–600 V < 400 A 240 kW 30 min Off-board Standardization of EV charging connectors is required to provide safeSAE/DC and efﬁcient charging. Charging connectors follow the standards of SAE, IEC, and IEEE [126–128]. 4.Different EV Charging types ofConnectors standards are deﬁned by different countries, and connectors are not only usedStandardization for power transfer, of EV but charging also for connectors communication is required and sensing. to provide Charging safe and connectors efficient charging.provide the Charging details ofconnectors the electricity follow consumed the standards by the of connectedSAE, IEC, and vehicle. IEEE Different [126–128]. charg- Dif- ferenting connectors types of standards are used forare ACdefined and DCby different charging, countries, also based and on connectors the levels are of chargingnot only usedconnector for power types. transfer, but also for communication and sensing. Charging connectors provideThe the charging details characteristicsof the electricity and consumed requirements by the ofconnected EV-charging vehicle. differ Different based oncharg- the ingtypes connectors of vehicle are and used capacity for AC of the and battery. DC charging, The charging also based range ofon the the battery levels isof calculated charging connectoras the percentage types. of SOC. The range of SOC in 20–30% is taken as low and 80–90% as high value. The charging time of the battery depends on the battery capacity, battery The charging characteristics and requirements of EV-charging differ based on the type, and charging level [86]. The charging strategy has two categories—constant current types of vehicle and capacity of the battery. The charging range of the battery is calculated (CC) charging and constant voltage (CV) charging. The battery can be effectively charged as the percentage of SOC. The range of SOC in 20–30% is taken as low and 80–90% as high with either of these strategies. Sometimes, both strategies are used. Most EVs depend value. The charging time of the battery depends on the battery capacity, battery type, and on lithium-ion battery technology, which uses CC followed by CV charging. During CC charging level [86]. The charging strategy has two categories—constant current (CC) charging, the current is regulated till the cell voltage reaches the threshold level. Then, the charging and constant voltage (CV) charging. The battery can be effectively charged with

Electronics 2021, 10, x FOR PEER REVIEW 22 of 46 Electronics 2021, 10, x FOR PEER REVIEW 22 of 46

Electronics 2021, 10, 1895 21 of 45 either of these strategies. Sometimes, both strategies are used. Most EVs depend on lith- either of these strategies. Sometimes, both strategies are used. Most EVs depend on lithium-ion battery technology, which uses CC followed by CV charging. During CC charg- ium-ion battery technology, which uses CC followed by CV charging. During CC charging, the current is regulated till the cell voltage reaches the threshold level. Then, the ing, the current is regulated till the cell voltage reaches the threshold level. Then, the charging is done with CV charging.charging. The specifications speciﬁcations of di differentfferent EV charging connectors charging is done with CV charging. The specifications of different EV charging connectors are shown in Figure 1919.. The different types of charging connectors used for EV-charging, arebased shown on the in supplyFigure 19. system The anddifferent countries, types followof charging different connectors standards used [130 for,131 EV-charging,]. Different based on the supply system and countries, follow different standards [130,131]. Different basedtypes ofon connectors the supply are system shown and in countries, Figures 19 fo andllow 20 different. standards [130,131]. Different types of connectors are shown in Figures 19 and 20. types of connectors are shown in Figures 19 and 20.

SAE J1772 Mennekes SAE J1772 GB/T Mennekes N L N AC AC L: Line (Line 1) PD N: Neutral PD PE: Earth / Ground PE CP PE: Earth / Ground PE CP: Control Pilot DC- DC+ CP: Control Pilot PD:Proximity Detection CP DC CP PD PE DC+ DC- SAE J1772 CCS GB/T Tesla SAE J1772 CCS GB/T

Figure 19. Specifications of different EV charging connectors. Figure 19. SpeciﬁcationsSpecifications of different EV chargingcharging connectors.connectors.

Type USA JAPAN EU CHINA Type USA JAPAN EU CHINA

Single Single Phase/ Phase/ Three PhaseThree AC Phase AC charging charging SAE J1772 Level 1, level SAE J1772 Level IEC 62196 IEC 62196-2 IEC 62196 SAE J1772 Level 1, level SAE J1772 Level IEC 62196 IEC 62196-2 IEC 62196 2 Single Phase 1, level 2 Single Level 1 Level 2,3 Level 1,2 2 Single Phase 1, levelPhase 2 Single SingleLevel Phase 1 Single/Tree Level 2,3 Phase Single/TreeLevel 1,2 Phase Phase Single Phase Single/Tree Phase Single/Tree Phase

DC Fast ChargingDC Fast Charging AC-DC AC-DC Combo ChargingCombo Level 1 Level 2 JEVS G105 GB/T 20234.3 Charging Level 1 Level 2 JEVS G105 GB/T 20234.3 +DC +DC 1993 CHAdeMO IEC 62196-3 Hybrid 2011 +DC +DC 1993 CHAdeMO IEC 62196-3 Hybrid 2011 SAE J1772Combo DC Combo DC Fast Charging SAE J1772Combo Fast ChargingDC Combo DC Fast Charging Fast Charging (a) (a)

Figure 20. Cont.

Electronics 2021, 10, 1895 22 of 45 Electronics 2021, 10, x FOR PEER REVIEW 23 of 46

Type USA JAPAN EU CHINA

Single Phase/ Three Phase AC charging IEC 62196 SAE J1772 Level 1, level SAE J1772 Level IEC 62196-2 Level 1,2 2 Single Phase 1, level 2 Single Level 2,3 Single/Tree Phase Phase Single/Tree Phase

DC Fast Charging AC-DC Combo Charging Level 1 Tesla GB/T 20234.3 +DC Super CHAdeMO DC IEC 62196-3 Hybrid 2011 SAE J1772 Charger Fast Charging Combo DC Fast Charging

(b)

Figure 20. Types of connectors used in different countries.countries. (a).). Male connectors (b). Female connectors.

5. Power Power Electronic Electronic Converters Converters for for Charging Charging Stations Stations There are are different different types types of ofconverter converter topologi topologieses used used in an in EV an charging EV charging station. station. Con- verterConverter types types include include an AC-AC an AC-AC converter, converter, DC-DC DC-DC converter, converter, switching-mode switching-mode inverters, invert- anders, rectifiers. and rectifiers. More MoreDC-DC DC-DC converters converters are used are in used charging in charging stations, stations, classified classified as unidi- as rectionalunidirectional and bidirectional and bidirectional converters. converters. These Theseconverters converters also work also as work a cascaded as a cascaded buck- boostbuck-boost converter converter and operate and operate with low with electrical low electrical and thermal and thermal stress. Moreover, stress. Moreover, half bridge half convertersbridge converters are used are because used becauseof their high of their efficiency, high efficiency, but their output but their current output is discontin- current is uousdiscontinuous in nature in[132]. nature Bidirectional [132]. Bidirectional DC-DC converters DC-DC converters are mainly are used mainly because used they because pro- videthey regenerative provide regenerative braking in braking EV and in also EV andoffer also V2G offer charging. V2G charging. The performance The performance of the con- of verterthe converter is increased is increased by reducing by reducing the ripples the ripples at the atoutput the output using using a switched-capacitor a switched-capacitor or a combinationor a combination of a switch of a switch and a capacitor. and a capacitor. A comparison A comparison of traditional of traditional converters converters revealed thatrevealed those that with those a minimum with a minimum number of number switches of with switches low voltage with low stress voltage provide stress high provide effi- ciencyhigh efficiency [133]. The [133 different]. The different types of types converte of converterr used for used EV for charging, EV charging, control control scheme scheme and featuresand features presented presented in Table in Tabless 13–15. 13 – 15. Power electronic electronic converters, converters, which which act act as asthe the interface interface between between the thegrid grid and andthe bat- the teries,batteries, have have the potential the potential to act to actively act actively by pr byoviding providing a variety a variety of active of active functions functions or ancil- or laryancillary services services to support to support power power system system operations, operations, such suchas frequency as frequency control, control, voltage voltage control,control, operating operating reserve, reserve, controllable controllable load, load, and andpower power quality quality (PQ) (PQ) improvement. improvement. This Thisis in theis in context the context of the of vehicle-to-grid the vehicle-to-grid (V2G) (V2G) concept. concept. Even without Even without a car connected a car connected for charg- for charging, the AC/DC converter, which includes DC link capacitors, is suitable for use as ing, the AC/DC converter, which includes DC link capacitors, is suitable for use as an an active filter in off-board chargers. active filter in off-board chargers. 5.1. DC-DC Converters 5.1. DC-DC Converters The battery in an EV requires a regulated DC supply to efficiently utilize the generated power.The Different battery in DC-DC an EV converters requires a connectedregulated DC between supply the to RES efficiently and DC utilize bus regulate the gener- the atedoutput power. and Different also provide DC-DC better converters control inconnected all parameters between to the maintain RES and maximum DC bus regulate power thepoint output [134]. and Some also applications provide better with control bidirectional in all parameters DC-DC converters to maintain are maximum used to maintain power pointpower [134]. flow Some in both applications directions. Generally,with bidirectional bidirectional DC-DC converters converters are are classified used to as maintain isolated powerand non-isolated flow in both converters. directions. Generally, In an isolated bidirectional converter, converters input and are output classified are isolated as isolated by andusing non-isolated a transformer converters. [110]. In an isolated converter, input and output are isolated by using a transformer [110].

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The charging module connected between the DC bus and the battery controls all the The charging module connected between the DC bus and the battery controls all charging parameters. The different DC-DC converters investigated are half bridge, inter- the charging parameters. The different DC-DC converters investigated are half bridge, leaved half bridge, and full bridge with isolation [132]. The half bridge topology is the interleaved half bridge, and full bridge with isolation [132]. The half bridge topology is the basicbasic one one with with a a minimum minimum number number of of components, components, providing providing low low switching switching losses. losses. In In the the basicbasic half half bridge bridge converter converter topology, topology, switching switching stresses stresses are are high. high. In In interleaved interleaved converter converter technology,technology, the the ripple ripple generated generated in in the the inductor inductor and and input input current current are are reduced reduced [135 [135].]. TheThe resonant resonant circuit circuit in in the the converter converter reduces reduces the the switching switching stress stress through through zero zero current cur- switchingrent switching (ZCS) (ZCS) and zeroand zero voltage voltage switching switchin (ZVS)g (ZVS) [133 [133].]. In In full full bridge, bridge, ZVS ZVS and and ZCS ZCS usesuses soft soft switchingswitching byby reducingreducing di/dt and and dv/dt dv/dt ratios. ratios. In Infull full bridge bridge topology, topology, inbuilt inbuilt iso- isolationlation transformers transformers are are used used to provide to provide low low switching switching stress stress and and high high efficiency. efﬁciency. The Thebasic basicbidirectional bidirectional converter converter operates operates as a as two-quad a two-quadrantrant charger, charger, because because it itoperates operates both both in inboost boost and and buck buck mode mode [133]. [133]. The The interleaving interleaving and and resonant resonant topology providesprovides lessless ripple.ripple. TheThe conduction conduction losses losses and and ripple ripple current current are are controlled controlled by by the the switches switches in in continuous continuous conductionconduction mode. mode. A A dual dual active active bridge bridge (DAB) (DAB) is used is used for thefor EVthe application EV application due to due its ZVSto its andZVS snubber and snubber capacitor. capacitor. The non-isolated The non-isolated topology topology is of low is of cost low and cost the and isolated the isolated converter con- isverter efﬁcient is efficient in nature. in nature.

5.2.5.2. AC-DC AC-DC Converters Converters TheThe AC-DC AC-DC converter converter injects injects current current harmonics harmonics into theinto grid the andgrid creates and creates a poor powera poor factorpower on factor the grid on the side; grid thus, side; power thus, factor power correction factor correction is required. is required. In general, In thegeneral, AC-DC the converterAC-DC converter used in EV used charging in EV stations charging is operatedstations inis bidirectionaloperated in bidirectional mode. It can alsomode. operate It can asalso a rectifier operate and as a inverting rectifier and mode. inverting mode. TheThe AC-DCAC-DC convertersconverters usedused inin the the following following configurations configurations areare basic basic buck-boost, buck-boost, bridgelessbridgeless pseudo-boost, pseudo-boost, and and SEPIC SEPIC [136 [136].]. The The basic basic topology topology uses uses the the diode, diode, which which leads leads toto high high losses losses across across the the diode. diode. The The bridgeless bridgeless pseudo-boost pseudo-boost and and SEPIC SEPIC are are the the modified modified formsforms of of the the converter. converter. Due Due to to electromagnetic electromagnetic emission emission and and high high inrush inrush current, current, SEPIC SEPIC convertersconverters are are used.used. In addition, single single phase phase and and three three phase phase converters converters are are also also used used for forEV-charging. EV-charging. However, However, in inmost most high high power power charging charging stations, stations, three-phase three-phase full full bridge bridge to- topologypology is is used used due due to to its its fewer fewer switching switching losses. losses. In Inaddition, addition, multi-level multi-level topology topology was was also alsoinvestigated investigated due due to its to lower its lower voltage voltage stress stress and and switching switching losses losses [137]. [137 The]. The three-level three-level fly- flyinging capacitor capacitor multilevel multilevel inverter inverter was was analyzed analyzed and found to provideprovide fixedfixedvoltage voltage stress stress andand less less DC DC link link voltage. voltage. Even Even though though it is it of is complex of complex topology, topology, it provides it provides better better efficiency. effi- Theciency. charging The charging infrastructure infrastructure in India isin shown India is in shown Figure in 21 Figure[138–140 21]. [138–140].

EV charging in Mumbai EV charging in Hyderabad

EV charging in Delhi EV charging in Chennai FigureFigure 21. 21.Charging Charging infrastructure infrastructure in in India. India.

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There are many factors influencing the selection of converter topology, such as power quality level (permitted level of power factor and THD), type of output (variable or constant), power flow direction (unidirectional or bidirectional), and nature of output (isolated and non-isolated), as shown in Figure 22. Other factors include size, cost, efficiency, Electronics 2021, 10, x FOR PEERreliability, REVIEW and number of input and output. The main factors considered in the selection25 of 46 are magnetic components and control methodology. Electronics 2021, 10, x FOR PEER REVIEW 25 of 46 Electronics 2021, 10, x FOR PEER REVIEW 25 of 46

Table 13. Grid side AC-DC converters. Table 13. Grid side AC-DC converters. Reference ConverterTable 13. Grid side Control AC-DC converters. Feature Reference Converter Table 13. Grid Control side AC-DC converters. Feature Reference Converter Control Feature Reference Converter C Control Feature Vd1 • Suitable• Suitable for high for power high power rating rating and andcan support C can support several charging points. A C IVd3d1 • several charging points. Vd1 • Suitable• for high power rating and can support B d1 •• SuitableIn for the high step-down power condition, rating and it can can support [135,141,1 A Id3 Voltage Ori- In the step-down condition, it can operate un- CA Id3 Voltage Oriented operateseveral under charging balanced points. mode and [135,141,142] BA C Id3 several charging points. 42] B ented Control •der balancedalso mode support and storage also support units. storage units. [135,141,1 CB VoltageControl Ori- • In the step-down condition, it can operate un- [135,141,1 C Vd2 Voltage Ori- • In the step-down condition, it can operate un- [135,141,1 C C Voltage Ori- • InIt• canthe Itstep-downsupport can support a PEV condition, a PEV uninterrupted uninterrupted it can operate charging un- 42] C Id2 ented Control der balanced mode and also support storage units. 42] Vd2 ented Control der balancedcharging mode facilityand also through support the storage units. 42] Vd2 ented Control der balanced mode and also support storage units. Vd2 • Itfacility can support through a PEV the balancinguninterrupted circuit. charging Id2 • It canbalancing support a circuit. PEV uninterrupted charging Balancing circuit Id2 • It can support a PEV uninterrupted charging Id2 facility through the balancing circuit. Three-Level Three-PhaseBalancing circuit NPC facility through the balancing circuit. Balancing circuit Converter • This can provide bidirectional power flow + • from and to the utility grid. + • This can provide bidirectional power flow + • • •ThisThis can canprovide provide bidirectional bidirectional power power flow A + It reducesfrom the and filter to thesize utility and stress grid. on the active [137,143] Vdc Sliding mode flowfrom from and and to the to the utility utility grid. grid. B • from and devicesto the utility grid. CA • It reduces• It reduces the filter the filtersize and size andstress stress on onthe active A Vdc control • It reduces the filter size and stress on the active [137,143] B Vdc Sliding mode • It reduces the filter size and stress on the active [137[137,143],143] B Vdc Sliding mode mode control The converterthe active boost devicesdevices inductor and high-power CB control • devices C - control • stress areThe controlled converter through boost inductor control and schemes. control • The converter boost inductor and high-power - The converterhigh-power boost stress inductor are controlled and high-power - stress arethrough controlled control through schemes. control schemes. stress are controlled through control schemes. Three-phase bridgeless boost converter D1 D2 C S 1 • This converter reduces THD from AC input 2 • This converter reduces THD from AC AC D1 D2 • D1 D2 C1 Vo • Minimize the ripple from the regulated DC D1 D2 S2 C1 • This converterinput reduces THD from AC input S2 C1 PWM based • This converter reduces THD from AC input [144] S2 • • Minimize theoutputs ripple from the AC S1 • Minimize the ripple from the regulated DC AC C2 Vo control • Minimize the ripple from the regulated DC AC Vo PWM based • StressMinimizeregulated on the the power DCripple outputs components from the regulated is reduced. DC [144] D3 D4 PWM based outputs [144[144]] S1 PWM based control outputs [144] S1 C2 control • • Stress on theoutputs power components is S1 C2 control • Low switching losses and high efficiency. D C2 control • Stress on the power components is reduced. D3 D4 • Stressreduced. on the power components is reduced. D3 D4 Stress on the power components is reduced. 3 D4 . • •Low switching losses and high efficiency. • LowLow switching switching losses losses and and high high efficiency. . • Swingefficiency. voltage is further reduced, and a diode UnidirectionalD1 multilevel.D 2 converter. • Swing voltage isis further saved. reduced, and a diode AC D1 D2 • Swing voltage is further reduced, and a diode D1 D2 C • Swing voltage is further reduced, and a diode D1 D2 1 Vo PWM based The condition of neglecting the power loss of [145] • Swing voltageisis saved.saved. is further reduced, and AC is saved. AC C1 Vo PWMcontrol based • The conditioncontrol and of neglecting driver circuits. the power loss of C1 Vo • a diode is saved. [145] C1 Vo PWM based • The condition of neglecting the power loss of [145] PWM based • TheThe• highestconditionThe condition efficiency of neglecting of neglecting of the theproposed the power power systemloss of [145] control control and driver circuits. [145] PWMcontrol based control losscontrol of control and driver and driver circuits. circuits. • The highest efficiencywas 96.76%. of the proposed system • The• highestThe highest efficiency efficiency of the of theproposed proposed system was 96.76%. system waswas 96.76%. 96.76%. Symmetrical two-device unidirectionalTable 14. PV side DC-DC converters. was 96.76%. boost converter Table 14. PV side DC-DC converters. Ref. Converter Table 14. Control PV side DC-DC converters. Feature Ref. Converter Control • The control MPPT operated Feature on the basis of the volt- Ref. Converter Control Feature L1 • ageThe oncontrol the PV MPPT output operated and weather on the conditions.basis of the volt- C1 Maximum • The control MPPT operated on the basis of the volt- PV 1 • TheThis control cuk converter MPPT operated operated on under the basis two ofloop the con- volt- L1 age on the PV output and weather conditions. L1 C1 Maximumpower age on the PV output and weather conditions. [146] C L1 C1 PV 1 Maximum • trols such as voltage and current control. C1 PV 1 Maximum • This cuk converter operated under two loop con- pointpower track- • This cuk converter operated under two loop con- [146] C C2 PV 2 power Thetrols control such asmethodology voltage and cancurrent track control. closely to the [146] C L2 ing (MPPT) trols such as voltage and current control. [146] point track- DWSSA• trolspredicted such MPPas voltage voltages and with current accuracy control. and mini- C2 PV 2 point track- • The control methodology can track closely to the L2 C2 PV 2 • The control methodology can track closely to the L2 C2 ing (MPPT) The control methodology can track closely to the L2 ing (MPPT) DWSSA predicted MPPmum voltages oscillation with accuracy and mini- DWSSA predicted MPP voltages with accuracy and mini-

L2S L1S mum oscillation mum oscillation L • A DC-DC converter was developed from a two- 1P L2S L1S L2S L1S L2S L1S phase• IBC consisting of one CI in each phase, a voltage lift L1P • A DC-DC converter was developed from a two- L1P CM Vo • A DC-DC converter was developed from a two- L1P Voltage lift phasecapacitor IBC consistingClift, and one of one VMC CI networkin each phase, connected a voltage across lift [147] CM phase IBC consisting of one CI in each phase, a voltage lift L2P CM Vo technique the secondary windings. L CM Vo Voltage lift capacitor Clift, and one VMC network connected across [147] 2 Voltage lift • capacitor Clift, and one VMC network connected across [147] L2P technique The converterthe confirmedsecondary thewindings. delivered power of 225 L2P technique the secondary windings. L2P L2 technique the secondary windings. L2 • TheW converterto the load confirmed at 91.6% fullthe loaddelivered efficiency. power of 225 2 • The converter confirmed the delivered power of 225 W to the load at 91.6% full load efficiency. W to the load at 91.6% full load efficiency.

Electronics 2021, 10, x FOR PEER REVIEW 25 of 46

Table 13. Grid side AC-DC converters.

Reference Converter Table 13. Grid Control side AC-DC converters. Feature Reference Converter Control Feature C Vd1 • Suitable for high power rating and can support C A VId3d1 • Suitable forseveral high charging power rating points. and can support B • [135,141,1 AC Id3 Voltage Ori- In the step-downseveral charging condition, points. it can operate un- 42] B C ented Control •der balanced mode and also support storage units. [135,141,1 C V Voltage Ori- In the step-down condition, it can operate un- C d2 • 42] Id2 ented Control der balancedIt can support mode and a PEV also uninterrupted support storage charging units. Vd2 • Itfacility can support through a PEV the balancinguninterrupted circuit. charging Balancing circuit Id2 facility through the balancing circuit.

Balancing circuit • + This can provide bidirectional power flow • from and to the utility grid. + This can provide bidirectional power flow • A It reducesfrom the and filter to thesize utility and stress grid. on the active [137,143] Vdc Sliding mode B • devices AC It reduces the filter size and stress on the active [137,143] Vdc Slidingcontrol mode • B The converter boostdevices inductor and high-power C - control • stressThe are converter controlled boost through inductor control and high-powerschemes. - stress are controlled through control schemes.

D1 D2 C1 • S2 This converter reduces THD from AC input AC D1 D2 • C1 Vo • ThisMinimize converter the ripple reduces from THD the from regulated AC input DC S2 PWM based [144] • outputs AC S1 Minimize the ripple from the regulated DC C2 Vo control PWM based • Stress on the power components is reduced. [144] D3 D4 outputs S1 C2 control • • Low switching losses and high efficiency. D3 D Stress on the power components is reduced. 4 . • Low switching losses and high efficiency. . • Swing voltage is further reduced, and a diode D1 D2 • Swing voltage isis further saved. reduced, and a diode AC D1 D2 C • 1 Vo PWM based The condition ofis neglectingsaved. the power loss of [145] AC C1 Vo PWMcontrol based • The conditioncontrol andof neglecting driver circuits. the power loss of Electronics[145]2021 , 10, 1895 • The highest efficiency of the proposed25 system of 45 control control and driver circuits. • The highest efficiencywas 96.76%. of the proposed system

was 96.76%. TableTable 14. 14.PV PV side side DC-DC DC-DC converters. converters.

Ref.Ref. Converter Table 14. Control PV side Control DC-DC converters. Feature Feature Ref. Converter Control • The control• MPPTThe control operated Feature MPPT on operated the basis on theof the basis volt- of the voltage on the PV output and L1 • Theage oncontrol the PV MPPT output operated and weather on the conditions.basis of the volt- C1 Maximum weather conditions. PV 1 • This cuk converter operated under two loop con- L1 power age on the• PVThis output cuk converter and weather operated conditions. under two C C1 Maximum [146] PV 1 Maximum power• pointThistrols cuk such converter loopas voltage controls operated and such current asunder voltage control. two and loop con- [146] pointpower track- • [146] C C2 PV 2 tracking (MPPT) Thetrols control such currentas methodology voltage control. and cancurrent track control. closely to the L2 pointing (MPPT) track- • The control methodology can track C PV 2 DWSSA• The predicted control MPPmethodology voltages canwith track accuracy closely and to mini- the L 2 closely to the DWSSA predicted MPP Bidirectional2 ing (MPPT) DWSSA predicted voltagesMPPmum voltages withoscillation accuracy with accuracy and minimum and mini- PV-Cuk converter oscillation L2S L1S mum oscillation L • A DC-DC converter was developed from a two- 1P L2S L1S • A DC-DC converter was developed phase• IBCA DC-DCconsisting converter of one CIwas in developed each phase, from a voltage a two- lift L1P CM Vo from a two-phase IBC consisting of one Voltage lift phasecapacitor IBC consistingClift, and oneof one VMC CI innetwork each phase, connected a voltage across lift [147] CM CI in each phase, a voltage lift capacitor L2P Vo technique the secondary windings. L Voltage lift capacitor Clift, andClift, one and VMC one network VMC network connected connected across [147[147]] 2 Voltage lift• technique L2P technique The convertertheacross confirmedsecondary the secondary windings.the delivered windings. power of 225 ElectronicsElectronics 2021 2021, ,10 10, ,x x FOR FOR PEER PEERL2 REVIEW REVIEW • The converter conﬁrmed the delivered2626 ofof 4646 • TheW converter to the load confirmed at 91.6% fullthe deliveredload efficiency. power of 225 power of 225 W to the load at 91.6% full W to the loadload at efﬁciency. 91.6% full load efficiency. Interleaved high gain DC-DC converter •• TheThe SC-based SC-based• The converters converters SC-based attain convertersattain a a creditable creditable attain a position position forfor standalone standalone PV PV creditablesystems systems op positionoperatingerating for at at standalone a a higher higher voltage PVvoltage CC11 CC1212 11 CC1n1n DD1n1n level,level, above above 300 300 V, V,systems in in comparison comparison operating to to at the athe higher conventional conventional voltage B- B- DD1111 DD1212 level, aboveBBCs.BBCs. 300 V, in comparison to the conventional B-BBCs. VoltageVoltage •• TheThe structure structure allows allows increasing increasing the the SC SC cells cells in in order order [113][113] • The structure allows increasing the SC [113] controlcontrolVoltage to controlto obtain obtain the the reduced reducedcells voltage voltage in order stress stress to obtain on on switching theswitching reduced compo- compo- DD2121 DD2222 DD2m2m CC2121 CC2222 CC2m2m nentsnents with voltagewith a a higher higher stress on voltage voltage switching gain. gain. components •• with a higher voltage gain. ItIt allows allows the the converter converter to to achieve achieve a a higher higher gain, gain, • It allows the converter to achieve a whichwhich is is otherwise otherwise not not possible possible in in conventional conventional convert- convert- Transformer less high gain boost and higher gain, which is otherwise not buck-boost DC-DC converters possible iners.ers. conventional converters.

LL1 1 ●● ProvideProvide higher higher output output voltage. voltage. CC11 • Provide higher output voltage. ●● OutputOutput •current current is is continuous continuous and and minimizing minimizing the the CC0 MPPT and Output current is continuous and 0 VoVo MPPT and [148][148] LL22 switchingswitching stress stressminimizing also also reduce reduce thess switchingthe the ripple ripple stress in in output. output. also [148] FuzzyFuzzyMPPT and Fuzzy VVDCDC ●● TheThe converter converterreduces topology topology the used rippleused to into attain output.attain the the maximum maximum • The converterpower.power. topology used to attain the maximum power. Single-Ended Primary Inductance Converter (SEPIC) TableTable 15. 15. Charging Charging side side DC-DC DC-DC converters. converters.

ChargingCharging side side DC-DC DC-DC converters converters RefRef Converter Converter Control Control Feature Feature

L1L1 ●● TheThe converter converter was was able able to to quickly quickly scale scale the the system system on on

L2L2 Discrete-Discrete- interleavedinterleaved topology topology employing employing a a higher higher number number of of legs legs ++ TimeTime stackedstacked together. together. [149,1[149,1 L3L3 CC1 1 ModelModel ●● TheThe load load resistance resistance (representing (representing the the battery) battery) varied varied 50]50] VoVo VdcVdc CC2 2 andand Direct Direct during during charging; charging; future future work work might might investigate investigate the the possibil- possibil- - - ControlControl ityity of of improving improving the the versatilit versatilityy of of the the proposed proposed procedure procedure byby employing employing a a real-time real-time meas measure/estimationure/estimation of of the the load load

●● Fast-chargingFast-charging converter converter uses uses soft soft switching switching character- character- isticsistics in in the the ZVS ZVS region region PFMPFM ●● TheThe EV EV system system has has a a fast-charging fast-charging scheme scheme with with an an LIB LIB CrC r (Pulse(Pulse Fre- Fre- ofof 800 800 Wh Wh and and SC SC of of 50 50 Wh. Wh. C1C1 C2C2 quencyquency ●● TheThe charging charging time time through through the the fast-charging fast-charging SC SC sys- sys- [151][151] VdcVdc VoVo Modula-Modula- temtem has has a a charging charging time time of of about about 20 20 min. min. tion)tion) and and ●● TheThe transient transient period period during during the the conversion conversion from from the the

PIPI control. control. CCCC mode mode to to the the CV CV mode. mode. ●● TheThe maximum maximum conversion conversion efficiency efficiency of of the the output output cur- cur- rentrent of of 30 30 A A in in the the CC CC mode mode is is 96.4%. 96.4%. ●● TheThe converter converter reduces reduces the the charging charging time time and and allows allows forfor wide wide LS-EVs LS-EVs adoption. adoption. ●● TheThe multi-module multi-module DC-DC DC-DC converters converters for for LS-EVs LS-EVs fast fast CurrentCurrent chargerschargers are are used used due due to to the the fact fact that that modular modular power power con- con- C1C1 C2C2 [152][152] VdcVdc VoVo sharingsharing vertersverters offer offer easy easy maintenance, maintenance, redundancy, redundancy, and and scalability. scalability. controlcontrol ●● TheThe higher higher switching switching frequency frequency can can be be achieved achieved in in the the ACAC link, link, which which results results in in weight weight and and size size reduction. reduction. ●● TheThe employment employment of of soft soft swit switchingching techniques; techniques; losses losses of of

thethe converter converter are are reduced reduced

Electronics 2021, 10, x FOR PEER REVIEW 26 of 46

Electronics 2021, 10, x FOR PEER REVIEW 26 of 46 • The SC-based converters attain a creditable position for standalone PV systems operating at a higher voltage C C12 11 C1n D1n level, above 300 V, in comparison to the conventional B- D11 D12 • Electronics 2021, 10, x FOR PEER REVIEW The SC-based converters attain a creditable position26 of 46 BBCs. •for standalone PV systems operating at a higher voltage C C12 Voltage The structure allows increasing the SC cells in order 11 C1n [113] D1n level, above 300 V, in comparison to the conventional B- D11 D12 control to obtain the reduced voltage stress on switching compo- D21 D22 D2m BBCs. C21 • nents with a higher voltage gain. C22 C2m • The SC-based converters attain a creditable position Voltage • The structure allows increasing the SC cells in order [113] for standaloneIt allows PV the systems converter op eratingto achieve at a a higher higher voltage gain, C C12 control to obtain the reduced voltage stress on switching compo- 11 D CD1n which is otherwise not possible in conventional convert- D21 D 22 D 2m D1n level, above 300 V, in comparison to the conventional B- C2111 12 nents with a higher voltage gain. C22 C2m BBCs.ers. • It allows the converter to achieve a higher gain, Voltage • The structure allows increasing the SC cells in order [113] which is otherwise not possible in conventional convert- L1 control to obtain the● reducedProvide voltage higher stress output on voltage. switching compo- D21 D22C1 D2m ers. C21 C22 C2m ● Outputnents current with is a continuous higher voltage and gain.minimizing the C 0 Vo MPPT and • It allows the converter to achieve a higher gain, [148] L2 switching stress also reduces the ripple in output. Electronics 2021, 10, 1895 L1 Fuzzy ● Provide higher output voltage.26 of 45 VDC C1 ●which The is otherwiseconverter topologynot possible used in toconventional attain the maximum convert- ● Output current is continuousers. and minimizing the C0 MPPT and power. [148] L Vo switching stress also reduces the ripple in output. 2 Fuzzy VDC Table 15. Charging side DC-DC● converters.The converter topology used to attain the maximum L1 ● Provide higher output voltage. C1 power. ChargingTable 15. Side Charging DC-DC side Converters DC-DC● Output converters. current is continuous and minimizing the C0 MPPT and [148] L Vo switching stress also reduces the ripple in output. Ref Converter2 ChargingFuzzy side ControlDC-DC converters Feature VDC ● The converter topology used to attain the maximum Ref Converter Table 15. ControlCharging side DC-DC converters.• The Feature converter was able to quickly power. L1 scale the system on interleaved Charging side DC-DC● The converters converter wastopology able to employingquickly scale a higher the system on

Ref ConverterL2 Discrete- Control interleaved topology employingnumber Feature of legs a higher stacked number together. of legs • The load resistance (representing L1 +Table 15. ChargingTime side DC-DC converters. stacked together. [149,1 L Discrete-Time● ModelThe andconverter wasthe able battery) to quickly varied duringscale the system on [149,150] C 3 Model ● The load resistance (representing the battery) varied 1 Direct Control charging; future work might 50] L2 Vo ChargingDiscrete- side DC-DCinterleaved converters topology employing a higher number of legs Vdc C2 and Direct during charging; futureinvestigate work might the investigate possibility of the possibil- Ref Converter + ControlTime stacked Feature together. [149,1 - Control ity of improving the versatilitimprovingy of the the versatility proposed of theprocedure L1 L3 C1 Model ● The load resistanceproposed (representing procedure the by battery) varied 50] Vo ●by employingThe converter a real-time was able meas to ure/estimationquickly scale the of systemthe load on Vdc C2 and Direct during charging; futureemploying work might a real-time investigate the possibil- Multiple Interleaved Buck Converters Discrete- interleaved topology employing a higher number of legs L2 ● Fast-charging convertermeasure/estimation uses soft switching of the load character- - Control ity of improving the versatility of the proposed procedure + Time stacked together. [149,1 by employing a real-timeistics• Fast-charging in the meas ZVSure/estimation region converter uses of soft the load L3 Model ● The load resistance (representing the battery) varied 50] C1 PFM ● The EV system hasswitching a fast-charging characteristics scheme in thewith an LIB C Vo Vdc Cr 2 and Direct during● Fast-charging charging; future converter work mightuses soft investigate switching the character- possibil- (Pulse Fre- of 800 WhZVS and region SC of 50 Wh. Control ity of improving theistics• versatilitThe in the EV systemZVSy of theregion has proposed a fast-charging procedure C1 C2 - quency ● The charging time through the fast-charging SC sys- [151] Vdc Vo PFM ● by employingThe EV system a real-time hasscheme a fast-charging meas withure/estimation an LIB scheme of 800 Whof with the anload LIB C Modula- tem has a charging time of about 20 min. r (Pulse Fre- of 800 Whand and SC of SC 50 of Wh. 50 Wh. tion) and ●● Fast-chargingThe transient• periodconverterThe chargingduring uses thesoft time conversion switching through the character-from the C1 C2 quencyPFM (Pulse ● FrequencyThe charging time through the fast-charging SC sys- [151[151]] Vdc Vo PI control. CCistics modefast-charging in tothe the ZVS CV region SC mode. system has a Modula-Modulation) and PI control.tem has a charging time of about 20 min. PFM ●● TheThe maximumEV system conversion hascharging a fast-charging timeefficiency of about scheme of 20 the min. outputwith an cur- LIB Cr tion) and ● The transient• periodThe transientduring the period conversion during the from the (Pulse Fre- rent ofof 30800 A Wh in the and CC SC mode of 50 isWh. 96.4%. PI control. CC modeconversion to the CV from mode. the CC mode to Full-BridgeC1 LLC Resonant ConverterC2 quency ● The convertercharging timereducesthe through CV the mode. charging the fast-charging time and SCallows sys- [151] Vdc Vo ● The maximum conversion efficiency of the output cur- Modula- tem hasfor a• chargingwideThe LS-EVs maximum time adoption. of conversionabout 20 min. rent of 30 A in the CC mode is 96.4%. tion) and ●● TheThe multi-module transient periodefﬁciency DC-DC during of converters the the output conversion for current LS-EVs from of fastthe ● The converter reduces30 A in the the CCcharging mode istime 96.4%. and allows PICurrent control. chargers are usedCC due mode to the to factthe CVthat mode. modular power con- C1 C2 for wide LS-EVs adoption. [152] Vdc Vo sharing verters● The offer maximum easy maintenance,• conversionThe converter redundancy, efficiency reduces of the andthe outputscalability. cur- ● The multi-modulecharging DC-DC time converters and allows for for LS-EVs wide fast control ● The higherrent of switching 30 A in the frequency CC mode can is be96.4%. achieved in the Current chargers are used due LS-EVsto the fact adoption. that modular power con- C1 C2 ● ACThe link, converter which• results reducesThe in multi-module weight the charging and DC-DC size time reduction. and allows [152] Vo sharing verters offer easy maintenance, redundancy, and scalability. Vdc ● The employmentfor wide ofconverters soft LS-EVs swit foradoption.ching LS-EVs techniques; fast losses of control ● The higher switching frequency can be achieved in the ● The multi-modulethe converterchargers DC-DC are are converters reduced used due to for the LS-EVs fact fast AC link, which resultsthat in modular weight power and size converters reduction. Current chargers are used due to the fact that modular power con- C1 C2 ● The employment ofoffer soft easy swit maintenance,ching techniques; losses of [152[152]] Vdc Vo sharingCurrent sharingverters control offer easy maintenance, redundancy, and scalability. the converterredundancy, are reduced and scalability. control ● The higher switching• The higherfrequency switching can be frequency achieved in the AC link, which resultscan bein achievedweight and in the size AC reduction. link, ● The employment whichof soft results switching in weight techniques; and size losses of Dual Active Bridge reduction. Electronics 2021, 10, x FOR PEER REVIEW the converter are reduced 27 of 46 • The employment of soft switching techniques; losses of the converter are reduced • The extension of drive FCs to the e PWM ● The extension ofadditional drive FCs battery to the charging additional battery modula- functionality of EVs and mobile C1 charging functionality of EVs and mobile electric work mation factor electric work machines allows the [153] C1 Vo e PWM modulationchines allows factor the distribution of battery charging stations. [153] Vdc value and distribution of battery charging C2 value and thereby● Existing controls converterstations. electric drives can be adapted for thereby • batteryExisting charging. converter electric drives controls can be adapted for battery DC/AC/DC Converter charging.

L L1 1 ● Here, the current-sharing mechanism is possible L2 En- ● Losses are very low when the current sharing condi- [154] Vdc C C1 Vo hancedI2 tion is achieved. control ● The series-capacitor voltage is used to achieve current sharing between phases without a current sensing circuit

There are many factors influencing the selection of converter topology, such as power quality level (permitted level of power factor and THD), type of output (variable or constant), power flow direction (unidirectional or bidirectional), and nature of output (isolated and non-isolated), as shown in Figure 22. Other factors include size, cost, efficiency, reliability, and number of input and output. The main factors considered in the selection are magnetic components and control methodology.

Electronics 2021, 10, x FOR PEER REVIEW 27 of 46

Electronics 2021, 10, 1895 e PWM 27 of 45 ● The extension of drive FCs to the additional battery modula- C1 charging functionality of EVs and mobile electric work mation factor C1 Vo [153] Vdc chines allows the distribution of battery charging stations. Tablevalue 15.andCont. C2 ● Existing converter electric drives can be adapted for thereby battery charging. Charging Sidecontrols DC-DC Converters

Ref Converter Control Feature • Here, the current-sharing L L1 1 ● Here, the current-sharingmechanism is mechanism possible is possible L2 En- ● Losses are very• lowLosses when are verythe current low when sharing the condi- [154] Vdc C C1 Vo hancedI2 tioncurrent is achieved. sharing condition is [154] controlEnhancedI2 ● controlThe series-capacitorachieved. voltage is used to achieve current • The series-capacitor voltage is sharing between phases without a current sensing circuit used to achieve current sharing Electronics 2021, 10, xTwo-Phase FOR PEER REVIEW Series-Capacitor DC-DC between phases without a current28 of 46 Buck Converter sensing circuit There are many factors influencing the selection of converter topology, such as power quality level (permitted level of power factor and THD), type of output (variable or con- Termsstant), power flow directionVariable (unidirectional or bidirectional),Aspects and nature of output (isolated and non-isolated), as shown in Figure 22. Other factors include size, cost, efficiency, Total Capacity and Number of Vehicles reliability, and number of input and output. The main Connectedfactors considered in the selection are magnetic components and control methodology. Harmonic Mitigation and Devices Reactive Power STATCOM, Capacitor and DVR

Reactance , Series and Line Impedance

Grounding High and Low Resistance Grounding AC-DC Fault Regulation Switches and resistances

220 V, 380 V, 440 V, 3 kV and 10 kV

DC 48 V, 375 V, 750 V and 20 kV Voltage Grid Integration Power Frequency and High Frequency

AC/DC load, Location, Power Quality and Generation, Storage, Load Reliability

DC Microrid RES Integration, DC Load Ratio

Unified/ Decentralized

Star/ Ring/ Radial Network Design AC/DC, DC/AC

AC-DC/AC, DC-AC/DC

Load Demand and Power Transfer Margins Location Converter Role Generation/Load/Group Type CSI/VSI, DC/ Electronics Tranformer

Figure 22. Typical MG system planning requirements.

6. Control Strategy of Charging Stations A charging station’s microgrid voltage is regulated for effective utilization of charge. The optimization algorithm and nonlinear disturbance observer (NDO)-based control provides better voltage regulation along with its filter circuit. This section discusses the various control techniques investigated in the EV charging station control.

6.1. Rule-based Control Supervisory control improves charging station performance through system control based on engineering knowledge, mathematical model, and predefined power required by the charging port [155]. This rule-based control is operated with a deterministic or fuzzy based method. In the deterministic method, a lookup table-based control is used for the output control; it is not based on real-time data. In addition, it is operated in an on/off- based control strategy. The on/off control strategy is easy and simple; it is based on charging or discharging. Input sources are turned on/off based on the battery parameter SOC and voltage.

Electronics 2021, 10, 1895 28 of 45

6. Control Strategy of Charging Stations A charging station’s microgrid voltage is regulated for effective utilization of charge. The optimization algorithm and nonlinear disturbance observer (NDO)-based control provides better voltage regulation along with its ﬁlter circuit. This section discusses the various control techniques investigated in the EV charging station control.

6.1. Rule-Based Control Supervisory control improves charging station performance through system control based on engineering knowledge, mathematical model, and predeﬁned power required by the charging port [155]. This rule-based control is operated with a deterministic or fuzzy based method. In the deterministic method, a lookup table-based control is used for the output control; it is not based on real-time data. In addition, it is operated in an on/off-based control strategy. The on/off control strategy is easy and simple; it is based on charging or discharging. Input sources are turned on/off based on the battery parameter SOC and voltage.

6.2. Fuzzy Logic Control of Charging Stations The charging station requires an energy management strategy to control power ﬂow; fuzzy-based optimization provides a better solution for vehicle control [156]. The fuzzy- based priority charging or discharging ESU at the charging station is also presented. Peng and Jessy proposed a simulation study for parallel hybrid electric vehicles; it suggests that the fuzzy logic increases fuel economy and fast charging of EV [157]. Better energy management in hybrid vehicles with hybrid energy sources like ultra-capacitor, fuel cells, and battery storage is done through the fuzzy logic system. In addition, split energy management includes a hybrid electric vehicle and a controller used as an auto boxed SPACE platform. A comparison between conventional controllers revealed that a fuzzy logic controller is a suitable tool to control the power converter at the charging station. The energy management strategy is based on decentralized microgrid voltage control using the fuzzy logic technique [158]. In addition, charging and discharging priority of hybrid source-connected EVs are controlled through fuzzy-based control. Decentralized fuzzy control is used with a coordinated operation between DC bus voltage, power ﬂow, and energy storage device SOC. Based on charging station architecture, power rating of EV, renewable energy sources, and different types of isolation, bidirectional or unidirectional converter comparisons are shown in Table 16.

Table 16. Charging Station Control.

Charging Station Control Power Converter Topologies Used and Ref. System Architecture Strategy Control Methodology Rule-based control using power A dedicated DC-DC converter is used. Isolated DC microgrid for balancing control, optimal power Differential evaluation based on SOC [159,160] EV-charging self-scheduling of power predicted that charging minimizes total balancing control energy cost PV-based battery charging stations considering service Dual stage-controlled DC-DC converter Grid-connected RES powered [86,161,162] availability, DC link power along with the optimization algorithm DC microgrid for EV-charging predictive model (PPM) for for charging charging station control Dedicated DC-DC converter for PHEVs RES-powered DC microgrid [163] DC link voltage-based control charging; it uses minimum energy from with ESU the utility grid Charging of EV with AC-DC AC bus-coupled charging Control area network (CAN) [164,165] bidirectional converters; it works in both station architecture -based charging station control inverter and rectifying mode Electronics 2021, 10, 1895 29 of 45

Table 16. Cont.

Charging Station Control Power Converter Topologies Used and Ref. System Architecture Strategy Control Methodology SOC-based charging of EV is done based Power oscillation damping on DC link voltage and is controlled control implemented for both through a DC-DC converter. Charging [166,167] Hybrid AC-DC Microgrid ESU and other devices connected and discharging of ESU operates inner to the microgrid current loop control outer voltage/frequency control DC bus voltage is maintained Two loop control of battery charging is Multiport converter-based DC [168–170] through the PI and fuzzy followed as constant current and constant micro grid controller. voltage control Dual active bridge-based, Multiport converter-based AC Dual output port controlled for charging [171] control-based current sharing micro grid based on the realized DC voltage error compensation

The design of a charging station with minimum conversion losses between the sources to load provides high efﬁciency. Charging cords are isolated from the power sources due to safety reasons. Charging stations with ESU provide uninterrupted supply for charging. A bidirectional converter on the vehicle side provides an option for vehicle-to-anything charging. PV power generation is generally intermittent in nature, which tends to the variation of microgrid voltages. The regulation of microgrid voltage is done by adding available power sources to the DC bus. Direct connection of the battery to the DC microgrid creates ﬂuctuations in DC bus voltage and is controlled by the DC-DC converter. When a large- scale signal is applied, the proportional-integral controller is unable to predict the load voltage and increases the rise time and reduces oscillation. A fuzzy logic controller is thus a suitable tool to control the power converter at the charging station. Moreover, it provides coordinated operation between the DC bus voltage and SOC ESU.

7. Energy Management in DC Microgrid-Based Charging Stations DC microgrid-based EV charging stations reduce conversion losses in recent power systems. A microgrid with RES provides effective reduction in emissions; effective utilization is done through the EMS. The development of charging stations with multiport charging terminals creates overloading in the microgrid and utility grid. In addition, multiport charging stations address the following technical issues: Development of an energy management strategy to control the power ﬂow among available sources and charging terminals for effective utilization of the generated renewable power. In a PV-powered charging station, the power produced by the PV is variable in nature and it creates a DC bus voltage variation in the microgrid, irrespective of overloading and irradiation on the PV. The charging port with the closed loop control provides constant current and constant voltage charging, and reduces charging time. The main motive of the EMS and its control is to maximize the usage of RES consumption. In addition, the energy stored in ESU, when excess power is produced by RES or EV charging ports, is idle.

7.1. Microgrid-Connected EV Charging Stations The EV-charging station’s architecture in microgrids is shown in Figure 23. The charging station is connected to a PV system by an MPPT-controlled DC-DC converter [157]. Three charging ports in the station are connected via a bidirectional DC-DC converter. In addition, utility grids are connected to the charging stations by a distribution transformer Electronics 2021, 10, 1895 30 of 45

AC-DC bidirectional converter. EV charging stations and local loads are connected with secondary distribution transformers. BMS in the charging station is used to disconnect the vehicle during abnormal conditions. It can increase the safety of the system by monitoring and controlling effective utilization of power input. In addition, BMS predicts the state of health, providing safe and quality operation and performance [172]. Furthermore, IC is Electronics 2021, 10, x FOR PEER REVIEW 31 of 46 designed to be operated by an external BMS control unit via a serial peripheral interface (SPI), through which acquired data can also be retrieved [173].

Start

V REF-1 ,VREF-2,VREF-3,PPV,PCS,SOCESU,SOCPHEV 1-3,PBEV1-3 and IDTR

Terminate BEV Yes Yes SOC Limit Yes BEVs PPV= 0 IDTR > IDTR- MAX No Minimum=25% Present Maximum=90% No No No No SOCESU ≥ SOCBEV ≥ 50% 40% A B No Yes Yes MODE 11 Yes VDC Link < VREF-1 PV gives an output to charge one EV BEV to BEV

Yes Yes SOCBEV ≤ 90% IDTR ≥ IDTR- MAX VREF-1 ≤ VDC Link ≤ VREF-2 PV gives an output No MODE 4 to charge All EV MODE 1 Yes No No Bidirectional AC/DC MODE 10 Terminate BEV No Converter open

VREF-2 ≤ VDC Link ≤ VREF-3 ESU to BEV PV gives Yes No an output Yes IDTR < IDTR- MAX Yes to charge No All EV and SOCESU ≤ 50% 18%≤ SOCESU ≤ 40% SOCBEV >90% Yes ESU B BEVs No No BEV No Yes Yes Present ESU 20%≤ SOC ≤ No Present Yes 50% E No No BEV No Present Yes BEVs BEVs SOCESU > 90% Yes Present Present E No Yes Yes ESU to BEVs ISW -5= Open Yes LOCAL LOAD No Yes Present Yes No PV to MODE 6 Yes SOCESU ≤ 50% No GRID No PPV>PBEV PPV>PBEV Yes Yes A Yes PV to GRID PPV>PBEV PPV>PBEV D Yes No D MODE 8 No No E MODE 3 MODE 1 MODE 7 MODE 4 MODE 2 MODE 5 PV to DC GRID TO BEV PV TO PV to EV PV to ESU PV to BEV ESU to Particular ISW -1= Open Charge BEV Bus to and ESU Grid to ESU Particular BEV PV to ESU through DC through DC through DC BEV through DC Terminate PV from from PV and and Grid charge THROUGH DC trough DC Bus THROUGH Bus bus BUS BUS charging point ESU ALL BEV GRID DC Bus

No No No BEV 1 BEV 1 BEV 1 Present Present Present 5 Yes Yes 6 Yes 7 Constant Yes Discharging Discharging Discharging BEV charging/ BEV charging/ BEV charging/ Current SOC ESU < 70% Discharging Discharging Discharging Charging

C C C Yes Charging Charging Charging No Yes 4 Yes

Constant Voltage Charging Constant Yes Constant Yes Constant Yes BEV Current SOC -1< 70% Current SOC BEV-1 < 70% Current SOC BEV-1 < 70% Charging Charging Charging No

SOC ESU ≥ 90% No No No

Constant Voltage Charging Constant Voltage Charging Constant Voltage Charging

ISW -4= Open BEV SOC -1 ≥ 95% SOC BEV-1 ≥ 95% SOC BEV-1 ≥ 95% 5 6 7 Yes Yes Yes

ISW-2 Open ISW-3 Open ISW-4 Open

FigureFigure 23.23.The The chargingcharging station’sstation’s EMSEMS ﬂowchart.flowchart.

Electronics 2021, 10, 1895 31 of 45

7.1.1. Energy Management Modes The charging stations’ operating modes are developed based on renewable source (PV) availability and considered as a prime source. When the utility grid is in peak demand for electricity and PV power is less than or equal to the power capacity of the single charging point, the charging station operates in modes 1 to 4. These modes’ EV charging power is utilized along with other available sources, such as ESU and utility grid shared with all or any EV available at the charging station. The next four modes are operated as PV power, rather than charge for all the EVs available at the charging point. PV power greater than or equal to the storage connected to the charging station as EV and ESU at the charging point can be considered Modes 9 to 11. Based on the above considerations, the charging station operates under 11 different modes, as shown in Figure 23, along with their corresponding considerations.

7.1.2. Experimental Implementation The charging station’s architecture and practical feasibility were tested with the help of a laboratory prototype. The energy management strategy and control of 240 W rating of charging stations was developed. This charging station consisted of three EV ports, one ESU unit, separate inverter circuit for testing utility grid integration, and a PV integrated boost converter; an FPFA controller is used for overall control. The PV is connected to the DC charging system through a MPPT boost converter. The charging port uses a bidirectional DC-DC converter and ESU of 12 V, 80 Ah lead acid battery. The MPPT extracted from the PV is 240 W during full operating conditions, the corresponding voltage and current measured as 18 V and 13.34 A. The P&O with MPPT algorithm was used to attain the maximum power output from the PV system. On comparison with the original system, 5% losses were considered compared to the lab scale prototype. The DC bus voltage is varied based on the irradiation over the PV. The experimental modes selected were based on the selection of the threshold value of the PV power. There were three reference power levels selected—low, medium, and high. The values are PPV REF 1 = 120 W, PPV REF 2 = 160 W, and PPV REF 3 = 230 W, respectively. Based on this, the charging station’s operating modes are selected, as explained below.

• Mode 1: ESU and PV to EV charging (PPV ≤ 120 W). • Mode 2: Particular EV charging by PV (PPV ≤ 120 W). • Mode 3: Utility grid and PV-based ESU charging (PPV ≤ 120 W). • Mode 4: Utility grid and PV-connected EV charging (PPV ≤ 120 W). • Mode 5: Utility grid and PV-powered EV and ESU charging (120 W ≤ PPV ≤ 160 w) • Mode 6: PV-powered EV charging (120 W ≤ PPV ≤ 160 W). • Mode 7: Utility grid powering (120 W ≤ PPV ≤ 160 W). • Mode 8: PV-powered EV and ESU charging (160 W ≤ PPV ≤ 230 W). • Mode 9: PV-powered energy storage (PPV ≥ 230 W). • Mode 10: Energy storage to EV charging (PPV < PPV MIN). • Mode 11: EV to EV (V2V) charging (PPV < PPV MIN). The PV produces less power than or equal to reference level 1, which is not sufﬁcient to charge all the EVs and ESUs at the charging station. This is operated in four different modes (mode 1 to mode 4). Among the ﬁrst four modes, mode 2 and mode 4 are presented for experimental results.

1. Mode 2: Particular EV Charging by PV (PPV ≤ 120 W) The side bidirectional converter in modes 1 and 2 charging work under buck mode to charge the EV. By considering mode 2, the PV system delivers an output to the particular EV available at the charging point. The different parameters of the charging station monitored under this mode are DC bus power, PV voltage and current, distribution to the local load, and corresponding EV port current, as shown in Figure 24. Electronics 2021, 10, x FOR PEER REVIEW 32 of 46

from the PV is 240 W during full operating conditions, the corresponding voltage and current measured as 18 V and 13.34 A. The P&O with MPPT algorithm was used to attain the maximum power output from the PV system. On comparison with the original system, 5% losses were considered compared to the lab scale prototype. The DC bus voltage is varied based on the irradiation over the PV. The experimental modes selected were based on the selection of the threshold value of the PV power. There were three reference power levels selected—low, medium, and high. The values are PPV REF 1 = 120 W, PPV REF 2 = 160 W, and PPV REF 3 = 230 W, respectively. Based on this, the charging station’s operating modes are selected, as explained below.

• Mode 1: ESU and PV to EV charging (PPV ≤ 120 W). • Mode 2: Particular EV charging by PV (PPV ≤ 120 W). • Mode 3: Utility grid and PV-based ESU charging (PPV ≤ 120 W). • Mode 4: Utility grid and PV-connected EV charging (PPV ≤ 120 W). • Mode 5: Utility grid and PV-powered EV and ESU charging (120 W ≤ PPV ≤ 160 w) • Mode 6: PV-powered EV charging (120 W ≤ PPV ≤ 160 W). • Mode 7: Utility grid powering (120 W ≤ PPV ≤ 160 W). • Mode 8: PV-powered EV and ESU charging (160 W ≤ PPV ≤ 230 W). • Mode 9: PV-powered energy storage (PPV ≥ 230 W). • Mode 10: Energy storage to EV charging (PPV < PPV MIN). • Mode 11: EV to EV (V2V) charging (PPV < PPV MIN). The PV produces less power than or equal to reference level 1, which is not sufficient to charge all the EVs and ESUs at the charging station. This is operated in four different modes (mode 1 to mode 4). Among the first four modes, mode 2 and mode 4 are presented for experimental results.

1. Mode 2: Particular EV Charging by PV (PPV ≤ 120 W) The side bidirectional converter in modes 1 and 2 charging work under buck mode to charge the EV. By considering mode 2, the PV system delivers an output to the partic- Electronics 2021, 10, 1895 ular EV available at the charging point. The different parameters of the charging station32 of 45 monitored under this mode are DC bus power, PV voltage and current, distribution to the local load, and corresponding EV port current, as shown in Figure 24.

VPV=12.21V (V) 10 V/Div PV

V IPV= 10.5A

(A) 10A/Div PV I

IBEV-1= 2.96A Electronics 2021, 10, x FOR PEER REVIEW (A) 3A/Div 33 of 46

BEV-1 I

I DTR=5A In this PV-powered energy storage mode, batteries5A/Div connected to the charging points (A) are fully charged and the PV voltage and current are measured as (VPV = 18.1 V, IPV = 12.7

Electronics 2021, 10, x FOR PEER REVIEW DTR 33 of 46 A), I as shown in Figure 26. Time (200 ms)

3. Mode-11: EV to EV (V2V) Charging (PPV < PPV MIN) FigureFigure 24. 24. PVPV to toparticular particular EV EV charging. charging. InIn this this PV-powered mode, all the energy sources storage at the mode, charging batteries station connected are unable to theto supply charging the points power arefor fully EV-charging;Mode charged 6: PV-powered and only the particular PV voltage EV chargingports and allo current (120w EV-charging W are≤ measuredP ≤ by160 exchanging as W). (VPV In = this18.1 power mode,V, IPV between = the 12.7 PV Mode 6: PV-powered EV charging (120 W ≤ PPV ≤ 160PV W). In this mode, the PV deliv- EVs. The power provided in this mode is from one vehicle to the other; the corresponding A),ersdelivers asthe shown power the in power of Figure 150 of W 15026. (VPV W (VPV = 13.09 = 13.09 V, IPV V, IPV= 11.53 = 11.53 A). A).The The DC DC bus bus voltage voltage is ismeasured measured valuesas 22 Vare and measured the bidirectional as 13.9 V, converter 1.9 A from operated the supply in buck vehicle mode to tothe charge charging the vehicle, EV battery as 3.as 22Mode-11: V and the EV bidirectional to EV (V2V) converter Charging operat(PPV < edPPV in MIN buck) mode to charge the EV battery shownconnected in Figure to the 27. charging point, as shown in Figure 25. connectedIn this to mode, the charging all the sources point, asat shownthe charging in Figure station 25. are unable to supply the power for2. EV-charging;Mode 9: PV-powered only particular Energy ports Storage allow (P EV-chargingPV ≥ 230 W) by exchanging power between V PV=13.09V EVs. The(V) power provided in this mode is from one10 vehicleV/Div to the other; the corresponding PV

valuesV are measured as 13.9 V, 1.9 A from the supply vehicle to the charging vehicle, as

shown(v) in Figure 27.

PV PV IPV= 11.53A 10A/Div I

(A) V PV=13.09V (V) 10 V/Div PV I BEV 1= 3.3A 3A/Div BEV 1 BEV V I I DTR=5A 10A/Div (v) PV PV (A) IPV= 11.53A 10A/Div I DTR I Time (100 ms) (A) I BEV 1= 3.3A 3A/Div BEV 1 BEV

FigureFigureI 25. 25. PV-poweredPV-powered EV EV charging. charging. I DTR=5A 10A/Div 2. Mode 9: PV-powered Energy Storage (P ≥ 230 W) (A) PV

(V) V PV=18.1V DTR I PV In this PV-powered energy storageTime mode, (100 batteries 20V/Divms) connected to the charging points are

V fully charged and the PV voltage and current are measured as (VPV = 18.1 V, IPV = 12.7 A), IPV= 12.70A 10A/Div Figureas shown (v) 25. PV-powered in Figure EV26. charging. PV PV I

I ESU= 4.67A 5A/Div

(V) PV (A) V =18.1V

PV 20V/Div ESU V I VESU=13.6V IPV= 12.70A 10A/Div20A/Div (v) (V) PV PV

I ESU ESU V I = 4.67A 5A/Div

(A) Time (100 ms)

ESU I Figure 26. PV-powered energyV ESUstorage.=13.6V 20A/Div (V)

ESU (V) V V BEV 1= 13.9VTime (100 ms) 10 V/Div BEV 1 BEV

FigureFigure 26.V 26. PV-poweredPV-powered energy energy storage. storage. I BEV 1= 1.9A (A) 2A/Div 3. Mode-11: EV to EV (V2V) Charging (PPV < PPV MIN) BEV 1 BEV I

(V) In this mode, all the sources at the charging station are unable to supply the power V BEV 1= 13.9V V B2= 13.8V 1010V/Div V/Div

for EV-charging;(V) only particular ports allow EV-charging by exchanging power between BEV 1 BEV V

BEV 2 I B2= 1.81A V I BEV 1= 1.9A 5A/Div (A) 2A/Div (A) BEV 1 BEV I

BEV 2 BEV Time (300 ms) I V B2= 13.8V 10V/Div (V)

FigureBEV 2 27. EV-EV (V2V) chargingI B2= 1.81A (PPV < PPV MIN). V 5A/Div

(A) The main objective of the EMS is to avoid the overloading of distribution transform-

BEV 2 BEV Time (300 ms)

ers, I effective usage of renewable energy, and uninterrupted supply for charging. This type

Figure 27. EV-EV (V2V) charging (PPV < PPV MIN).

The main objective of the EMS is to avoid the overloading of distribution transformers, effective usage of renewable energy, and uninterrupted supply for charging. This type

Electronics 2021, 10, x FOR PEER REVIEW 33 of 46

In this PV-powered energy storage mode, batteries connected to the charging points are fully charged and the PV voltage and current are measured as (VPV = 18.1 V, IPV = 12.7 A), as shown in Figure 26.

3. Mode-11: EV to EV (V2V) Charging (PPV < PPV MIN) In this mode, all the sources at the charging station are unable to supply the power for EV-charging; only particular ports allow EV-charging by exchanging power between EVs. The power provided in this mode is from one vehicle to the other; the corresponding values are measured as 13.9 V, 1.9 A from the supply vehicle to the charging vehicle, as shown in Figure 27.

V PV=13.09V (V) 10 V/Div PV V (v)

PV PV IPV= 11.53A 10A/Div I (A) I BEV 1= 3.3A 3A/Div BEV 1 BEV I I DTR=5A 10A/Div (A) DTR I Time (100 ms)

Figure 25. PV-powered EV charging.

(V) V PV=18.1V

PV 20V/Div V

IPV= 12.70A 10A/Div (v) PV PV I

I ESU= 4.67A 5A/Div (A) ESU

Electronics 2021, 10, 1895 I 33 of 45 VESU=13.6V 20A/Div (V)

ESU

EVs.V The power provided in this modeTime is from(100 onems) vehicle to the other; the corresponding values are measured as 13.9 V, 1.9 A from the supply vehicle to the charging vehicle, as Figureshown 26. PV-powered in Figure 27 energy. storage.

(V) V BEV 1= 13.9V 10 V/Div BEV 1 BEV V

I BEV 1= 1.9A (A) 2A/Div BEV 1 BEV I

V B2= 13.8V 10V/Div (V)

BEV 2 I B2= 1.81A V 5A/Div (A)

BEV 2 BEV Time (300 ms) I

FigureFigure 27. 27.EV-EVEV-EV (V2V) (V2V) charging charging (PPV (P

TheThe main main objective objective of ofthe the EMS EMS is is to to avoid avoid the the overloading overloading of of distribution distribution transform- transformers, ers,effective effective usage usage of of renewable renewable energy, energy, and and uninterrupted uninterrupted supply supply for for charging. charging. ThisThis typetype of charging architecture is more suitable for workplace-based charging, because the small amount of power generated from the PV is directly utilized by the EV that can accept the power level offered in vehicle-to-vehicle charging. All the modes provide uninterrupted charging based on EV availability through the EMS approach. The EMS at the charging station maintains DC bus voltage and power requirements for all the conditions of utility grid local load increasing or overloading. The energy management modes in the charging station provide continuous supply to the charging point. Even though the utility grid is fully loaded due to local demand and irradiation variation on PV, EV-charging is not delayed. The 11 modes based on load and source availability offer EV charging terminals an uninterrupted supply for charging. The total power demand is maintained by the available vehicle and ESU. A MATLAB model was developed for the charging station to analyze different types of charging stations, including multiport offerings. The experimental setup analyzes the practical feasibility of charging stations. In addition, the different modes proposed in the EMS showcase better performance and maintain DC bus power effectively by the effective utilization of PV power generation; in addition, renewable generated power is fully stored.

7.2. A DC Microgrid-Based Charging Station and Its IoT-Based Monitoring System The practical feasibility of a charging station with three charging ports was examined to check the time it took to charge an EV. In addition, different capacities of electric vehicles were chosen to test practical conditions. A power converter was used for the charging mode and DC grid side, controlled by a digital controller. Different types of and different ratings of the swapping batteries were selected to analyze the performance through an IOT module (SB1 = 12.5 V, SB 2= 12 V and SB 3 = 12.7 V). The experimental results for different operating conditions of the charging system, such as grid to vehicle (G2V) and vehicle to grid (V2G), are presented. The experimental results of all operations of the system are shown below in Figures 28 and 29. Electronics 2021, 10, x FOR PEER REVIEW 34 of 46

of charging architecture is more suitable for workplace-based charging, because the small amount of power generated from the PV is directly utilized by the EV that can accept the power level offered in vehicle-to-vehicle charging. All the modes provide uninterrupted charging based on EV availability through the EMS approach. The EMS at the charging station maintains DC bus voltage and power requirements for all the conditions of utility grid local load increasing or overloading. The energy management modes in the charging station provide continuous supply to the charging point.  Even though the utility grid is fully loaded due to local demand and irradiation variation on PV, EV-charging is not delayed.  The 11 modes based on load and source availability offer EV charging terminals an uninterrupted supply for charging.  The total power demand is maintained by the available vehicle and ESU.  A MATLAB model was developed for the charging station to analyze different types of charging stations, including multiport offerings.  The experimental setup analyzes the practical feasibility of charging stations. In addition, the different modes proposed in the EMS showcase better performance and maintain DC bus power effectively by the effective utilization of PV power generation; in addition, renewable generated power is fully stored.

7.2. A DC Microgrid-based Charging Station and its IoT-based Monitoring System The practical feasibility of a charging station with three charging ports was examined to check the time it took to charge an EV. In addition, different capacities of electric vehicles were chosen to test practical conditions. A power converter was used for the charging mode and DC grid side, controlled by a digital controller. Different types of and different ratings of the swapping batteries were selected to analyze the performance through an IOT module (SB1 = 12.5 V, SB 2= 12 V and SB 3 = 12.7 V). The experimental results for different operating conditions of the charging system, such as grid to vehicle (G2V) and Electronics 2021, 10, 1895 vehicle to grid (V2G), are presented. 34 of 45 The experimental results of all operations of the system are shown below in Figures 28 and 29.

V ESU= 48V 50 V/Div (V) ESU V

SB 1 10V/Div

(A) V = 12.1 V SB1 V (V) VSB 2= 12.7V 10V/Div SB2 V Electronics 2021, 10, x FOR PEER REVIEW 35 of 46

(V) VSB 3= 12.3V 10V/Div SB 3 I In this mode, all the PEVs are fully charged, and the charging station's total power is

given to the ESU. Figure 29 shows the MEV-to-grid powering mode, where 48 V is given FiguretoFigure the 28. ESU. 28. ExperimentalExperimental result result of ofSB SB and and ESU ESU voltage. voltage.

The DC bus voltage is maintained at 48 V and the SBs are charged with 12 V with the V DC BUS=50 V 40 V/Div switching currentV DCof theBUS =48converter V at 1.5 A. From the results, it is observed that the EV charging(V) system is functioning satisfactorily in mobile EV-to-EV charging mode. In vehi-

BUS BUS SB 1+SB 2 SB 1+SB 2+ SB 3

cle-to-vehicleV and vehicle-to-mobile electric vehicle ESU charging mode, the DC bus volt-

age is maintained as 12 VV.SB1 Swapping= 12.7V battery voltage and ESU voltages are measured, as

(A) 10V/Div

shownSB1 in Figure 28. V 1A/Div

(A) I DC BUS= 3.5A I DC BUS= 3A BUS I

(V) VESU =48 A 50 V/Div ESU V

FigureFigure 29. 29. ExperimentalExperimental result result of of MEV-to-grid MEV-to-grid charging. charging.

DifferentThe DC busmodes voltage of PEV is maintained charging stations at 48 V were and thedeveloped SBs are and charged the function with 12 Vof withthe systemthe switching tested for current modes of like the G2V, converter MEV2G, at 1.5 V2 A.V, Fromand battery the results, swapping it is observedis verified. that In theall theEV modes, charging DC system bus voltage is functioning is maintained. satisfactorily The IoT-based in mobile measurement EV-to-EV charging data collected mode. by In thevehicle-to-vehicle DAQ system to andthe cloud vehicle-to-mobile is shown in Figure electric 30a,b. vehicle ESU charging mode, the DC bus voltage is maintained as 12 V. Swapping battery voltage and ESU voltages are measured, as shown in Figure 28. In this mode, all the PEVs are fully charged, and the charging station’s total power is given to the ESU. Figure 29 shows the MEV-to-grid powering mode, where 48 V is given to Battery monitored thefor ESU.its complete life cycle Battery monitored for its complete life cycle Different modes of PEV charging stations were developed and the function of the system tested for modes like G2V, MEV2G, V2V, and battery swapping is veriﬁed. In all the modes, DC bus voltage is maintained. The IoT-based measurement data collected by the DAQ system to the cloud is shown in Figure 30a,b. This channel provides information on battery voltage, current drawn, SOC, SOH, Current Voltage and battery pack temperature required for safe operation of the battery and swapping details. The battery data is stored and used for monitoring and analyzing. Whenever any abnormality occurs, a notiﬁcation is sent to the mobile electric vehicle charging station. Time Time

(a) (b) Figure 30. Discharge response of the battery: (a) Voltage discharge profile, (b) Current discharge profile.

This channel provides information on battery voltage, current drawn, SOC, SOH, and battery pack temperature required for safe operation of the battery and swapping details. The battery data is stored and used for monitoring and analyzing. Whenever any abnormality occurs, a notification is sent to the mobile electric vehicle charging station. An autonomous system of the battery and BSS parameters for estimation and analysis using IoT is designed:  The cloud delivers a swapping battery condition report to the driver’s mobile device directly after detecting unusual conditions in the battery.  The battery swapping parameters’ detection and storage in the cloud helps develop the business model.

Electronics 2021, 10, x FOR PEER REVIEW 35 of 46

In this mode, all the PEVs are fully charged, and the charging station's total power is given to the ESU. Figure 29 shows the MEV-to-grid powering mode, where 48 V is given to the ESU.

V DC BUS=50 V 40 V/Div V DC BUS=48 V (V)

BUS BUS SB 1+SB 2 SB 1+SB 2+ SB 3 V

VSB1= 12.7V

(A) 10V/Div SB1 V 1A/Div

(A) I DC BUS= 3.5A I DC BUS= 3A BUS I (V) VESU =48 A 50 V/Div ESU V

Figure 29. Experimental result of MEV-to-grid charging.

Different modes of PEV charging stations were developed and the function of the Electronics 2021, 10, 1895 system tested for modes like G2V, MEV2G, V2V, and battery swapping is verified.35 In of all 45 the modes, DC bus voltage is maintained. The IoT-based measurement data collected by the DAQ system to the cloud is shown in Figure 30a,b.

Battery monitored for its complete life cycle Battery monitored for its complete life cycle Current Voltage

Time Time

(a) (b)

FigureFigure 30. 30. DischargeDischarge response response of of the the battery: battery: ( (aa)) Voltage Voltage discharge discharge profile, proﬁle, ( b) Current discharge proﬁle.profile.

ThisAn autonomous channel provides system information of the battery on battery and BSS voltage, parameters current for drawn, estimation SOC, and SOH, analysis and batteryusing IoT pack is designed:temperature required for safe operation of the battery and swapping details. The batteryThe cloud data delivers is stored a swappingand used for battery monitoring condition and report analyzing. to the driver’sWhenever mobile any abnor- device malitydirectly occurs, after a notification detecting is unusual sent to conditionsthe mobile inelectric the battery. vehicle charging station. AnThe autonomous battery swapping system parameters’ of the battery detection and BSS and parameters storage in for the estimation cloud helps and develop analysis usingthe IoT business is designed: model.  TheThe cloud measured delivers data a andswapping testing battery conditions condition of the report developed to the prototype driver’s mobile show that device the proposeddirectly scheme after isdetecting an efficient unusual alternative conditions method in the to minimize battery. charging time and increase driver The comfort. battery swapping parameters’ detection and storage in the cloud helps develop the business model. 7.3. Energy Management Strategy for a DC Microgrid with Maximum Penetration of RES A laboratory-scale DC microgrid with PV, fuel cell, and wind generator are taken into consideration. These sources are connected to the DC bus by a suitable converter. The main objective is to maintain DC bus voltage through the battery connected in parallel. The hardware details of the DC microgrid are presented in Table 17. The EMS is tested under the following cases: 1. PG > PL (the priority load is the lighting load that charges during the day and discharges at night) 2. PG = PL (the load is raised at 9 am to 10 am to generate power, load power is 440 W, load current is 2 A, and the generated power continuously supplies the load without any interruption) 3. PG < PL (from 8 pm to 12 am, the load is considered to be 900 W and available generation is 600 W) The results satisfy the EMS for DC microgrid architecture that is scalable for 15 kW to satisfy the load demand in rural communities. The experimental results at different time periods are presented in Table 18. Experimental studies on DC microgrids with the proposed EMS clearly indicate that power is dissipated to consumers throughout the day with maximum renewable energy penetration and batteries without any divergence in the system. Thus, the proposed EMS is verified through a laboratory-scale, real-time DC microgrid experimental setup, confirming its merits. Electronics 2021, 10, 1895 36 of 45

Table 17. Description of the hardware components used.

DC Bus Capacity of Wind Capacity of Capacity of Fuel DC-DC Diesel Maximum Parameters Battery Type Battery Capacity Lamp Loads Load Bus Voltage Generator PV Panel Cell Power Converter Generator Current Tall tubular Speciﬁcation 24 V 200 W 200 W 100 W 14 Ah/12 V 24 V/220 V 500 W 220 V 500 W 3 A C10

Table 18. Experimental results at different time periods.

Loads (W) Batteries PV Power Wind Power Fuel Cell Power Generated Power (W) Priority Load Commercial Load Net Lads Priority Load Commercial Load Time (W) (W) (W) P I P I P I P I P I P I (W) (A) (W) (A) (W) (A) (W) (A) (W) (A) (W) (A) 8 p.m.–12 a.m. 0 150 100 250 1.13 100 0.5 250 1.1 350 1.6 100 0.5 100 0.45 9 a.m.–10 a.m. 200 140 100 440 2 100 0.5 340 1.5 440 2 −100 −0.5 0 0 7 a.m.–8 a.m. 100 170 100 370 1.68 100 0.5 150 0.7 250 1.2 −100 −0.5 −120 −0.7 Electronics 2021, 10, 1895 37 of 45

8. Challenges and Opportunities for Charging Station Infrastructures The commercialization of EV usage and its success lies is the development of a charging station infrastructure with standard equipment. In addition, the charging connector should be easily accessible. Accessibility depends on the power levels of the vehicle being connected. The charging station studied was found to have some challenges and opportunities in the selection of architecture, adaptation of renewable sources, maintaining voltage ﬂuctuation, and development of control schemes.

8.1. Challenges • Optimal location for electric vehicle charging stations: An ideal location is critical in reducing the range of anxiety experienced by electric vehicle buyers. Several factors influence the location of charging stations, including drivers’ satisfaction with charging, operators’ economic problems, vehicle power loss, power grid safety, and transportation system and traffic congestion. • The following problems were assumed while conducting the research: a. Charging demand (mainly by BEV) b. Charging facility features c. Charging fees and electricity cost d. Cost of station installation, operation, maintenance, and land acquisition • Development of fast charging, ultra-fast charging, and battery swapping stations to reduce EV-charging time. • Provision of queue management based on the strategy at the charging station. • To maintain energy balance, a proper grid integration structure with charging stations is required. • A communication system between the charging point to EV and grid management and the ability to quickly identify a vehicle and make the billing process as simple as possible. Charge cost optimization by selecting the most appropriate time and charging rates. Optimize grid load by adjusting charger capacity in response to grid demand. Use V2G technology to support grid operation during peak loads. • Energy storage technologies in EV charging stations, particularly fast and ultra-fast charging stations, support stable operation, and improve customer satisfaction. In order to define their contribution level in terms of energy supply for EV-charging, ESS sizing is critical in charging station infrastructure. • Architecture: Efficiency of the existing utility grid system is decreased due to EV charging loads. • Adaptation of renewable sources: High cost of installation of renewable energy sources and compliance with multiple charging protocols. • Accommodation of land space: The promotions of EV charging stations with renewable energy sources occupy large land areas that can increase the cost of installation. • Energy management: There are many challenges in EMS, such as low efficiency at charging port, reliability and prediction of batteries from overcharging. • DC link interfacing: Interfacing of renewables creates voltage fluctuation in DC links and requires a regulated output from the sources. • Selection of converters: Number of switching devices and storage devices in converter topology lead to more losses in conversion. In addition, control of the converter requires a suitable control. • Control scheme: The control methodology selection for a suitable power rating application requires detailed analysis and dedicated control.

8.2. Opportunities • Optimal selection of charging stations based on the demand for charging (primarily from BEVs), charging facility features, fees and electricity costs, station installation, operation, and maintenance costs, as well as land acquisition costs; there is literature available on optimization. Electronics 2021, 10, 1895 38 of 45

• The optimal development of charging infrastructure necessitates careful planning in terms of charging station location and size. A large charging station can accommodate a greater number of chargers to accommodate more EVs; nevertheless, it will also require more electrical energy and construction costs. • The charging infrastructure requirements are highly dependent on EV battery sizes and power rates, both of which are expected to rise in the future. • Development of fast-charging, ultra-fast charging, and battery swapping stations are being developed to reduce EV-charging time. • The rate of EV adoption has a significant impact on the development of fast- and ultra-fast charging stations, as well as the profitability of their operation. • It is critical to regulate and schedule the available charging stations in order to charge an electric vehicle. This will aid in the strategic management of EV queues at charging stations. The management of queues will be aided by an effective communication network. • Several communication protocols and standards are available for billing and managing the charging. • Architecture: The charging station, by connecting renewable sources and bidirectional power converter in the charging station architecture, meets grid demand through V2G technology. • Adaptation of renewable sources: Charging pricing decreases, emission reduces, and the utility grid does not overload. • Accommodation of land space: The installations of more charging stations lead to an increase in the number of vehicles on the road. • Energy management: The energy management system in charging stations with renewable sources makes full use of solar energy; thus, the operating cost is reduced, resulting in maximum benefit. • DC link interfacing: EV-charging depends on storage devices; effectively connected DC energy storages provide better efficiency. • Selection of converters: A suitable converter and control strategy leads to increased efficiency and reduces charging time. • Control scheme: The conventional and intelligent control, the optimization algorithm, is used to provide better control.

9. Conclusions This paper summarized a recent literature study on renewable energy sources-powered EV charging stations’ topologies and the importance of different types of charging station architectures. The different categories of architecture are classified based on RES integration with DC, AC, and hybrid microgrids. Charging converter topologies and their bidirectional power flow options for DC bus voltage regulations are also presented. The main difficulty in a charging station is the connection of the DC and AC loads, i.e. charging points of DC levels. Different control strategies in microgrid charging stations were also reviewed, which can provide guidance in selecting an apt control technology. The AC microgrid-based charging station follows droop control characteristics, which gives higher stability at higher gains compared to frequency droop control; however, it requires a communication channel. The DC microgrid-based charging station with simple DC–DC conversion, on the other hand, provides a fast-charging option and reduces conversion loss. Different connectors are also presented, which help in selecting a charging connector based on charging levels. The following observations have been made in this review: • In AC microgrids, the controller can manage frequency, voltage regulation, and real and reactive power control. • The DC microgrid-based charging station is more suitable due to conversion losses. • The energy storage unit in the charging station provides uninterrupted EV-charging and ESU provides an option for effective usage of renewable energy sources. Electronics 2021, 10, 1895 39 of 45

• ESU integration with microgrids through dedicated converters enables fast charging and discharging. • DC microgrid-based charging station control through DC link voltage and power prediction provides better efficiency. • Controlling of microgrids through fuzzy logic and optimization technique-based energy management strategy provides better regulation and optimal management of fast charging. • Charging side converters with bidirectional power flow support grid voltage regulation through constant current and voltage charging. The adoption of new technologies for EV-charging, such as V2G, smart grid, smart charging technique, etc., will be extremely beneficial in maintaining the power system’s energy balance and maximizing the use of available renewable energy. It will also assist in achieving customer satisfaction as well as ensuring cost-effective charging rates. The development of an efficient optimization unit for shorter charging times, as well as a prediction unit to aid in the best possible optimization, are critical for the efficient operation of EV charging infrastructure. To meet the goals of reduced reliance on fossil fuels and zero emissions of environmentally polluting gases, a stable distributed or microgrid system network with maximized energy generation from a renewable energy system must be promoted to feed increasing and dynamic electrical loads in the form of EVs.

Author Contributions: Conceptualization, D.S.A. and B.C.; methodology, D.S.A., B.C., L.M.-P. and N.R.; software, S.R.G.T.R.; validation, D.S.A., K.M.S., B.C. and N.R.; formal analysis, D.S.A., B.C. and N.R.; investigation, D.S.A., B.C. and N.R.; resources, L.M.-P. and N.R.; data curation, D.S.A., K.M.S., B.C. and N.R.; writing—original draft preparation, D.S.A., B.C. and N.R.; writing—review and editing, D.S.A., B.C. and L.M.-P.; visualization, L.K.; supervision, B.C. and L.M.-P.; project administration, R.V.; funding acquisition, L.M.-P. and B.C. All authors have read and agreed to the published version of the manuscript. Funding: This research is associated with the project of Deanship of Scientific Research at King Khalid University, Kingdom of Saudi Arabia, for funding this work through the General Research Project under grant number RGP. 1/262/42. Data Availability Statement: Experimental data is available upon request. Conflicts of Interest: The authors declare no conflict of interest.

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