Low-Voltage Ride-Through Techniques in DFIG-Based Wind Turbines: a Review

applied sciences

Review Low-Voltage Ride-Through Techniques in DFIG-Based Wind Turbines: A Review

Boyu Qin 1,* , Hengyi Li 1 , Xingyue Zhou 1, Jing Li 2 and Wansong Liu 1

1 State Key Laboratory of Electrical Insulation and Power Equipment, and the school of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi Province, China; [email protected] (H.L.); [email protected] (X.Z.); [email protected] (W.L.) 2 Jiangsu Nuclear Power Co., Ltd., Lianyungang 222000, Jiangsu Province, China; [email protected] * Correspondence: [email protected]; Tel.: +86-136-5925-0017

Received: 15 February 2020; Accepted: 15 March 2020; Published: 22 March 2020

Abstract: In recent years, considerable advances were made in wind power generation. The growing penetration of wind power makes it necessary for wind turbines to maintain continuous operation during voltage dips, which is stated as the low-voltage ride-through (LVRT) capability. Doubly fed induction generator (DFIG)-based wind turbines (DFIG-WTs), which are widely used in wind power generation, are sensitive to disturbances from the power grid. Therefore, several kinds of protection circuits and control methods are applied to DFIG-WTs for LVRT capability enhancement. This paper gives a comprehensive review and evaluation of the proposed LVRT solutions used in DFIG-WTs, including external retroﬁt methods and internal control techniques. In addition, future trends of LVRT solutions are also discussed in this paper.

Keywords: Doubly fed induction generator; low-voltage ride through; wind turbine; rotor-side converter; grid-side converter

1. Introduction In recent years, renewable energy generation made great progress worldwide to meet the challenge of drastic climate changes and increasing energy demands [1]. Among various renewable energy sources, wind energy is the most rapidly growing one around the world [2,3]. In 2018, the 51.3 GW of new installations brought total cumulative installations of wind power up to 591 GW, and the Global Wind Energy Council (GWEC) expects that more than 55 GW of new capacity will be added each year until 2023. In 2019, with the 25.7 GW of new wind power installations, the cumulative wind power installations of China increased by 14% to 210 GW. With the development of wind energy, wind turbines (WTs) are more widely used in electrical power systems. WTs can be classiﬁed into four basic categories based on the ways of speed control: ﬁxed-speed wind turbines, limited variable-speed-controlled wind turbines, doubly fed induction generator (DFIG)-based wind turbines (DFIG-WTs), and full variable-speed-controlled wind turbines [4,5]. Among these four types of wind turbines, DFIG-WTs are mostly preferred in practical applications due to their advantages including simple installation, low cost, variable-speed constant frequency, and independent control of active and reactive power [6]. Figure1 shows the schematic diagram of a DFIG. The stator is connected to the power grid directly, and the rotor is linked to the power grid through a back-to-back voltage source converter, which comprises a rotor-side converter (RSC) and a grid-side converter (GSC). RSC is used to control the active power and reactive power delivered and to achieve the maximum power capture. GSC is used to control the active power and reactive power delivered, as well as maintain the direct current (DC)-link voltage [1,7].

Appl. Sci. 2020, 10, 2154; doi:10.3390/app10062154 www.mdpi.com/journal/applsci Appl. Sci. 2020, 2, x FOR PEER REVIEW 2 of 25

perturbations will induce large voltages in rotor windings and lead to uncontrolled rotor current. The rotor overcurrent may cause a large increase in DC‐link voltage. Both rotor inrush current and DC‐link overvoltage can damage wind turbines [8] and may lead to tripping of DFIG‐WTs. The Appl.ability Sci. to2020 ensure, 10, 2154 continuous operation during voltage dips, as well as minimize re‐synchronization2 of 25 Appl. Sci. 2020, 2, x FOR PEER REVIEW 2 of 25 problems after the clearance of faults, is stated as low‐voltage ride‐through (LVRT) capability. perturbations will induce large voltages in rotor windings and lead to uncontrolled rotor current. The rotor overcurrent may cause a large increase in DC-link voltage. Both rotor inrush current and DFIG DC-link overvoltageGearbox can damage wind turbines [8] and may lead to tripping of DFIG-WTs. The ability to ensure continuous operation during voltage dips, as well as minimize re-synchronization RSC GSC Power System problems after the clearance of faults, is stated as DClow Link-voltage ride-through (LVRT) capability.

Gearbox DFIG FigureFigure 1. 1. BasicBasic structurestructure ofof doublydoubly fedfed induction induction generator generator (DFIG). (DFIG).

DFIG-WTs are susceptible to grid voltageRSC dips and disturbances.GSC DuringPower gridSystem voltage dips, the In the past, the penetration level of wind DCenergy Link was extremely small compared with stator and GSC are inﬂuenced directly by the sudden change in DFIG bus voltage. Stator perturbations conventional generation systems [9]. Therefore, wind turbines were allowed to disconnect from the will induce large voltages in rotor windings and lead to uncontrolled rotor current. The rotor power grid during voltage dips to avoid overcurrent. However, with the penetration of wind energy overcurrent may cause a large increase in DC-link voltage. Both rotor inrush current and DC-link in electrical power systems increasing, a sudden loss of wind power during voltage dips can result overvoltage can damageFigure wind 1. turbinesBasic structure [8] and of maydoubl leady fed to induction tripping generator of DFIG-WTs. (DFIG The). ability to ensure in control problems of system frequency and voltage, which leads to a system collapse in the worst continuous operation during voltage dips, as well as minimize re-synchronization problems after the case [10,11]. Hence, it is ideal to make wind turbines stay connected to the power grid and provide clearanceIn ofthe faults, past, is the stated penetration as low-voltage level ride-through of wind energy (LVRT) was capability. extremely small compared with support during transient periods. Therefore, the LVRT capability in wind turbines is now mandatory. conventionalIn the past, generation the penetration systems level [9 of]. windTherefore, energy wind was turbines extremely were small allowed compared to disconnect with conventional from the With emphasis on the LVRT capability of DFIG‐WTs, many countries updated their grid code generationpower grid systems during [voltage9]. Therefore, dips to windavoid turbinesovercurrent. were However, allowed towith disconnect the penetration from the of powerwind energy grid requirements (GCRs). The German LVRT grid criteria are mostly preferred over other GCRs in duringin electrical voltage power dipsto systems avoid overcurrent.increasing, a However, sudden loss with of the wind penetration power durin of windg voltage energy dips in electricalcan result practical applications [1]; they require that (1) wind turbines shall remain connected to grid for at powerin control systems problems increasing, of system a sudden frequency loss ofand wind voltage, power which during leads voltage to a system dips can collapse result in in the control worst least 0.65 s after fault inception, (2) the permitted fault voltage is 15% of its rated voltage, and (3) the case [10,11]. Hence, it is ideal to make wind turbines stay connected to the power grid and provide problemsvoltage should of system recover frequency to 90% and of its voltage, rated voltage which leads within to a3 systems after the collapse clearance in the of worst faults. case According [10,11]. support during transient periods. Therefore, the LVRT capability in wind turbines is now mandatory. Hence,to the GCR it is ideal for LVRT, to make wind wind turbines turbines should stay connected have the ability to the power to ensure grid continuous and provide operation support duringduring With emphasis on the LVRT capability of DFIG-WTs, many countries updated their grid code transientvoltage dips, periods. and Therefore,the voltage the should LVRT be capability kept above in wind the curve turbines shown is now in mandatory.Figure 2. In Withaddition, emphasis most requirements (GCRs). The German LVRT grid criteria are mostly preferred over other GCRs in onGCRs the LVRTrequire capability reactive ofcurrent DFIG-WTs, injection many during countries voltage updated dips. their The gridE.ON code requirement requirements for (GCRs).voltage practical applications [1]; they require that (1) wind turbines shall remain connected to grid for at Thesupport German proposes LVRT gridthat criteriawind turbines are mostly should preferred provide over reactive other GCRs current in practicalwhen the applications voltage reaches [1]; they the least 0.65 s after fault inception, (2) the permitted fault voltage is 15% of its rated voltage, and (3) the requiredead band that limit, (1) wind and turbineslarger reactive shall remain current connected injection is to needed grid for when at least the 0.65 voltage s after dips fault go inception,deeper, as voltage should recover to 90% of its rated voltage within 3 s after the clearance of faults. According (2)presented the permitted in Figure fault 3. voltage The reactive is 15% of current its rated support voltage, in and response (3) the voltageto severe should voltage recover dips to must 90% ofbe to the GCR for LVRT, wind turbines should have the ability to ensure continuous operation during itsachieved rated voltage within within 20 ms 3of s fault after detection the clearance [12]. of faults. According to the GCR for LVRT, wind turbines shouldvoltage have dips, the and ability the tovoltage ensure should continuous be kept operation above duringthe curve voltage shown dips, in Fig andure the 2. voltage In addition, should most be keptGCRs above requi there curve reactive shown current in Figure injection2. In during addition, voltage most dips.GCRs The require E.ON reactive requirement current for injection voltage duringsupport voltage proposes dips. that The wind E.ON turbines requirement should for provide voltage reactive support current proposes when that the wind voltage turbines reaches should the providedead band reactive limit, current and larger when reactive the voltage current reaches injection the is dead needed band when limit, the and voltage larger dips reactive go deeper current, as injectionpresented is needed in Figure when 3. The the voltage reactive dips current go deeper, support as in presented response in to Figure severe3. The voltage reactive dips current must be supportachieved in responsewithin 20 toms severe of fault voltage detection dips [12] must. be achieved within 20 ms of fault detection [12].

Figure 2. Wind turbine German low‐voltage ride‐through (LVRT) grid criteria for voltage dips.

To meet the demands of GCRs, the behavior of the DFIG during and after voltage dips should be controlled as follows: (1) overcurrent and overvoltage in stator and rotor windings should be restrained to avoid damaging the power converters; (2) speed of turbines and electromagnetic torques should be properly designed to guarantee safe operation [13]; (3) DC‐link voltage should be FigureFigure 2. 2.Wind Wind turbine turbine German German low-voltage low-voltage ride-through ride-through (LVRT) (LVRT grid) grid criteria criteria for for voltage voltage dips. dips. kept constant to ensure the desirable operation of RSC and GSC [14]; (4) enough reactive power is To meet the demands of GCRs, the behavior of the DFIG during and after voltage dips should be controlled as follows: (1) overcurrent and overvoltage in stator and rotor windings should be restrained to avoid damaging the power converters; (2) speed of turbines and electromagnetic torques should be properly designed to guarantee safe operation [13]; (3) DC-link voltage should be kept constant to ensure the desirable operation of RSC and GSC [14]; (4) enough reactive power is Appl. Sci. 2020, 2, x FOR PEER REVIEW 3 of 25

needed to promote grid recovery [15]. To solve these problems, various methods including external retrofit techniques and internal control techniques were proposed to ameliorate the LVRT capability Appl. Sci. 2020, 10, 2154 3 of 25 of DFIG-WTs.

Dead band

-50% -20% 20% 50%

FigureFigure 3. 3.Reactive Reactive power power requirement requirement as as per per E.ON E.ON grid grid code. code. To meet the demands of GCRs, the behavior of the DFIG during and after voltage dips should be The review of LVRT techniques can serve as a reference to analyze the existing control methods controlled as follows: (1) overcurrent and overvoltage in stator and rotor windings should be restrained and identify the prospects for further improvements in this area. In Reference [9], a comprehensive to avoid damaging the power converters; (2) speed of turbines and electromagnetic torques should be review about the grid code regulations enforced on large wind power plants was presented. However, properly designed to guarantee safe operation [13]; (3) DC-link voltage should be kept constant to the specific LVRT techniques which fulfill various grid code requirements were not covered in this ensure the desirable operation of RSC and GSC [14]; (4) enough reactive power is needed to promote paper. A technical review of LVRT techniques applied in DFIG-WTs was discussed in Reference [4], grid recovery [15]. To solve these problems, various methods including external retrofit techniques while a significant portion was dedicated to LVRT protection circuits and hybrid combinations. In and internal control techniques were proposed to ameliorate the LVRT capability of DFIG-WTs. Reference [16], relevant literature about LVRT strategies was studied regarding when DFIG-WTs are The review of LVRT techniques can serve as a reference to analyze the existing control methods connected to an alternating current (AC) network or voltage source converter (VSC) based high and identify the prospects for further improvements in this area. In Reference [9], a comprehensive voltage direct current (HVDC). However, this paper mainly focused on the power surplus of two- review about the grid code regulations enforced on large wind power plants was presented. However, terminal systems and DC voltage control of multi-terminal systems. A review on LVRT techniques the specific LVRT techniques which fulfill various grid code requirements were not covered in this for improving the transient stability of DFIG-WTs was proposed in Reference [1], where LVRT paper. A technical review of LVRT techniques applied in DFIG-WTs was discussed in Reference [4], solutions were divided into protection circuits, Flexible Alternating Current Transmission System while a significant portion was dedicated to LVRT protection circuits and hybrid combinations. In (FACTS) device-based methods, traditional control methods, and advanced control techniques. Reference [16], relevant literature about LVRT strategies was studied regarding when DFIG-WTs However, the lack of comparative analysis results may restrict its application value. are connected to an alternating current (AC) network or voltage source converter (VSC) based This paper gives a comprehensive review and evaluation of the proposed LVRT techniques used high voltage direct current (HVDC). However, this paper mainly focused on the power surplus of in DFIG-WTs to analyze the current level of LVRT techniques. The main contributions of this paper two-terminal systems and DC voltage control of multi-terminal systems. A review on LVRT techniques are summarized as follows: firstly, this paper offers a remarkably up-to-date review of the state of for improving the transient stability of DFIG-WTs was proposed in Reference [1], where LVRT solutions LVRT techniques applied in DFIG-WTs, especially the latest achievements in this area. Secondly, were divided into protection circuits, Flexible Alternating Current Transmission System (FACTS) novel classifications are proposed in this paper based on internal/external retrofits and the location device-based methods, traditional control methods, and advanced control techniques. However, the of modifications. The analysis results and discussions show the potential of LVRT techniques and lack of comparative analysis results may restrict its application value. provide future research directions based on this work. This paper gives a comprehensive review and evaluation of the proposed LVRT techniques used This paper is organized as follows: Section 2 presents the classification of LVRT techniques. in DFIG-WTs to analyze the current level of LVRT techniques. The main contributions of this paper Section 3 gives an introduction of rotor-side external retrofit techniques. Stator-side external retrofit are summarized as follows: firstly, this paper offers a remarkably up-to-date review of the state of techniques are described in Section 4. Section 5 introduces internal control techniques. Section 6 LVRT techniques applied in DFIG-WTs, especially the latest achievements in this area. Secondly, concludes this paper and discusses future research trends. novel classifications are proposed in this paper based on internal/external retrofits and the location of2. modifications. Classification Theof LVRT analysis techniques results and discussions show the potential of LVRT techniques and provide future research directions based on this work. ThisExternal paper retrofit is organized techniques as follows: can effectively Section2 presents solve these the classification problems and of enhan LVRTce techniques. the LVRT Sectioncapability3 gives by an installing introduction devices of rotor-side to DFIG-WTs. external Different retrofit device techniques. topologies Stator-side and installed external locations retrofit techniquesdetermine arethe describedway external in Section retrofit4 .techniques Section5 introduceswork. For example, internal controlthe rotor techniques.-side crowbar Section circuit6 is concludesused to restrain this paper rotor and current discusses increase future and research protect trends. the RSC [17], while a DC-chopper can limit the overcharge during grid voltage dips [18]. According to the above factors, external retrofit techniques 2.are Classification classified into of LVRTtwo parts: Techniques rotor-side external retrofit techniques and stator-side external retrofit External retrofit techniques can effectively solve these problems and enhance the LVRT capability by installing devices to DFIG-WTs. Different device topologies and installed locations determine the Appl. Sci. 2020, 10, 2154 4 of 25 way external retrofit techniques work. For example, the rotor-side crowbar circuit is used to restrain Appl.rotor Sci. current 2020, 2, increasex FOR PEER and REVIEW protect the RSC [17], while a DC-chopper can limit the overcharge during4 of 25 grid voltage dips [18]. According to the above factors, external retrofit techniques are classified into techniques.two parts: rotor-side Internal control external techniques retrofit techniques are prioritized and stator-side choices for external new installations retrofit techniques. of wind turbines Internal [19].control Through techniques properly are prioritized designed choicescontrol forstrategies, new installations internal control of wind techniques turbines [19 make]. Through full use properly of the DFIG’sdesigned own control capability strategies, to enhance internal the control LVRT capability techniques of make DFIG full‐WTs. use Thus, of the they DFIG’s are more own capability economical to byenhance avoiding the LVRTextra capabilityhardware of devices. DFIG-WTs. Many Thus, advanced they are control more economical techniques by are avoiding proposed extra to hardware provide betterdevices. dynamic Many performances advanced control and techniquesenhance the are LVRT proposed capability to provide of DFIG better‐WTs. dynamic performances and enhance the LVRT capability of DFIG-WTs. 3. Rotor‐Side External Retrofit Techniques 3. Rotor-Side External Retrofit Techniques Due to the coupling between stator and rotor, grid voltage dips may result in rotor overcurrent and overvoltage.Due to the coupling Crowbars, between DC‐choppers, stator and series rotor, dynamic grid voltage resistors dips may (SDRs), result and in rotorDC‐link overcurrent energy storageand overvoltage. systems (ESSs) Crowbars, can DC-choppers,be installed on series the dynamic rotor side resistors to restrain (SDRs), the and inrush DC-link current energy and storage the increasingsystems (ESSs) voltage. can beThese installed extra ondevice the rotorinstallations side to restrainare classified the inrush as rotor current‐side and external the increasing retrofit techniques.voltage. These extra device installations are classified as rotor-side external retrofit techniques.

3.1.3.1. Crowbar Crowbar Method Method TheThe crowbar crowbar is is the the most most prevalent prevalent LVRT LVRT protection circuit adopted in DFIG DFIG-WTs.‐WTs. A A crowbar crowbar circuitcircuit comprises comprises a a set set of of paralleled paralleled resistors resistors installed installed between between rotor rotor windings windings and and the theAC‐ AC-sideside of the of RSCthe RSC [18,20], [18, 20as], shown as shown in inFigure Figure 4.4 When. When rotor rotor overcurrent overcurrent or or DC DC-link‐link overvoltage occurs, thethe crowbarcrowbar is is triggered triggered and and a a low low-resistance‐resistance path path is is created created to to allow allow high high currents currents to to flow flow through. In In thisthis operation operation mode, mode, the the RSC RSC is is disconnected disconnected from from rotor rotor windings windings via via the the closed closed crowbar crowbar switch switch [21] [21] andand the the damage damage of of overcurrent overcurrent can can be be avoided. avoided. With With the the crowbar crowbar circuit circuit triggered, triggered, the the DFIG DFIG-WT‐WT is is transformedtransformed into into a a standard standard induction induction motor motor and and the the excitation excitation control control is is lost. lost. In In this this case, case, the the DFIG DFIG consumesconsumes a a large large amount amount of of reactive reactive power power from from the the power power grid grid via via stator stator windings, windings, which which may may deterioratedeteriorate the the terminal terminal voltage voltage dynamics. dynamics. In In general, general, removing removing the the crowbar crowbar in in a a timely timely manner manner can can makemake the the RSC RSC provide provide reactive reactive power power to to the the grid grid as as soon soon as as possible possible to to accelerate accelerate the the grid grid voltage voltage recovery.recovery. However,However, more more crowbar crowbar connection connection time time is needed is needed to ensure to ensure that the that natural the natural flux is damped flux is dampedup to a desirableup to a desirable level so thatlevel the so RSCthat the can RSC recover can therecover DFIG the control DFIG [ 22control]. Therefore, [22]. Therefore, the crowbar the crowbarconnection connection time should time be should chosen be carefully chosen carefully to improve to improve the transient the transient stability stability of DFIG-WTs. of DFIG Another‐WTs. Anotherimportant important factor which factor influences which influences the crowbar the performancecrowbar performance is the crowbar is the resistance. crowbar resistance. On one hand, On onelow hand, crowbar low resistance crowbar canresistance lead to can serious lead rotor to serious overcurrent rotor overcurrent and electromagnetic and electromagnetic torque with torque a large withpeak a [ 22large]. On peak the [22]. other On hand, the other increased hand, crowbar increased resistance crowbar can resistance lead to rotor can lead overvoltage to rotor overvoltage and DC-link andovervoltage, DC‐link overvoltage, which may cause which damage may cause to rotor damage windings. to rotor Thus, windings. an appropriate Thus, an appropriate value of the value crowbar of theresistance crowbar should resistance be selected should to be meet selected the aforementioned to meet the aforementioned issues. issues.

Gearbox DFIG

Power System Crowbar RSC GSC Rc DC Link

Rotor Filter Grid Filter

controller FigureFigure 4. 4. CrowbarCrowbar protection protection circuit. circuit.

To improve the control effect based on these important factors, many crowbar modifications were proposed in terms of both configuration and strategy. An analytical expression for crowbar resistance was derived in Reference [23]. By taking grid impedance into consideration, the appropriate crowbar resistance can be calculated, which shows desirable performance in practical applications. Crowbar resistance chosen via the traditional method is close to the margin of LVRT Appl. Sci. 2020, 10, 2154 5 of 25

To improve the control effect based on these important factors, many crowbar modifications were proposed in terms of both configuration and strategy. An analytical expression for crowbar Appl.resistance Sci. 2020 was, 2, x derivedFOR PEER in REVIEW Reference [23]. By taking grid impedance into consideration, the appropriate5 of 25 Appl.crowbar Sci. 2020 resistance, 2, x FOR PEER can be REVIEW calculated, which shows desirable performance in practical applications.5 of 25 securityCrowbar region. resistance The chosenparallel via R‐L the configuration traditional methodcrowbar is with close series to the R‐ marginL circuit, of as LVRT shown security in Figure region. 5, wassecurityThe proposed parallel region. R-L in The configurationReference parallel [20]. R‐ crowbarL Duringconfiguration with the transient series crowbar R-L period, circuit, with seriesthe as parallel shown R‐L circuit, in R‐ FigureL configuration as 5shown, was proposed in Figurecrowbar in5, iswasReference activated proposed [ 20and]. in Duringmaintains Reference the a transient[20]. connection During period, tothe the transient the terminal parallel period, of R-L rotor configurationthe windings parallel R and‐ crowbarL configuration the RSC. is activated The crowbarsmooth and transitionismaintains activated can a and connection be maintainsguaranteed to thea throughconnection terminal the of to injection rotor the terminal windings of the of proposed and rotor the windings RSC. circuit. The and smooth the RSC. transition The smooth can be transitionguaranteed can through be guaranteed the injection through of the the proposed injection circuit. of the proposed circuit. rotor

shaft rotor DFIG Steady-state R-crowbar shaft DFIG + R-crowbar Steady-state + GSC Z - RSC GSC L-crowbar Z - RSC L-crowbar Figure 5. Parallel R‐L configuration crowbar. FigureFigure 5. 5. ParallelParallel R R-L‐L configuration configuration crowbar. crowbar. 3.2. DC Chopper 3.2. DC Chopper 3.2. DCA DC Chopper‐chopper is a resistance circuit connected in parallel with a DC‐link capacitor between the A DC-chopper is a resistance circuit connected in parallel with a DC-link capacitor between the RSC Aand DC GSC,‐chopper as shown is a resistance in Figure circuit 6. A connectedbraking resistor in parallel is used with to a DCmaintain‐link capacitor DC‐link between voltage theby RSC and GSC, as shown in Figure6. A braking resistor is used to maintain DC-link voltage by accepting acceptingRSC and transientGSC, as shownrotor overcurrent in Figure 6.[24]. A Thebraking insulated resistor gate is bipolar used to transistor maintain (IGBT), DC‐link in seriesvoltage with by transient rotor overcurrent [24]. The insulated gate bipolar transistor (IGBT), in series with resistors, resistors,accepting controls transient the rotor inserting overcurrent and quitting [24]. The time insulated of the gate DC bipolarchopper transistor circuit. Whenever (IGBT), in gridseries faults with controls the inserting and quitting time of the DC chopper circuit. Whenever grid faults occur, the rapid occur,resistors, the controlsrapid increase the inserting in rotor and current quitting will time lead ofto theDC ‐DClink chopper overvoltage circuit. by Whenevercharging the grid DC faults‐link increase in rotor current will lead to DC-link overvoltage by charging the DC-link capacitor. When capacitor.occur, the Whenrapid DCincrease‐link voltagein rotor exceeds current thewill threshold lead to DC value,‐link the overvoltage IGBT should by chargingbe closed theto short DC‐ linkthe DC-link voltage exceeds the threshold value, the IGBT should be closed to short the link with braking linkcapacitor. with braking When DC resistors‐link voltage [25,26]. exceeds As a result, the thresholdDC‐link voltage value, thecan IGBTbe adjusted should to be an closed acceptable to short level. the resistors [25,26]. As a result, DC-link voltage can be adjusted to an acceptable level. However, the rotor However,link with braking the rotor resistors transient [25,26]. overcurrent As a result, and DC ‐DClink‐link voltage overcurrent can be adjusted cannot to be an restrainedacceptable level.by a transient overcurrent and DC-link overcurrent cannot be restrained by a conventional DC-chopper. In conventionalHowever, the DC rotor‐chopper. transient In this overcurrent situation, theand RSC DC switching‐link overcurrent is stopped. cannot A modified be restrained DC‐chopper by a this situation, the RSC switching is stopped. A modified DC-chopper was proposed not only to restrain wasconventional proposed DC not‐chopper. only to In restrain this situation, DC‐link the voltage RSC switching on a desirable is stopped. level, A modifiedbut also DCrestrict‐chopper the DC-link voltage on a desirable level, but also restrict the overcurrent on both the rotor side and the overcurrentwas proposed on bothnot theonly rotor to restrainside and DCthe‐ linkstator voltage side in Referenceon a desirable [27]. The level, modified but also DC ‐restrictchopper the is stator side in Reference [27]. The modified DC-chopper is placed between the DC-link capacitor and placedovercurrent between on boththe DC the‐ linkrotor capacitor side and and the statorRSC, and side DC in Referencechopper resistance [27]. The modifiedcan be inserted DC‐chopper through is RSC, and DC chopper resistance can be inserted through three extra semiconductors. By employing threeplaced extra between semiconductors. the DC‐link By capacitor employing and the RSC, modified and DC DC chopper chopper, resistance rotor current can be and inserted stator throughcurrent, the modified DC chopper, rotor current and stator current, as well as DC-link voltage, can be controlled asthree well extra as DC semiconductors.‐link voltage, canBy employing be controlled the withoutmodified utilizing DC chopper, the extra rotor fault current‐current and ‐statorlimit methodcurrent, without utilizing the extra fault-current-limit method and voltage sag compensation approach [27]. andas well voltage as DC sag‐link compensation voltage, can approach be controlled [27]. without utilizing the extra fault‐current‐limit method and voltage sag compensation approach [27].

Gearbox DFIG

Gearbox DFIG (a)The general configuration Wind Turbine of DC chopper (a)The general configuration Wind Turbine of DC chopper

RSC GSC (b) The modified DC chopper RSC GSC S1 (b) The modified DC chopper S4 GSC RSC S1 S4 S2 S3 GSC RSC

Figure 6. Direct current (DC)S2 chopper protectionS3 circuit. Figure 6. Direct current (DC) chopper protection circuit. Figure 6. Direct current (DC) chopper protection circuit. 3.3. Series Dynamic Resistor 3.3. SeriesThe SDR Dynamic is a configuration Resistor where a dynamic resistor is put in series between rotor windings and the RSCThe to SDR limit is arotor configuration overcurrent where during a dynamic grid voltage resistor dips, is put as inshown series in between Figure 7.rotor The windings operation and of thethe SDRRSC isto controlled limit rotor by overcurrent a power electronic during grid switch: voltage (1) in dips, steady as‐ shownstate conditions, in Figure the7. The switch operation is closed of andthe SDRthe seriesis controlled resistor by is abypassed; power electronic (2) during switch: fault (1) conditions, in steady‐ statethe switch conditions, turns the open switch and isa closedseries and the series resistor is bypassed; (2) during fault conditions, the switch turns open and a series Appl. Sci. 2020, 10, 2154 6 of 25

3.3. Series Dynamic Resistor The SDR is a configuration where a dynamic resistor is put in series between rotor windings and the RSC to limit rotor overcurrent during grid voltage dips, as shown in Figure7. The operation of the Appl. Sci. 2020, 2, x FOR PEER REVIEW 6 of 25 SDR is controlled by a power electronic switch: (1) in steady-state conditions, the switch is closed and theresistor series is resistor connected is bypassed; between (2) the during RSC and fault rotor conditions, windings, the switchwhere turnsrotor opencurrent and flows a series through resistor the is connectedresistor [28]. between Due to the the RSC specific and rotortopology, windings, the SDR where can rotor directly current control flows the through magnitude the resistor of current. [28]. DueFurthermore, to the specific the limitation topology, of the rotor SDR current can directly can reduce control the thecharging magnitude current of to current. a DC‐link Furthermore, capacitor; thethus, limitation DC‐link ofovervoltage rotor current can can be reduceavoided the [29,30]. charging During current voltage to a DC-link dips, the capacitor; high rotor thus, voltage DC-link is overvoltageshared by the can series be avoided resistor, [29 ,30and]. Duringthe continuous voltage dips, operation the high of rotorconverter voltage can is sharedbe guaranteed. by the series By resistor,maximizing and the operation continuous time operation of the RSC, of converter more reactive can be power guaranteed. can be Bygenerated maximizing from theDFIG operation‐WTs to timesupport of the the RSC, terminal more voltage. reactive power can be generated from DFIG-WTs to support the terminal voltage.

Gearbox DFIG

DC Link

RSC GSC RSC Figure 7. Series dynamic resistance (SDR) protection circuit. A coordinated control approach which consists of a series rotor-side crowbar (SRSC), DC-link A coordinated control approach which consists of a series rotor‐side crowbar (SRSC), DC‐link capacitor crowbar (DCCC), and circuit breaking (CB) was proposed in Reference [31]. In this scheme, capacitor crowbar (DCCC), and circuit breaking (CB) was proposed in Reference [31]. In this scheme, the SRSC is in parallel with CB, and it connects in series with rotor windings and the RSC. The DCCC the SRSC is in parallel with CB, and it connects in series with rotor windings and the RSC. The DCCC is connected in parallel with the DC-link capacitor with the function of restricting DC-link overvoltage. is connected in parallel with the DC‐link capacitor with the function of restricting DC‐link Figure8 shows the conﬁguration of the coordinated control approach. In steady-state conditions, the overvoltage. Figure 8 shows the configuration of the coordinated control approach. In steady‐state rotor is connected with the RSC through CB, and the normal operation of the generator can be ensured. conditions, the rotor is connected with the RSC through CB, and the normal operation of the During grid voltage dips, the SRSC and DCCC operate in coordination to protect the RSC and DC-link generator can be ensured. During grid voltage dips, the SRSC and DCCC operate in coordination to capacitor. Furthermore, the RSC can be connected with rotor windings through the SRSC and maintain protect the RSC and DC‐link capacitor. Furthermore, the RSC can be connected with rotor windings operation to provide control eﬀorts. through the SRSC and maintain operation to provide control efforts.

Gearbox DFIG

DC Link

RSC GSC DCCC

CB SRSC

Figure 8. Coordinated control of a series rotor-side crowbar (SRSC) and DC-link capacitor Figure 8. Coordinated control of a series rotor‐side crowbar (SRSC) and DC‐link capacitor crowbar crowbar (DCCC). (DCCC). 3.4. DC-Link Energy Storage System 3.4. DC‐Link Energy Storage System The ESS acts as a buffer in DFIG-WTs with the function of keeping the DC-link power flow The ESS acts as a buffer in DFIG‐WTs with the function of keeping the DC‐link power flow throughThe charging ESS acts/discharging as a buffer [in32 ].DFIG Typical‐WTs DC-link with the ESS function configurations of keeping include the indirectDC‐link DC-link power ESSsflow through charging/discharging [32]. Typical DC‐link ESS configurations include indirect DC‐link ESSs andthrough direct charging/discharging DC-link ESSs. Figure [32].9 shows Typical the DC topology‐link ESS of configurations an indirect DC-link include ESS, indirect which DC comprises‐link ESSs a and direct DC‐link ESSs. Figure 9 shows the topology of an indirect DC‐link ESS, which comprises a batteryand direct and DC a buck-boost‐link ESSs. converter Figure 9 shows acting the as a topology DC/DC converter. of an indirect The batteryDC‐link is ESS, connected which in comprises paralleled a battery and a buck‐boost converter acting as a DC/DC converter. The battery is connected in withbattery the and DC-link, a buck and‐boost the energy converter storage acting system as isa ableDC/DC to share converter. the DC-link The battery with the is GSC connected when it in is paralleled with the DC‐link, and the energy storage system is able to share the DC‐link with the GSC when it is activated [33]. By applying an indirect ESS to the DC‐link, DFIG‐WTs have better capability to control DC‐link voltage and balance the power flow in the system [34]. Due to the specific structure, the ESS can smooth the steady‐state active power output. During transient periods, the ESS can endow the RSC with the ability to provide enough reactive power support via additional demagnetizing current injection. Thus, the terminal voltage dynamics can be improved. To reduce costs and increase efficiency by avoiding converter losses, a direct DC‐link ESS configuration was Appl. Sci. 2020, 2, x FOR PEER REVIEW 7 of 25 proposed in Reference [35]. In this topology, the energy storage is connected in parallel with the DC‐ link without any extra converter. Moreover, the direct DC‐link ESS also shows the desirable capability to smooth output power during voltage dips. However, the demagnetizing current which is injected from converters into the rotor circuit may result in rotor overcurrent.

Appl. Sci. 2020, 10, 2154 7 of 25

Gearbox DFIG activated [33]. By applying an indirect ESS to the DC-link, DFIG-WTs have better capability to control DC-link voltage and balance the power flow in the system [34].ESS Due to the specific structure, the ESS can smooth the steady-state active power output. During transient periods, the ESS can endow the RSC with the ability to provide enough reactive power support via additional demagnetizing current injection. Thus, the terminal voltage dynamics can be improved. To reduce costs and increase efficiency Appl. Sci. 2020, 2, x FOR PEER REVIEW DC Link 7 of 25 by avoiding converter losses, a direct DC-link ESS configuration was proposed in Reference [35]. In this proposedtopology, in the Reference energy storage [35]. In is this connected topology, in the parallel energy with storage the DC-link is connected without in parallel any extra with converter. the DC‐ linkMoreover, without the any direct extra DC-link converter.RSC ESS also Moreover, shows the the desirable direct capability DC‐link toESS smooth GSCalso outputshows powerthe desirable during

capabilityvoltage dips. to smooth However, output the power demagnetizing during voltage current dips. which However, is injected the fromdemagnetizing converters current into the which rotor iscircuit injected may from result converters inFigure rotor overcurrent. into9. Indirect the rotor DC circuit‐link mayenergy result storage in rotor system overcurrent. (ESS).

An advanced auxiliary ESS control system was proposed in Reference [36], and the auxiliary control configuration isGearbox shown DFIGin Figure 10. The operation of the ESS is controlled by power electronic switches. During normal operation, power electronic switchesESS SW1 and SW2 are off and the control system is disabled. When disturbances occur in the output power, the primary control system can participate in smoothing the output power by switching SW1 off and SW2 on. During the fault condition, the auxiliary control system is activated DCby Link switching SW1 on and SW2 off to restrict DC link overvoltage. This auxiliary ESS control system provides different functions based on different RSC GSC conditions. Therefore, both steady‐state and transient stability can be enhanced by applying this Figure 9. Indirect DC-link energy storage system (ESS). control system. Figure 9. Indirect DC‐link energy storage system (ESS). In the DC‐link ESS, super‐capacitors can replace traditional energy storages due to the function An advanced auxiliary ESS control system was proposed in Reference [36], and the auxiliary An advanced auxiliary ESS control system was proposed in Reference [36], and the auxiliary of matchingcontrol the configuration intermittent is shownwind inenergy. Figure 10Super. The‐ operationcapacitors of the do ESS not is have controlled an electrochemical by power electronic reaction control configuration is shown in Figure 10. The operation of the ESS is controlled by power electronic but have switches.electric Duringcharge normal absorption operation, and power desorption electronic during switches charging SW1 and SW2and aredischarging off and the control periods [37]. switches. During normal operation, power electronic switches SW1 and SW2 are off and the control system is disabled. When disturbances occur in the output power, the primary control system can Super‐capacitorssystem is disabled.can be Whenconnected disturbances with occurthe DCin the‐link output capacitor power, thedirectly primary or control via systeman interface. can By participate in smoothing the output power by switching SW1 off and SW2 on. During the fault implementingparticipate a super in smoothing‐capacitor the to output the DC power‐link, by the switching oscillation SW1 offof statorand SW2 current, on. During stator the voltage, fault and condition, the auxiliary control system is activated by switching SW1 on and SW2 off to restrict DC DC‐link voltagecondition, can the beauxiliary significantly control system restrained is activated [38]. by Although switching theSW1 super on and‐capacitor SW2 off to imposesrestrict DC a higher link overvoltage. This auxiliary ESS control system provides different functions based on different link overvoltage. This auxiliary ESS control system provides different functions based on different cost, it canconditions. enhance Therefore, the transient both steady-state stability of and the transient DFIG, stabilityand the can operation be enhanced of the by applying machine this does not conditions. Therefore, both steady‐state and transient stability can be enhanced by applying this need to becontrol modified system. [39]. control system. In the DC‐link ESS, super‐capacitors can replace traditional energy storages due to the function of matching the intermittent wind energy. Super‐capacitors do not have an electrochemical reaction but have electric charge absorptionGearbox DFIG and desorption during charging and discharging periods [37]. Super‐capacitors can be connected with the DC‐link capacitor directly or via an interface. By implementing a super‐capacitor to the DC‐link, the oscillation of stator current, stator voltage, and DC‐link voltage can be significantly restrained [38]. Although the super‐capacitor imposes a higher SW2 DC Link cost, it can enhance the transient stability of the DFIG, and the operation of the machine does not need to be modified [39].

RSC GSC

Gearbox DFIG

SW2SW1 DC Link FigureFigure 10. 10.AuxiliaryAuxiliary control control systemsystem of of ESS. ESS.

RSC GSC 3.5. Comparative Results Analysis of Rotor‐Side External Retrofit Techniques

Rotor‐side external retrofit techniquesSW1 have a desirable control effect in restraining rotor overcurrent, controlling DC‐link voltage, and improving terminal voltage dynamics. To compare the control effect of various rotor‐side Figureexternal 10. Auxiliary retrofit control techniques, system of aESS. single ‐machine infinite bus (SMIB) 3.5. Comparative Results Analysis of Rotor‐Side External Retrofit Techniques Rotor‐side external retrofit techniques have a desirable control effect in restraining rotor overcurrent, controlling DC‐link voltage, and improving terminal voltage dynamics. To compare the control effect of various rotor‐side external retrofit techniques, a single‐machine infinite bus (SMIB) Appl. Sci. 2020, 10, 2154 8 of 25

In the DC-link ESS, super-capacitors can replace traditional energy storages due to the function of matching the intermittent wind energy. Super-capacitors do not have an electrochemical reaction but have electric charge absorption and desorption during charging and discharging periods [37]. Super-capacitors can be connected with the DC-link capacitor directly or via an interface. By implementing a super-capacitor to the DC-link, the oscillation of stator current, stator voltage, and DC-link voltage can be signiﬁcantly restrained [38]. Although the super-capacitor imposes a higher cost, it can enhance the transient stability of the DFIG, and the operation of the machine does not need to be modiﬁed [39].

3.5. Comparative Results Analysis of Rotor-Side External Retroﬁt Techniques

Appl. Sci.Rotor-side 2020, 2, x externalFOR PEER retrofit REVIEW techniques have a desirable control effect in restraining rotor overcurrent,8 of 25 controlling DC-link voltage, and improving terminal voltage dynamics. To compare the control effect systemof various was rotor-side established, external as depicted retrofit in techniques,Figure 11. The a single-machine rated active power infinite of the bus wind (SMIB) farm system was 9 MW was withestablished, six 1.5 as MW depicted DFIG in‐ FigureWTs, and11. Thethe rated detailed active models power ofof the DFIG wind‐WTs farm were was 9provided MW with sixby 1.5Matlab/Simulink MW DFIG-WTs, R2016a. and the The detailed wind speed models in of the DFIG-WTs wind farm were was provided maintained by Matlab as a constant/Simulink of R2016a.15 m/s. AThe three wind‐phase speed to inground the wind fault farm with was grounding maintained resistance as a constant of 0.5 Ω ofoccurs 15 m /ats. the A three-phase end of 25 kV to feeder ground at 0fault s, and with the grounding fault is cleared resistance at 0.1 of s. 0.5 AccordingΩ occurs to at the the GCR end for of 25 LVRT, kV feeder the maximum at 0 s, and limit the of fault rotor is currentcleared was at 0.1 set s. as According 2 p.u. Typical to the models GCR for of an LVRT, active the crowbar maximum [20], limit SDR of [30], rotor ESS current [36], and was DC set chopper as 2 p.u. [25]Typical were models implemented of an active in MATLAB/Simulink crowbar [20], SDR [ 30for], ESSsimulation. [36], and The DC durations chopper [ 25of] the were active implemented crowbar circuitin MATLAB and SDR/Simulink were forset simulation.as 0.02 s, while The durationsthe ESS and of theDC active‐chopper crowbar remained circuit activated and SDR during were set grid as fault.0.02 s, The while comparative the ESS and results DC-chopper are shown remained in Figure activated 12. during grid fault. The comparative results are shownFigure in 12a Figure shows 12 the. control effect in restricting rotor current. Since the ESS and a conventional DC chopperFigure 12 maya shows result the in rotor control overcurrent, effect in restricting they are not rotor included. current. As Since shown the in ESS Figure and a 12a, conventional the active crowbarDC chopper circuit may shows result better in rotor performance overcurrent, in restricting they are not rotor included. overcurrent As showncompared in Figure with the 12 a,SDR, the becauseactive crowbar it creates circuit a low shows‐resistance better path performance to allow high in restricting‐current flow rotor through overcurrent during compared transient withperiods. the TheSDR, desirable because control it creates effect a low-resistance in restricting path rotor to current allow high-currentendows the active flow through crowbar during circuit transientwith the abilityperiods. to The avoid desirable overcharging control in eff DCect incapacitors, restricting such rotor that current the active endows crowbar the active circuit crowbar can restrain circuit withDC‐ linkthe ability overvoltage to avoid better overcharging than the SDR, in DC as shown capacitors, in Figure such 12b. that The the DC active‐chopper crowbar can circuit maintain can DC restrain‐link voltageDC-link in overvoltage an acceptable better range than by thequickly SDR, discharging as shown in the Figure excess 12 activeb. The power DC-chopper in the DC can‐link. maintain Figure 12cDC-link shows voltage the comparative in an acceptable results range of transient by quickly reactive discharging power support. the excess Since active the power SDR maximizes in the DC-link. the operationFigure 12c time shows of thethe comparativeRSC, more reactive results ofpower transient is generated reactive powerwhen the support. SDR is Since applied the SDRcompared maximizes with the operationactive crowbar time ofcircuit. the RSC, The more ESS reactiveoffers more power transient is generated reactive when power the SDRsupport is applied during compared transient withperiods, the which active crowbarresults in circuit. better terminal The ESS ovoltageffers more dynamics, transient as reactive shown in power Figure support 12d. Since during the transient reactive powerperiods, generation which results mainly in better depends terminal on RSC voltage control, dynamics, the active as shown crowbar in Figure circuit 12 andd. Since SDR the provide reactive a similarpower generation control effect mainly for depends supporting on RSC terminal control, voltage. the active The crowbar DC‐chopper circuit andhas SDRno control provide effect a similar on reactivecontrol e powerffect for support supporting and terminal voltage dynamics voltage. The improvement; DC-chopper thus, has no it controlis not presented effect on reactive in Figure power 12c andsupport d. and voltage dynamics improvement; thus, it is not presented in Figure 12c,d.

FigureFigure 11. The structurestructure ofof single-machinesingle‐machine inﬁniteinfinite busbus (SMIB)(SMIB) system.system.

1.1 2.5 Crowbar Crowbar SDR SDR 1.05 ESS 2 DC Chopper

1.5 1 Ir/p.u. 1 Vdc/p.u. 0.95

0.5 0.9

0 0.85 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 t/s t/s

(a) (b) Appl. Sci. 2020, 2, x FOR PEER REVIEW 8 of 25 system was established, as depicted in Figure 11. The rated active power of the wind farm was 9 MW with six 1.5 MW DFIG‐WTs, and the detailed models of DFIG‐WTs were provided by Matlab/Simulink R2016a. The wind speed in the wind farm was maintained as a constant of 15 m/s. A three‐phase to ground fault with grounding resistance of 0.5 Ω occurs at the end of 25 kV feeder at 0 s, and the fault is cleared at 0.1 s. According to the GCR for LVRT, the maximum limit of rotor current was set as 2 p.u. Typical models of an active crowbar [20], SDR [30], ESS [36], and DC chopper [25] were implemented in MATLAB/Simulink for simulation. The durations of the active crowbar circuit and SDR were set as 0.02 s, while the ESS and DC‐chopper remained activated during grid fault. The comparative results are shown in Figure 12. Figure 12a shows the control effect in restricting rotor current. Since the ESS and a conventional DC chopper may result in rotor overcurrent, they are not included. As shown in Figure 12a, the active crowbar circuit shows better performance in restricting rotor overcurrent compared with the SDR, because it creates a low‐resistance path to allow high‐current flow through during transient periods. The desirable control effect in restricting rotor current endows the active crowbar circuit with the ability to avoid overcharging in DC capacitors, such that the active crowbar circuit can restrain DC‐ link overvoltage better than the SDR, as shown in Figure 12b. The DC‐chopper can maintain DC‐link voltage in an acceptable range by quickly discharging the excess active power in the DC‐link. Figure 12c shows the comparative results of transient reactive power support. Since the SDR maximizes the operation time of the RSC, more reactive power is generated when the SDR is applied compared with the active crowbar circuit. The ESS offers more transient reactive power support during transient periods, which results in better terminal voltage dynamics, as shown in Figure 12d. Since the reactive power generation mainly depends on RSC control, the active crowbar circuit and SDR provide a similar control effect for supporting terminal voltage. The DC‐chopper has no control effect on reactive power support and voltage dynamics improvement; thus, it is not presented in Figure 12c and d.

Appl. Sci. 2020, 10, 2154Figure 11. The structure of single‐machine infinite bus (SMIB) system. 9 of 25

1.1 2.5 Crowbar Crowbar SDR SDR 1.05 ESS 2 DC Chopper

1.5 1 Ir/p.u. 1 Vdc/p.u. 0.95

0.5 0.9

0 0.85 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 t/s t/s

Appl. Sci. 2020, 2, x FOR PEER REVIEW(a) (b) 9 of 25

5 1.2

1 Crowbar 0.8 SDR ESS

0 0.6 Q/p.u. Us/p.u. Crowbar 0.4 SDR ESS 0.2

-5 0 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 t/s t/s

FigureFigure 12. 12. ControlControl effect eﬀect comparison comparison of of rotor rotor-side‐side external external retrofit retroﬁt techniques techniques in in ( (aa)) rotor rotor current, current, ( (bb)) DCDC-link‐link voltage, voltage, ( (cc)) reactive reactive power power support, support, and and ( (d)) terminal terminal voltage.

4.4. Stator Stator-Side‐Side External External Retrofit Retrofit Techniques Techniques DueDue to to the the direct direct connection connection between between the the stator stator and and power power grid, grid, grid grid voltage voltage dips dips can can lead lead to to statorstator overcurrentovercurrent and and voltage voltage sags sags at theat terminal.the terminal. Series Series dynamic dynamic braking braking resistors resistors (SDBRs), (SDBRs), reactive reactivepower compensation power compensation devices, faultdevices, current fault limiter current (FCLs), limiter and (FCLs), series grid-sideand series converter grid‐side (SGSCs) converter can (SGSCs)be installed can onbe theinstalled stator on side the of stator DFIGs side with of the DFIGs function with ofthe keeping function stator of keeping current stator and stator current voltage and statoron a desirable voltage on level. a desirable These external level. These devices external are classified devices asare stator-side classified as external stator‐ retrofitside external techniques. retrofit techniques. 4.1. Series Dynamic Braking Resistor 4.1. SeriesThe SDBRDynamic comprises Braking Resistor a resistor and a paralleled IGBT circuit. Generally, the SDBR is applied in series with line to restrain current during disturbances [40]. In a DFIG, the SDBR is installed between The SDBR comprises a resistor and a paralleled IGBT circuit. Generally, the SDBR is applied in the stator and the power grid, as shown in Figure 13. By dynamically inserting a resistor in the stator, series with line to restrain current during disturbances [40]. In a DFIG, the SDBR is installed between the SDBR can increase the terminal voltage during grid voltage dips. In addition, the destabilizing the stator and the power grid, as shown in Figure 13. By dynamically inserting a resistor in the stator, depression of electrical torque and power can be restricted [41]. However, when a traditional SDBR is the SDBR can increase the terminal voltage during grid voltage dips. In addition, the destabilizing activated, the back-to-back converter has to be disabled due to the hardship in synchronizing stator depression of electrical torque and power can be restricted [41]. However, when a traditional SDBR voltage, and the variable frequency switching scheme may result in undesirable voltage quality [42]. is activated, the back‐to‐back converter has to be disabled due to the hardship in synchronizing stator To solve these problems, a modulated series dynamic braking resistor (MSDBR) was proposed, which voltage, and the variable frequency switching scheme may result in undesirable voltage quality [42]. comprises a high-power braking resistor and two parallel IGBTs for each phase [43]. The MSDBR is To solve these problems, a modulated series dynamic braking resistor (MSDBR) was proposed, which also connected in series with the stator and grid, and this device is controlled by the fixed frequency comprises a high‐power braking resistor and two parallel IGBTs for each phase [43]. The MSDBR is also connected in series with the stator and grid, and this device is controlled by the fixed frequency Pulse‐Width Modulation (PWM) signals to dissipate excessive power and maintain the terminal voltage magnitude in a desirable range. Furthermore, a configuration which combines MSDBR with a parallel integrated capacitor was proposed in Reference [44] to improve the performance of DFIG‐WTs during the transient period. Due to the application of the proposed hybrid control scheme, the induced overvoltage in the stator will not cause the converter system to lose control. As a result, rotor overvoltage can be limited. At the same time, the DC‐link capacitor charging current can be controlled to restrain DC‐link overvoltage. Appl. Sci. 2020, 10, 2154 10 of 25

Pulse-Width Modulation (PWM) signals to dissipate excessive power and maintain the terminal voltage magnitudeAppl. Sci. 2020 in, 2 a, x desirable FOR PEER range. REVIEW 10 of 25

Bypass switch

Gearbox DFIG SDBR Appl. Sci. 2020, 2, x FOR PEER REVIEW 10 of 25 Power System Bypass switch RSC GSC DC Link Gearbox DFIG SDBR Rotor Grid Filter Filter Power System FigureFigure 13. 13.Series Series dynamic dynamic braking braking resistor resistor protection protection circuit. circuit. RSC GSC DC Link 4.2. Furthermore,Reactive Power a Compensation configuration Device which combines MSDBR with a parallel integrated capacitor was proposed in Reference [44] to improveRotor the performance of DFIG-WTsGrid during the transient period. Due The static var compensator (SVC)Filter is a widely used reactive power compensation device with the to the application of the proposed hybrid control scheme, theFilter induced overvoltage in the stator will configuration comprising a thyristor‐controlled reactor and thyristor‐switched capacitors. By not cause the converterFigure system 13. to Series lose dynamic control. braking As a result, resistor rotor protection overvoltage circuit. can be limited. At the injecting reactive power into the connected bus, the SVC can improve the transient stability of DFIG‐ same time, the DC-link capacitor charging current can be controlled to restrain DC-link overvoltage. 4.2.WTs Reactive and damp Power the Compensation power oscillations Device in transmission lines. 4.2. ReactiveThe static Power synchronous Compensation compensator Device (STATCOM) is a power electronic device based on the voltageThe staticsource var converter compensator principle. (SVC) Generally, is a widely the used STATCOM reactive comprises power compensation a two‐ or three device‐level with voltage the The static var compensator (SVC) is a widely used reactive power compensation device with the configurationsource converter comprising [45], as showna thyristor in Figure‐controlled 14. During reactor steady and ‐statethyristor conditions,‐switched this capacitors. device injects By configuration comprising a thyristor-controlled reactor and thyristor-switched capacitors. By injecting injectingreactive reactive power intopower or intoabsorbs the connectedreactive power bus, the from SVC the can power improve grid the to transientimprove stabilityvoltage regulationof DFIG‐ reactive power into the connected bus, the SVC can improve the transient stability of DFIG-WTs and WTsand andstability. damp Thus, the power the transfer oscillations capability in transmission can be enhanced. lines. To provide smooth and rapid reactive damp the power oscillations in transmission lines. powerThe compensationstatic synchronous for grid compensator voltage recovery, (STATCOM) the STATCOM is a power can electronic inject the device maximum based reactiveon the The static synchronous compensator (STATCOM) is a power electronic device based on the voltage voltagecurrent source into a converterpower grid principle. during fault Generally, conditions the STATCOM [1,45,46]. Compared comprises with a two thyristor‐ or three‐controlled‐level voltage static source converter principle. Generally, the STATCOM comprises a two- or three-level voltage source sourcevar compensators converter [45], (SVCs), as shown the STATCOM in Figure has14. Duringa similar steady capability‐state inconditions, stability improvement,this device injects but a converter [45], as shown in Figure 14. During steady-state conditions, this device injects reactive power reactivehigher controlpower intobandwidth. or absorbs Furthermore, reactive power the STATCOM from the powerimplemented grid to in improve DFIG‐WTs voltage can compensateregulation into or absorbs reactive power from the power grid to improve voltage regulation and stability. Thus, andcurrent stability. without Thus, depending the transfer on thecapability voltage can level be of enhanced. the connection To provide point, smooth which ensuresand rapid the reactive effective the transfer capability can be enhanced. To provide smooth and rapid reactive power compensation poweroperation compensation of the STATCOM for grid during voltage serious recovery, fault conditionsthe STATCOM [47–49]. can A GSCinject controller the maximum can successfully reactive for grid voltage recovery, the STATCOM can inject the maximum reactive current into a power grid currentreturn intothe DC a power‐link voltage grid during to a normal fault conditions value with [1,45,46]. the aid Comparedof the STATCOM with thyristor [50], which‐controlled is a necessary static during fault conditions [1,45,46]. Compared with thyristor-controlled static var compensators (SVCs), varcondition compensators for the RSC(SVCs), to restart. the STATCOM has a similar capability in stability improvement, but a thehigher STATCOM control bandwidth. has a similar Furthermore, capability inthe stability STATCOM improvement, implemented but in a DFIG higher‐WTs control can compensate bandwidth. Furthermore,current without the depending STATCOM on implemented the voltage inlevel DFIG-WTs of the connection can compensate point, which current ensures without the depending effective onoperation the voltage of the level STATCOM of theGearbox connection during seriousDFIG point, fault which conditions ensures [47–49]. the effective A GSC operation controller of can the successfully STATCOM duringreturn the serious DC‐link fault voltage conditions to a [ normal47–49]. value A GSC with controller the aid can of successfullythe STATCOM return [50], the which DC-link is a voltagenecessary to Power System acondition normal value for the with RSC the to aid restart. of the STATCOM [50], which is a necessary condition for the RSC to restart. RSC GSC DC Link

Rotor Gearbox DFIG Grid Filter Filter

Power System VSC RSC GSC Figure 14. Static synchronous compensatorDC Link (STATCOM) protection circuit.

Rotor Grid 4.3. Fault Current Limiter Filter Filter

Fault current limiters are now commonly used in large interconnectedVSC power systems to limit overcurrent during fault conditions. In this way, generators and transmission lines can be protected Figure 14. StaticStatic synchronous synchronous compensator (STATCOM) protection circuit.circuit. from being damaged. Furthermore, the FCL can maintain terminal voltage regulation by injecting 4.3.reasonable Fault Current resistance Limiter into a stator circuit and providing adaptive voltage compensation A superconducting fault current limiter (SFCL) is a typical configuration in the application of superconductors.Fault current limitersThe proposed are now SFCL commonly consists used of a incoupling large interconnected transformer (CT), power a controlled systems to switch, limit overcurrentand a superconducting during fault conditions. coil (SC), Inas this shown way, ingenerators Figure 15. and In transmission normal operation lines can conditions, be protected the fromcontrolled being damaged.switch is closed Furthermore, and the theflux FCL between can maintain the two CTterminal windings voltage can regulationoffset each by other. injecting Thus, reasonableonly the CTresistance operation into impedance a stator iscircuit implemented and providing in the circuit.adaptive During voltage transient compensation periods, theA superconducting fault current limiter (SFCL) is a typical configuration in the application of superconductors. The proposed SFCL consists of a coupling transformer (CT), a controlled switch, and a superconducting coil (SC), as shown in Figure 15. In normal operation conditions, the controlled switch is closed and the flux between the two CT windings can offset each other. Thus, only the CT operation impedance is implemented in the circuit. During transient periods, the Appl. Sci. 2020, 10, 2154 11 of 25

4.3. Fault Current Limiter Fault current limiters are now commonly used in large interconnected power systems to limit overcurrent during fault conditions. In this way, generators and transmission lines can be protected from being damaged. Furthermore, the FCL can maintain terminal voltage regulation by injecting reasonable resistance into a stator circuit and providing adaptive voltage compensation A superconducting fault current limiter (SFCL) is a typical conﬁguration in the application of superconductors. The proposed

SFCLAppl. Sci. consists 2020, 2, x of FOR a coupling PEER REVIEW transformer (CT), a controlled switch, and a superconducting coil (SC),11 of as25 Appl.shown Sci. in2020 Figure, 2, x FOR 15. PEER In normal REVIEW operation conditions, the controlled switch is closed and the ﬂux between11 of 25 thecontrolled two CT windingsswitch is can open, oﬀset which each other.results Thus, in onlythe thedestruction CT operation of non impedance‐inductive is implementedcoupling. The in controlled switch is open, which results in the destruction of non‐inductive coupling. The thesuperconducting circuit. During coil transient is quenched periods, to the its controlledhigh‐resistance switch state is open, to restrict which fault results current in the [51]. destruction Based on of superconducting coil is quenched to its high‐resistance state to restrict fault current [51]. Based on non-inductivethe operating principle coupling. of The the superconductingSFCL, this device coil is generally is quenched installed to its on high-resistance the stator side state of DFIG to restrict‐WTs the operating principle of the SFCL, this device is generally installed on the stator side of DFIG‐WTs faultto provide current high [51]. resistance Based on during the operating transient principle periods of [52]. the SFCL, Therefore, this device the stator is generally overcurrent installed can onbe to provide high resistance during transient periods [52]. Therefore, the stator overcurrent can be therestrained. stator side of DFIG-WTs to provide high resistance during transient periods [52]. Therefore, the restrained.stator overcurrent can be restrained.

SC SC

Figure 15. Superconducting fault current limiter. FigureFigure 15. 15. SuperconductingSuperconducting fault fault current current limiter. limiter. The saturated‐core fault current limiter (SCFCL) is another modified FCL topology which was TheThe saturated saturated-core‐core fault fault current current limiter limiter (SCFCL) (SCFCL) is is another modified modified FCL topology which was proposed in Reference [8]. The SCFCL is mainly composed of a superconducting DC coil and a proposed inin Reference Reference [8 [8].]. The The SCFCL SCFCL is mainly is mainly composed composed of a superconducting of a superconducting DC coil DC and coil a coupling and a coupling transformer in which AC coils are embedded in a saturated iron core. The DC coil is located couplingtransformer transformer in which in AC which coils areAC embeddedcoils are embedded in a saturated in a saturated iron core. iron The core. DC coil The is DC located coil is between located between two limbs of AC coils, as shown in Figure 16. In normal working conditions, the impedance betweentwo limbs two of limbs AC coils, of AC as coils, shown as in shown Figure in 16Figure. In normal 16. In normal working working conditions, conditions, the impedance the impedance of the of the SCFCL is negligible due to the saturated iron core. After a fault occurs, abruptly increased ofSCFCL the SCFCL is negligible is negligible due to due the saturatedto the saturated iron core. iron After core. a After fault occurs,a fault occurs, abruptly abruptly increased increased current current flows through AC coils, resulting in the iron core becoming unsaturated. Therefore, the currentflows through flows through AC coils, AC resulting coils, resulting in the iron in core the becomingiron core unsaturated.becoming unsaturated. Therefore, theTherefore, impedance the impedance of AC coils increases quickly, and the fault current can be restricted. The installation of impedanceof AC coils of increases AC coils quickly, increases and quickly, the fault and current the fault can current be restricted. can be Therestricted. installation The installation of an SCFCL of an SCFCL on the stator side, as shown in Figure 17, provides flexibility in stator resistance. Due to anon SCFCL the stator on side,the stator as shown side, in as Figure shown 17 in, providesFigure 17, flexibility provides in flexibility stator resistance. in stator Dueresistance. to the specificDue to the specific characteristics of the SCFCL, its external installation has no resistance under normal thecharacteristics specific characteristics of the SCFCL, of itsthe external SCFCL, installation its external has installation no resistance has underno resistance normal conditionsunder normal but conditions but provides a high resistance during transient periods to restrain stator overcurrent and conditionsprovides a but high provides resistance a high during resistance transient during periods transient to restrain periods stator to overcurrent restrain stator and overcurrent prevent voltage and prevent voltage dips in a DFIG [53]. preventdips in a voltage DFIG [ 53dips]. in a DFIG [53].

AC Coiling AC Coiling A A Grid Grid V DC Coiling V Iron Core DC Coiling Iron Core

Controller Controller Controllable CurrentControllable Supply Current Supply

Figure 16. SaturatedSaturated-core‐core fault current limiter (SCFCL). Figure 16. Saturated‐core fault current limiter (SCFCL).

Gearbox DFIG SCFCL Gearbox DFIG SCFCL Power System Power System Crowbar Crowbar RSC GSC RSC DC Link GSC DC Link

Rotor Grid Rotor Filter GridFilter Filter Filter Figure 17. DFIG system with SCFCL. Figure 17. DFIG system with SCFCL. A circuit configuration combining an FCL with a superconducting magnetic energy storage A circuit configuration combining an FCL with a superconducting magnetic energy storage (SMES) was proposed in Reference [54] to enhance the LVRT capability of DFIG‐WTs. During normal (SMES) was proposed in Reference [54] to enhance the LVRT capability of DFIG‐WTs. During normal Appl. Sci. 2020, 2, x FOR PEER REVIEW 11 of 25

controlled switch is open, which results in the destruction of non‐inductive coupling. The superconducting coil is quenched to its high‐resistance state to restrict fault current [51]. Based on the operating principle of the SFCL, this device is generally installed on the stator side of DFIG‐WTs to provide high resistance during transient periods [52]. Therefore, the stator overcurrent can be restrained.

Figure 15. Superconducting fault current limiter.

The saturated‐core fault current limiter (SCFCL) is another modified FCL topology which was proposed in Reference [8]. The SCFCL is mainly composed of a superconducting DC coil and a coupling transformer in which AC coils are embedded in a saturated iron core. The DC coil is located between two limbs of AC coils, as shown in Figure 16. In normal working conditions, the impedance of the SCFCL is negligible due to the saturated iron core. After a fault occurs, abruptly increased current flows through AC coils, resulting in the iron core becoming unsaturated. Therefore, the impedance of AC coils increases quickly, and the fault current can be restricted. The installation of an SCFCL on the stator side, as shown in Figure 17, provides flexibility in stator resistance. Due to the specific characteristics of the SCFCL, its external installation has no resistance under normal conditions but provides a high resistance during transient periods to restrain stator overcurrent and prevent voltage dips in a DFIG [53].

AC Coiling A Grid V DC Coiling Iron Core

Controller Controllable Current Supply

Appl. Sci. 2020, 10, 2154 12 of 25 Figure 16. Saturated‐core fault current limiter (SCFCL).

Appl. Sci. 2020, 2, x FOR PEER REVIEW 12 of 25

Gearbox DFIG SCFCL operation, the SMES circuit is activated to smooth the output power of wind turbines. When a fault occurs, the superconducting coil is injected into the circuit to restrain statorPower overcurrent. System In order to eliminate power fluctuationsCrowbar in power systems, the FCL resistance is reduced to its initial value during post‐fault periods, and the SMES‐FCLRSC returns to GSCoperating as an SMES [54]. DC Link

4.4. Series Grid‐Side Converter Rotor Grid Filter Filter The SGSC is a bidirectional power converter connected across the DC‐link, as shown in Figure 18. The SGSC has the capability toFigure regulateFigure 17. 17. DFIGDC DFIG‐link system system voltage with with by SCFCL. SCFCL. controlling the active power injection or absorption. Another function of the SGSC is to compensate for stator harmonic voltages by synthesizingAA circuit circuit a configurationharmonic configuration voltage combining combining content. an Inan FCL orderFCL with withto offset a a superconducting superconducting the influence of magnetic magneticnegative energy‐ sequenceenergy storage storage grid (SMES)voltage,(SMES) was the was proposedSGSC proposed is capable in in Reference Reference of injecting [54 [54]] to to series enhance enhance compensation the the LVRT LVRT capability capability voltage ofto of DFIG-WTs.balance DFIG‐WTs. the Duringstator During voltage. normal normal Theoperation, stator flux the SMEScan also circuit be controlled is activated by tothe smooth series voltage the output injection power through of wind the turbines. SGSC [55–58]. When a fault occurs,To theimprove superconducting the control coileffect is injectedof the SGSC, into the an circuit adaptive to restrain resonant stator controller overcurrent. was proposed In order in to Referenceeliminate power [59]. Through fluctuations voltage in power distortion systems, and the FCLunbalance resistance compensation, is reduced to the its initial hybrid value controller during post-faultregulates the periods, stator and voltage the SMES-FCL at its rated returns value toeven operating under asunbalanced an SMES [grid54]. voltage conditions. The simulation4.4. Series Grid-Side results proved Converter that this scheme could keep the stator voltage balanced and sinusoidal to enhance the generated power quality. A unified DFIG‐WT architecture comprising parallel grid‐side rectifiersThe SGSC(PGSRs) is a and bidirectional an SGSC powerwas proposed converter in connected Reference across [60]. In the this DC-link, scheme, as both shown the in PGSR Figure and 18. Thethe SGSC hasare theconnected capability to tothe regulate DC‐link DC-link and share voltage the by same controlling DC bus. the During active power sub‐synchronous injection or operation,absorption. the Another PGSR functionmaintains of the the DC SGSC‐link is voltage to compensate by injecting for stator power harmonic into the voltages rotor circuit. by synthesizing The SGSC maintainsa harmonic the voltage stator content.flux magnitude In order in to a odesirableffset the influencerange to meet of negative-sequence steady‐state power grid requirements. voltage, the TheSGSC hybrid is capable control of injectingscheme equips series compensationthe DFIG with voltage the capability to balance of unencumbered the stator voltage. power The processing stator flux andcan alsoenhances be controlled the LVRT by capability. the series voltage injection through the SGSC [55–58].

SGSC Power System

RSC GSC

Gearbox DFIG Grid DC Link Filter

Figure 18. Series grid-side converter protection circuit. Figure 18. Series grid‐side converter protection circuit. To improve the control effect of the SGSC, an adaptive resonant controller was proposed in 4.5.Reference Comparative [59]. Results Through Analysis voltage of Stator distortion‐Side External and unbalance Retrofit Techniques compensation, the hybrid controller regulatesStator the‐side stator external voltage retrofit at its techniques rated value show even underdesirable unbalanced performance grid voltagein maintaining conditions. the The terminalsimulation voltage results and proved restraining that this DC scheme‐link overvoltage. could keep To the compare stator voltage the control balanced effect and of sinusoidalvarious to statorenhance‐side the external generated retrofit power techniques, quality. A the unified same DFIG-WTtest system architecture and fault conditions comprising as parallel in Section grid-side 3 wererectifiers adopted. (PGSRs) Typical and an models SGSC wasof an proposed SDBR [44], in Reference STATCOM [60 [50],]. In FCL this scheme,[53], and both SGSC the [57] PGSR were and the implementedSGSC are connected in MATLAB/Simulink, to the DC-link and and share the results the same are DC shown bus. in During Figure sub-synchronous 19. Figure 19a shows operation, the PGSRcontrol maintains effect in maintaining the DC-link voltageterminal by voltage. injecting By power providing into thefast rotor and smooth circuit. Thereactive SGSC power maintains compensation,the stator flux magnitude the STATCOM in a desirableprovided range better to performance meet steady-state in improving power requirements. the terminal voltage The hybrid dynamicscontrol scheme than equipsthe SDBR. the DFIGIn addition, with the the capability FCL could of unencumbered maintain terminal power voltage processing regulation and enhances by injectingthe LVRT capability.reasonable resistance into the stator circuit and providing adaptive voltage compensation. The SGSC focuses on keeping terminal voltage balanced and sinusoidal; thus, it is not presented in Figure 19a. Figure 19b shows the control effect of maintaining DC‐link voltage. By controlling the active power injection or absorption, the SGSC can regulate DC‐link voltage during grid faults. The SDBR aids the RSC and GSC in maintaining controllability of DC‐link voltage. However, more serious DC‐link voltage drops occurred when the SDBR was implemented in DFIG‐WTs compared with the SGSC and STATCOM. Appl. Sci. 2020, 10, 2154 13 of 25

4.5. Comparative Results Analysis of Stator-Side External Retrofit Techniques Stator-side external retrofit techniques show desirable performance in maintaining the terminal voltage and restraining DC-link overvoltage. To compare the control effect of various stator-side external retrofit techniques, the same test system and fault conditions as in Section3 were adopted. Typical models of an SDBR [44], STATCOM [50], FCL [53], and SGSC [57] were implemented in MATLAB/Simulink, and the results are shown in Figure 19. Figure 19a shows the control effect in maintaining terminal voltage. By providing fast and smooth reactive power compensation, the STATCOM provided better performance in improving the terminal voltage dynamics than the SDBR. In addition, the FCL could maintain terminal voltage regulation by injecting reasonable resistance into the stator circuit and providing adaptive voltage compensation. The SGSC focuses on keeping terminal voltage balanced and sinusoidal; thus, it is not presented in Figure 19a. Figure 19b shows the control effect of maintaining DC-link voltage. By controlling the active power injection or absorption, the SGSC can regulate DC-link voltage during grid faults. The SDBR aids the RSC and GSC in maintaining Appl. Sci. 2020, 2, x FOR PEER REVIEW 13 of 25 controllability of DC-link voltage. However, more serious DC-link voltage drops occurred when the SDBR was implemented in DFIG-WTs compared with the SGSC and STATCOM.

1.2 1.1

1 1.05

SDBR 1 0.8 STATCOM FCL SDBR 0.6 0.95 STATCOM Vdc/p.u.

Us/p.u. SGSC 0.4 0.9

0.2 0.85

0 0.8 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 t/s t/s (a) (b)

FigureFigure 19. 19.Control Control eﬀ ecteffect comparison comparison of of stator-side stator‐side external external retroﬁt retrofit techniques techniques in (ina) ( terminala) terminal voltage, voltage, (b)( DC-linkb) DC‐link voltage. voltage.

5.5. Internal Internal Control Control Techniques Techniques

InIn addition addition to to implementing implementing extra extra devices devices to to enhance enhance the the LVRT LVRT capability capability of of DFIG-WTs, DFIG‐WTs, the the performanceperformance of DFIG-WTsof DFIG‐WTs during during transient transient periods periods can can be be improved improved by by optimizing optimizing control control strategies strategies andand parameters. parameters. Various Various advanced advanced control control methods methods were were applied applied in in wind wind turbines, turbines, RSCs, RSCs, or or GSCs GSCs toto make make full full use use of of the the DFIG’s DFIG’s own own control control capability. capability. The The application application of of linear linear control control methods methods and and nonlinearnonlinear control control methods methods can can provide provide support support during during grid grid voltage voltage dips. dips. 5.1. Wind Turbine Control 5.1. Wind Turbine Control In variable-speed wind turbines, the use of blade pitch angle (BPA) control is becoming common In variable‐speed wind turbines, the use of blade pitch angle (BPA) control is becoming common to control the output power and protect generators from sudden wind gusts. During normal operation, to control the output power and protect generators from sudden wind gusts. During normal when wind speed exceeds the threshold value, blade pitch angle controllers can modify the speed of operation, when wind speed exceeds the threshold value, blade pitch angle controllers can modify rotor windings to maintain tip speed ratio. In this case, output power can be kept at the rated value, the speed of rotor windings to maintain tip speed ratio. In this case, output power can be kept at the and the electrical and mechanical stress of wind turbines can be limited [61,62]. rated value, and the electrical and mechanical stress of wind turbines can be limited [61,62]. During the transient period, grid voltage dips can lead to imbalance between the high input power During the transient period, grid voltage dips can lead to imbalance between the high input from wind and the low electrical output power to the grid, which may result in overspeed. Blade pitch power from wind and the low electrical output power to the grid, which may result in overspeed. controllers can protect rotor windings from overspeed and possible runaway conditions. Furthermore, Blade pitch controllers can protect rotor windings from overspeed and possible runaway conditions. an advanced control technique was proposed to absorb the wind energy which cannot be injected to Furthermore, an advanced control technique was proposed to absorb the wind energy which cannot the power grid during faults. This control scheme converts additional power to kinetic energy of wind be injected to the power grid during faults. This control scheme converts additional power to kinetic energy of wind turbines by reducing rotor torque, which may also lead to the rotor speed increasing during grid faults. The blade pitch angle control is triggered once the rotor speed exceeds the threshold value. Therefore, the hybrid control scheme can improve wind energy efficiency on the premise of safe operation. To improve the control effect during severe disturbances and avoid the torque ripple caused by conventional proportion integration differentiation (PID) controllers, adaptive fuzzy‐PID controllers can be utilized in variable‐speed wind turbines [63]. Fuzzy‐PID controllers endow blade pitch angle controllers with the capability to maintain control even under unbalanced faults, as well as handle situations with unstable wind power.

5.2. RSC Control In recent years, various internal control methods were proposed from the perspective of RSC

control to enhance the LVRT capability of DFIG‐WTs, as shown in Figure 20. 𝑈 and 𝑈 denote the d‐ and q‐axis RSC output voltage. RSC control methods can be classified as linear methods and nonlinear methods. Appl. Sci. 2020, 10, 2154 14 of 25 turbines by reducing rotor torque, which may also lead to the rotor speed increasing during grid faults. The blade pitch angle control is triggered once the rotor speed exceeds the threshold value. Therefore, the hybrid control scheme can improve wind energy efficiency on the premise of safe operation. To improve the control effect during severe disturbances and avoid the torque ripple caused by conventional proportion integration differentiation (PID) controllers, adaptive fuzzy-PID controllers can be utilized in variable-speed wind turbines [63]. Fuzzy-PID controllers endow blade pitch angle controllers with the capability to maintain control even under unbalanced faults, as well as handle situations with unstable wind power.

5.2. RSC Control In recent years, various internal control methods were proposed from the perspective of RSC control to enhance the LVRT capability of DFIG-WTs, as shown in Figure 20. Urd and Urq denote the d- and q-axis RSC output voltage. RSC control methods can be classiﬁed as linear methods and nonlinearAppl. Sci. 2020 methods., 2, x FOR PEER REVIEW 14 of 25

FigureFigure 20.20. Rotor-sideRotor‐side converterconverter (RSC)(RSC) controller.controller.

5.2.1.5.2.1. LinearLinear RSCRSC ControlControl MethodsMethods VectorVector controlcontrol isis the the most most widely widely used used methodmethod inin DFIGDFIG toto achieveachieve thethe independentindependent controlcontrol ofof didifferentfferent system system parameters. parameters. InIn vectorvector control,control, thethe inductioninduction generatorgenerator isis controlledcontrolled inin aa synchronoussynchronous referencereference frameframe toto decoupledecouple electromagneticelectromagnetic torquetorque andand rotorrotor excitationexcitation currentcurrent [[64].64]. Moreover,Moreover, thethe currentcurrent vector vector is is decomposed decomposed into into components components of of stator stator active active power power and and stator stator reactive reactive power power in the in stator-flux-orientedthe stator‐flux‐oriented reference reference frame. frame. Typical Typical RSC proportional–integral RSC proportional–integral (PI) controllers (PI) controllers are capable are of regulatingcapable of rotor regulating speed, reactiverotor speed, power, reactive and rotor power, current and [65 ,66rotor]. Particle current swarm [65,66]. optimization Particle swarm (PSO) wasoptimization proposed (PSO) in Reference was proposed [67] to tune in Reference the PI controller [67] to parameters,tune the PI therebycontroller improving parameters, the controlthereby performanceimproving the of control PI controllers. performance of PI controllers. However,However, the the closed-loop closed‐loop structure structure of conventional of conventional vector controlvector maycontrol lead may to complex lead to calculation complex andcalculation low dynamic and low response dynamic in response DFIGs. in To DFIGs. solve To these solve problems, these problems, a combined a combined vector andvector direct and powerdirect power control control (CVDPC) (CVDPC) was proposed was proposed in Reference in Reference [66] to [66] provide to provide faster dynamic faster dynamic response, response, lower computation,lower computation, and simpler and simpler implementation. implementation. BasedBased onon thethe mechanismmechanism thatthat DCDC fluxflux cancan bebe transformed transformed intointo compensationcompensation current,current, currentcurrent compensationcompensation (CC) (CC) control control was was proposed proposed in in Reference Reference [68 [68]] to improve to improve the LVRTthe LVRT capability capability of DFIG-WTs. of DFIG‐ AsWTs. an As eff ectivean effective RSC control RSC control method, method, this control this control technique technique makes makes use use of compensation of compensation current current to counteractto counteract the the DC DC flux flux caused caused by faults;by faults; thus, thus, transient transient current current and and rotor rotor voltage voltage can can be limited. be limited. To coordinateTo coordinate with with the the current current compensation compensation control, control, a crowbara crowbar circuit circuit is is installed installed in in the the DC-link DC‐link toto actact asas aa releaserelease channelchannel forfor extra extra power. power. TheThe optimaloptimal activeactive currentcurrent compensationcompensation controllercontroller cancan eeffectivelyffectively improveimprove thethe LVRTLVRT capability capability of of DFIG-WTs. DFIG‐WTs. Due Due to to the the mechanism mechanism thatthat reasonablereasonable activeactive currentcurrent injectioninjection cancan significantlysignificantly improveimprove thethe smallsmall signalsignal stabilitystability ofof DFIG-WTs,DFIG‐WTs, aa novelnovel optimaloptimal activeactive andand reactivereactive currentcurrent proportionproportion methodmethod waswas proposedproposed inin Reference Reference [ [69].69]. TheThe optimaloptimal activeactive injectioninjection current current varies varies with with the the active active current current reference, reference, which which can be can calculated be calculated from the from fault the position. fault position. Simulation and experimental results showed that the proposed scheme can effectively reduce transient peak voltage and the voltage oscillations can be reduced during transient periods. A feedforward transient compensation (FFTC) controller was proposed in Reference [70]. Compensation terms for DFIG stator transient voltages are feedforward injected into the current control loop and power control loop to improve the control effect of DFIG‐WTs in transient conditions. For the current loop, complete transient voltages are injected into the rotor‐current control loop by feedforward terms to regulate the transient rotor current. For the power loop, 60‐ and 120‐ Hz components are feedforward injected into the rotor current; thus, desired tracking of the rotor current is achieved. In this case, an FFTC controller can improve the capability of transient rotor current control and minimizes the DFIG control lost during both balanced and unbalanced grid faults. Therefore, the LVRT capability of DFIG‐WTs can be enhanced. There are other two feedforward control methods that improve the transient capability of DFIG‐WTs by adding feedforward terms: (1) feedforward current reference control (FCRC) was proposed to achieve zero‐ error tracking for the rotor current references after grid faults. By introducing the additional rotor current feedforward to the current loop, the rotor current can be maintained in a balanced and sinusoidal manner [71]; (2) feed‐forward transient current control (FFTCC) was proposed to reduce Appl. Sci. 2020, 10, 2154 15 of 25

Simulation and experimental results showed that the proposed scheme can effectively reduce transient peak voltage and the voltage oscillations can be reduced during transient periods. A feedforward transient compensation (FFTC) controller was proposed in Reference [70]. Compensation terms for DFIG stator transient voltages are feedforward injected into the current control loop and power control loop to improve the control effect of DFIG-WTs in transient conditions. For the current loop, complete transient voltages are injected into the rotor-current control loop by feedforward terms to regulate the transient rotor current. For the power loop, 60- and 120-Hz components are feedforward injected into the rotor current; thus, desired tracking of the rotor current is achieved. In this case, an FFTC controller can improve the capability of transient rotor current control and minimizes the DFIG control lost during both balanced and unbalanced grid faults. Therefore, the LVRT capability of DFIG-WTs can be enhanced. There are other two feedforward control methods that improve the transient capability of DFIG-WTs by adding feedforward terms: (1) feedforward current reference control (FCRC) was proposed to achieve zero-error tracking for the rotor current references after grid faults. By introducing the additional rotor current feedforward to the current loop, the rotor current can be maintained in a balanced and sinusoidal manner [71]; (2) feed-forward transient current control (FFTCC) was proposed to reduce transient current stress on the RSC and ensure that the active and reactive power supply remains uninterrupted by adding the stator flux-related feedforward components to the current loop [72].

5.2.2. Nonlinear RSC Internal Control Since conventional PID controllers are designed based on linearized system models, they usually show poor performance during severe grid faults. Thus, many nonlinear control methods were proposed to provide better control effect for DFIG-WTs under large disturbances. Sliding mode control (SMC) is an intermittent control signal-based nonlinear control method. In Reference [73], SMC was applied in DFIG-WTs to restrict the oscillations in electromagnetic torque and stator active power, which improved the system robustness. However, the application of SMC may lead to the decreasing capability to generate power under large disturbances. A decentralized nonlinear control (DNC) method was proposed in Reference [74] to improve the transient stability of DFIG-WTs. Based on the differential geometry theory, this LVRT technique can linearize the complex DFIG model and realize optimal control for DFIG-WTs. This exact linearization based method can significantly improve the LVRT capability compared to the conventional PI controller. However, due to the fluctuation in stator terminal voltage, the strict assumption of exact linearization is hard to achieve, which limits the application of the proposed method. Input-to-state stability (ISS)-based control was applied to DFIG-WTs in Reference [75]. Based on the mechanism where the existence of ISS control Lyapunov functions can be used to demonstrate the input-to-state stability of nonlinear systems, a decentralized ISS control law was proposed for the RSC to enhance the transient stability of wind turbines. However, ISS-based controllers can only ensure the inverse optimality, while combination with other control techniques is needed to further optimize the ISS control parameters. By reasonably designing the state-dependent coefficient (SDC) matrices and weighting matrices, an effective RSC control scheme based on the state-dependent Riccati equation (SDRE) technique was proposed in Reference [19]. The implementation of the SDRE control method can simplify control and mitigate the impact of grid voltage dips. Moreover, different selections of SDC matrices lead to different control performance, whereas the extra freedom in selecting non-unique SDC matrices makes the robust H controller design more flexible [76]. ∞ Fuzzy control can provide desirable control performance for DFIG-WTs during severe disturbances such as three-phase short-circuit faults and sudden changes in wind speed, while linear control schemes cannot predict the behavior of DFIGs correctly in this case [77,78]. The fuzzy logic controller is known for its adaptability to nonlinear systems and the function of reducing computational complexity. In Reference [78], two fuzzy logic controllers were implemented in RSCs. One was applied to control the direct current proportional to the generated reactive power, and the other was applied to control the Appl. Sci. 2020, 10, 2154 16 of 25 quadrature current proportional to the generated active power. Simulation and experimental results showed that the fuzzy logic controller has a faster and a more accurate response when compared with PI controllers. Compared with the fuzzy logic controller, the interval type-2 fuzzy PI controller (IT2-FPI) is generally more robust and has a smoother control surface around the steady state [79]. Thus, the IT2-FPI can also be applied in DFIGs to provide ideal capability of handling uncertainties during grid voltage dips. The proposed strategy can provide a satisfactory performance in power oscillation damping without the need to retune controller parameters. Furthermore, a fuzzy operator was applied in DFIG-WTs to scale the variables of optimization including PI gains of DFIG controllers and SFCL parameters in Reference [53]. The fuzzy operator scaled the objective functions within the range of [0, 1], and the max-geometric mean operator aggregated the scaled objective functions. This optimization algorithm could smooth output power and improve the LVRT capability through multi-objective optimization.

5.3. GSC Control The GSC is an important device with the function of controlling DC-link voltage and the reactive power exchange with the grid. With properly designed GSC controllers, DC-link overvoltage can be restricted during large grid voltage disturbances, as shown in Figure 21. Ugd and Ugq denote the d- and q-Appl.axis Sci. GSC 2020 output, 2, x FOR voltage. PEER REVIEW 16 of 25

FigureFigure 21.21. Grid-sideGrid‐side converterconverter (GSC)(GSC) controller.controller.

InIn ReferenceReference [[80],80], aa coordinatedcoordinated controlcontrol methodmethod composedcomposed ofof aa PIPI controllercontroller andand resonantresonant compensationcompensation was was applied applied in in DFIG-WTs DFIG‐WTs to to restrict restrict torque torque and and active active power power oscillations. oscillations. Therefore, Therefore, the dynamicthe dynamic performance performance of DFIG-WTs of DFIG under‐WTs unbalanced under unbalanced conditions couldconditions be improved. could be In Referenceimproved. [81 In], aReference PI-DFR controller [81], a PI‐ comprisingDFR controller a PI comprising regulator and a PI a dual-frequencyregulator and a resonantdual‐frequency (DFR) wasresonant adopted (DFR) in DFIG-WTs.was adopted By in applying DFIG‐WTs. this coordinatedBy applying control, this coordinated the DC-link control, voltage the oscillations DC‐link voltage could be oscillations restricted, andcould constant be restricted, active powerand constant output active could power be ensured. output Both could the be RSC ensured. and the Both GSC the were RSC controlledand the GSC to providewere controlled a smooth to and provide optimized a smooth operation. and optimized operation. However,However, thethe linearlinear controlcontrol methodsmethods appliedapplied inin GSCGSC cannotcannot predictpredict thethe behaviorbehavior ofof DFIG-WTsDFIG‐WTs duringduring severesevere disturbances.disturbances. A A discrete discrete-time‐time inverse optimal control method based onon aa neuralneural networknetwork was proposed proposed in in Reference Reference [82] [82 to] toprovide provide better better dynamic dynamic performance performance for DFIG for DFIG-WTs.‐WTs. Due Dueto the to on the‐line on-line identification identiﬁcation property property of neural of neural networks, networks, the real the‐time real-time neural neuralinverse inverse optimal optimal (RNIO) (RNIO)controller controller design designcan be can achieved be achieved without without system system parameters parameters given, given, in in which which case case the the systemsystem robustnessrobustness cancan bebe improved.improved. SlidingSliding modemode controlcontrol cancan alsoalso bebe appliedapplied inin GSCGSC internalinternal controlcontrol toto alteralter thethe systemsystem dynamics. TheThe proposed proposed sliding sliding mode mode control control can can control control the GSCthe GSC to ensure to ensure desirable desirable DC-link DC voltage‐link voltage and steady and activesteady power active output.power output. In Reference In Reference [83], a nonlinear[83], a nonlinear coordinated coordinated control control method method was applied was applied to the GSCto the of GSC DFIG-WTs of DFIG to‐ stabilizeWTs to thestabilize internal the dynamics. internal dynamics. By using the By proposed using the method, proposed rotor method, overcurrent rotor andovercurrent DC-link overvoltageand DC‐link could overvoltage be restrained could and,be restrained thus, the LVRTand, thus, capability the LVRT could capability be enhanced. could be enhanced. Appl. Sci. 2020, 2, x FOR PEER REVIEW 17 of 25

5.4. Comparative Results Analysis of Internal Control Techniques Internal control techniques can be implemented in the RSC and GSC to restrain rotor current, restrict stator voltage oscillations, and maintain DC‐link voltage. To compare the control effect of various internal control techniques, the same test system and fault conditions as in Section 3 were adopted. Furthermore, all internal techniques adopted the same active crowbar circuits during the initial fault period to avoid the influence of protection circuits, and the duration of the active crowbar was set as 0.02 s. During the transient period, the simulation system applied five RSC control methods in DFIG‐WTs for comparison: conventional PI controller, SMC method [73], DNC method [74], ISS‐ based controller [75], and SDRE‐based controller [76]. The comparative results are shown in Figure 22. At the initial period, the terminal voltage dropped rapidly to around 0.1 p.u. due to the severe threeAppl.‐phase Sci. 2020 to ,ground10, 2154 fault, as shown in Figure 22c. Stator perturbations induced oscillations in rotor17 of 25 side, and the rotor current exceeded 2 p.u., as shown in Figure 22a. In this situation, the active crowbar circuit was activated. It can be seen from Figure 22a that the rotor current of the ISS controller and5.4. SDRE Comparative controller Results could Analysisbe maintained of Internal at a Controlhigh value Techniques to make full use of the control capability of converters.Internal In Figure control 22b, techniques the wind farm can bewith implemented an SDRE controller in the RSC could and provide GSC to more restrain reactive rotor power current, supportrestrict during stator the voltage transient oscillations, period. Thus, and maintainbetter voltage DC-link dynamics voltage. could To compare be achieved, the controlas shown eff ectin of Figurevarious 22c. internal control techniques, the same test system and fault conditions as in Section3 were adopted.Since the Furthermore, design of RSC all internal techniquescontrol methods adopted pays the less same attention active crowbaron DC‐link circuits overvoltage during the restriction,initial fault the periodSMC method, to avoid DNC the influence method, ofISS protection‐based controller, circuits, and theSDRE duration‐based ofcontroller the active are crowbar not presentedwas set asin 0.02the s.comparison During the of transient DC‐link period, voltage the simulationdynamics. systemThe conventional applied five PI RSC controller, control methods fuzzy in controlDFIG-WTs method for [79], comparison: and coordinated conventional control PI method controller, [83] SMC were method implemented [73], DNC in the method simulation, [74], ISS-based and the controllerresults are [75 ],shown and SDRE-based in Figure controller22d. The [76PI]. controller The comparative could resultseffectively are shown restrain in Figurethe DC 22‐.link At the overvoltageinitial period, in the the initial terminal fault voltageperiod, but dropped a large rapidly voltage to drop around occurred 0.1 p.u. later, due which to the could severe influence three-phase the tonormal ground operation fault, as of shown converters. in Figure Compared 22c. Stator with perturbations the PI controller, induced the coordinated oscillations control in rotor method side, and couldthe significantly rotor current restrict exceeded DC‐ 2link p.u., voltage as shown drops. in Figure Through 22a. parameter In this situation, optimization, the active the fluctuations crowbar circuit in DCwas‐link activated. voltage It could can be be seen limited from via Figure the fuzzy 22a that control the rotormethod current during of grid the ISS faults. controller It can be and seen SDRE fromcontroller Figure 22d could that be fluctuations maintained in at aDC high‐link value voltage to make occurred full use after of fault the control clearance. capability Thus, ofthe converters. fuzzy controlIn Figure method 22 b,should the wind be combined farm with with an SDREother protection controller couldcircuits provide such as more a DC‐ reactivechopper power to improve support the duringcontrol the performance. transient period. Thus, better voltage dynamics could be achieved, as shown in Figure 22c.

2.5 5 PI PI SMC 4 SMC DNC DNC 2 ISS 3 ISS SDRE SDRE 2

1.5 1

0 Ir/p.u. Q/p.u. 1 -1

-2 0.5 -3

-4 0 0 0.05 0.1 0.15 0.2 0.25 0.3 -5 0 0.05 0.1 0.15 0.2 0.25 0.3 t/s t/s .

Appl. Sci. 2020, 2, x FOR PEER REVIEW(a) (b) 18 of 25

1.2 1.2 PI Coordinated Control 1 Fuzzy Control

PI 0.8 SMC DNC ISS 1 Vdc/p.u. 0.6 SDRE Us/p.u.

0.4

0.2 0.8 0 0.05 0.1 0.15 0.2 0.25 0 0 0.05 0.1 0.15 0.2 0.25 0.3 t/s t/s (c) (d)

FigureFigure 22. 22.ControlControl effect eﬀ ectcomparison comparison of internal of internal control control techniques techniques in ( ina) (rotora) rotor current, current, (b) (reactiveb) reactive powerpower support, support, (c) (terminalc) terminal voltage, voltage, and and (d) ( DCd) DC-link‐link voltage. voltage.

6. Conclusion and Future Trend Exploration This paper gave a comprehensive review of the proposed LVRT solutions used in DFIG‐WTs, including the mature techniques applied in engineering practice and the latest theoretical achievements. All LVRT solutions were classified into three categories according to their location and the ways they work. Both advantages and disadvantages were discussed in this paper, and the modified topologies or methods were also given. External device installations inject specific circuits into DFIG‐WTs by controlling the state of electronic power switches during grid faults. In this way, the wind turbine configurations can be modified to restrain stator overcurrent and rotor overcurrent, provide power support, and restrict overvoltage, such that the LVRT capability of DFIG‐WTs can be enhanced. External retrofit techniques are widely used in pre‐installed wind turbines for simplicity. However, economic concerns still act as a major barrier for external retrofit techniques, and the difficulties in modifying the original control structure also restrict the application of external installations. Although external modifications are not the future direction of LVRT solutions, the installation of external LVRT devices with specific function is necessary. Internal control techniques apply advanced control theories to strengthen the LVRT capability of DFIG‐WTs. By making some compensations on voltage and current or improving the control performance in nonlinear uncertain systems, internal control modifications can provide better control effects during grid voltage dips. Because the LVRT capability of DFIG‐WTs can be enhanced without any external auxiliary circuits, internal control techniques have economic advantages over external retrofit techniques. Thus, internal control techniques are preferred in newly installed wind turbines, and they have broad prospects for future development. Inspired by the summary and comparative results in References [1] and [4], the above‐discussed LVRT techniques and their effects are presented in Table 1 and Table 2 to indicate the functions of various LVRT solutions. In this table, √ indicates that the chosen LVRT technique provides effective control to eliminate the grid fault effect, － indicates that the chosen LVRT solution has no effect on the fault, and ＊ indicates that the chosen LVRT technique may deteriorate the grid fault effect. In terms of advantages in the cost factor and the flexibility in modification, internal control techniques will make substantial progress in the future. New control theories will also be proposed to provide more flexible and effective performance during grid voltage dips. The LVRT solutions combined with artificial intelligence will be a hot field. For example, artificial intelligence techniques can endow DFIG‐WTs with the ability to achieve on‐line optimal tuning, and supplementary learning structures can be implemented in DFIG‐WTs to improve the LVRT capability. In addition, external protection circuits combined with internal control techniques should be studied in detail to meet the requirement of modifying existing DFIG‐WTs.

Appl. Sci. 2020, 10, 2154 18 of 25

Since the design of RSC internal control methods pays less attention on DC-link overvoltage restriction, the SMC method, DNC method, ISS-based controller, and SDRE-based controller are not presented in the comparison of DC-link voltage dynamics. The conventional PI controller, fuzzy control method [79], and coordinated control method [83] were implemented in the simulation, and the results are shown in Figure 22d. The PI controller could effectively restrain the DC-link overvoltage in the initial fault period, but a large voltage drop occurred later, which could influence the normal operation of converters. Compared with the PI controller, the coordinated control method could significantly restrict DC-link voltage drops. Through parameter optimization, the fluctuations in DC-link voltage could be limited via the fuzzy control method during grid faults. It can be seen from Figure 22d that fluctuations in DC-link voltage occurred after fault clearance. Thus, the fuzzy control method should be combined with other protection circuits such as a DC-chopper to improve the control performance.

6. Conclusions and Future Trend Exploration This paper gave a comprehensive review of the proposed LVRT solutions used in DFIG-WTs, including the mature techniques applied in engineering practice and the latest theoretical achievements. All LVRT solutions were classified into three categories according to their location and the ways they work. Both advantages and disadvantages were discussed in this paper, and the modified topologies or methods were also given. External device installations inject specific circuits into DFIG-WTs by controlling the state of electronic power switches during grid faults. In this way, the wind turbine configurations can be modified to restrain stator overcurrent and rotor overcurrent, provide power support, and restrict overvoltage, such that the LVRT capability of DFIG-WTs can be enhanced. External retrofit techniques are widely used in pre-installed wind turbines for simplicity. However, economic concerns still act as a major barrier for external retrofit techniques, and the difficulties in modifying the original control structure also restrict the application of external installations. Although external modifications are not the future direction of LVRT solutions, the installation of external LVRT devices with specific function is necessary. Internal control techniques apply advanced control theories to strengthen the LVRT capability of DFIG-WTs. By making some compensations on voltage and current or improving the control performance in nonlinear uncertain systems, internal control modifications can provide better control effects during grid voltage dips. Because the LVRT capability of DFIG-WTs can be enhanced without any external auxiliary circuits, internal control techniques have economic advantages over external retrofit techniques. Thus, internal control techniques are preferred in newly installed wind turbines, and they have broad prospects for future development. Inspired by the summary and comparative results in References [1] and [4], the above-discussed LVRT techniques and their effects are presented in Tables1 and2 to indicate the functions of various LVRT solutions. In this table, √ indicates that the chosen LVRT technique provides effective control to eliminate the grid fault effect, indicates that the chosen LVRT solution has no effect on the fault, and * − indicates that the chosen LVRT technique may deteriorate the grid fault effect. Appl. Sci. 2020, 10, 2154 19 of 25

Table 1. Inﬂuences of external LVRT techniques on low-voltage faults.

Active Reactive DC-Link High Rotor Stator Rotor Stator High High High LVRT Capacity Power Power Voltage DC-Link Voltage Voltage Current Current Rotor Rotor Stator Technique Support Support Oscillations Voltage Oscillations Oscillations Oscillations Oscillations Voltage Current Current * √ * √ √ Rotor crowbar − − − − − − √ √ * DC-chopper − − − − − − − − √ √ √ √ √ Series dynamic resistor − − − − − − √ √ √ √ √ * √ * √ Energy storage system − − Series braking √ √ √ √ − − − − − − − dynamic resistor √ √ √ √ * * STATCOM − − − − − √ √ Fault current limiter − − − − − − − − − Series grid side √ √ √ √ − − − − − − − converter Appl. Sci. 2020, 10, 2154 20 of 25

Table 2. Inﬂuences of internal LVRT techniques on low-voltage faults. BPA—blade pitch angle; CC—capacitor crowbar; SMC—sliding mode control; DNC—decentralized nonlinear control; ISS—input-to-state stability; SDRE—state-dependent Riccati equation; PI—proportional–integral; DFR—dual-frequency resonant; RNIO—real-time neural inverse optimal.

Active Reactive DC-Link High Rotor Stator Rotor Stator High High High LVRT Power Power Voltage DC-Link Voltage Voltage Current Current Rotor Rotor Stator Capacity Support Support Oscillations Voltage Oscillations Oscillations Oscillations Oscillations Voltage Current Current Technique BPA ** √ √ √ √ √ √ √ √ √ control Vector √ √ √ √ √ − − − − − − control √ √ √ √ √ √ CC control − − − − − Feedforward √ √ √ √ − − − − − − − control SMC √ √ √ √ − − − − − − − control DNC √ √ √ √ √ √ − − − − − control √ √ √ √ √ √ √ ISS control − − − − SDRE √ √ √ √ √ √ √ √ − − − control Fuzzy √ √ √ √ √ √ − − − − − control PI-R √ √ √ √ − − − − − − − control PI-DFR √ √ √ √ √ √ − − − − − control √ √ RNIO − − − − − − − − − -coordinated control in √ √ − − − − − − − − − Reference [83] Appl. Sci. 2020, 10, 2154 21 of 25

In terms of advantages in the cost factor and the flexibility in modification, internal control techniques will make substantial progress in the future. New control theories will also be proposed to provide more flexible and effective performance during grid voltage dips. The LVRT solutions combined with artificial intelligence will be a hot field. For example, artificial intelligence techniques can endow DFIG-WTs with the ability to achieve on-line optimal tuning, and supplementary learning structures can be implemented in DFIG-WTs to improve the LVRT capability. In addition, external protection circuits combined with internal control techniques should be studied in detail to meet the requirement of modifying existing DFIG-WTs.

Author Contributions: Conceptualization, B.Q.; methodology, B.Q., H.L., and W.L.; software, H.L. and B.Q.; validation, H.L. and X.Z.; investigation, H.L. and J.L.; writing—original draft preparation, B.Q., H.L. and X.Z.; writing—review and editing, H.L., X.Z., W.L., and J.L.; supervision, B.Q. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by the National Natural Science Foundation of China (51707147, U1766215), the Key Research and Development Program of Shaanxi (2017ZDCXL-GY-02-03), and the Project of Shaanxi Electric Power Corporation (B626KY190005) Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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