A Novel Fault-Tolerant DFIG-Based Wind Energy

8/9/2019 A Novel Fault-Tolerant DFIG-Based Wind Energy

1/10

1296 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 29, NO. 3, MAY 2014

A Novel Fault-Tolerant DFIG-Based Wind EnergyConversion System for Seamless Operation

During Grid FaultsParag Kanjiya, Bharath Babu Ambati, and Vinod Khadkikar , Member, IEEE

Abstract— A novel fault-tolerant configuration of doubly fedinduction generator (DFIG) for wind energy conversion systems

(WECSs) is proposed in this paper for the seamless operationduring all kinds of grid faults. The proposed configuration is de-

veloped by replacing the traditional six-switch grid-side converter(GSC) of DFIG with a nine-switch converter. With the additionalthree switches, the nine-switch converter can provide six inde-pendent output terminals. One set of three output terminals are

connected to the grid through interfacing inductors to realizenormal GSC operation while, the other set of three output termi-

nals are connected to neutral side of the stator windings to providefault ride-through (FRT) capability to the DFIG. An appropriatecontrol algorithm is developed for the proposed configurationthat: 1) achieves seamless fault ride-through during any kind of

grid faults and 2) strictly satisfies new grid codes requirements.

The effectiveness of the proposed configuration in riding throughdifferent kind of faults is evaluated through detailed simulationstudies on a 1.5-MW WECS.

Index Terms— Doubly f ed induction generator (DFIG), gridfaults, neutral side converter, seamless fault ride-through (seam-

less FRT), unbalance, wind turbine (WT).

I. I NTRODUCTION

T HE large-scale integration of wind power in today’s power system is increasing its proposition in total elec-tricity generated. To improve the reliability of the power systemwith large-scale wind power integration, different countrieshave specified fault ride-through (FRT) requirements for windturbine (WT) in their respective grid codes [1]–[4]. A typicalFRT requirement curve for WT, as per German and Irish gridcodes, is shown in Fig. 1. According to Fig. 1(a), WTs must stayconnected when the terminal voltage remains above the boldline. In addition to remaining connected to the transmissionsystem, recent grid codes put stringent requirements on thesupply of reactive current by WTs as per Fig. 1(b), in order toimprove the voltage security of the system. The general gridcode requirements can be summarized as follows.

1) The WTs must stay connected during voltage dips abovethe specified level for the time duration stated in Fig. 1(a).

Manuscript received May 23, 2013; revised September 12, 2013; acceptedOctober 11, 2013. Date of publication December 05, 2013; date of current ver-sion April 16, 2014. Paper no. TPWRS-00651-2013.

The authors are with the Institute Center for Energy, Masdar Instituteof Science and Technology, Abu Dhabi, United Arab Emirates (e-mail: [email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPWRS.2013.2290047

Fig. 1. Recent grid code requirements for grid connected wind turbines.(a) FRT requirement curve. (b) Reactive support requirement curve.

2) The WTs must support the grid voltage with additional re-active current during voltage dips, as per Fig. 1(b). More-over, the voltage control must take place within 20 ms after the fault recognition.

3) The active power output of the WTs must be continuedimmediately after the fault clearance and increased to theoriginal value with a gradient of at least 20% of the rated

power per second.The doubly fed induction generator (DFIG) is widely used

for the grid-connected variable-speed WTs because of high ef-ficiency and independent control of active and reactive power using partial capacity converters. In the conventional architec-ture of a DFIG, the stator windings are directly connected to thegrid and the rotor windings are connected to the grid through

back-to-back connected voltage-source converters (VSCs) [5],[6]. These two VSCs can be identified as: 1) rotor-side con-verter (RSC), connected between the rotor and dc link and 2)grid-side converter (GSC), connected between the grid and dclink through interfacing inductors. As the stator windings aredirectly connected to the grid, any amount of voltage dip at

the DFIG terminals resulting from the grid fault directly affectsthe air-gap flux and, hence, the energy conversion process. De- pending on the type of fault, the voltage dip may introduce dccomponent or combination of dc and reverse rotating ac compo-nent in the air-gap flux [7]. These flux components induces highvoltage in the rotor windings at rotational and/or double the ro-tational frequency. The RSC itself cannot limit these high-fre-quency voltages due the modulation index constraint and henceloses its current control capabilities. Unless the proper miti-gating measures are employed, the rotor currents under the gridfault condition can exceed the transient current rating of theRSC. Grid faults also cause severe mechanical stress on the

bearings and the gear box of WECS due to torque pulsation.

0885-8950 © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.


2/10

KANJIYA et al.: NOVEL FAULT-TOLERANT DFIG-BASED WECSS FOR SEAMLESS OPERATION DURING GRID FAULTS 1297

Fig. 2. Proposed fault-tolerant DFIG wind turbine configuration for seamless FRT.

Many solutions have been proposed to provide or improve theFRT capability of the DFIG wind turbines. To limit the transientcurrents, the rotor crowbar that disables the RSC and short cir-cuit the rotor windings through resistors during the grid faults is

discussed in [8] and [9]. The main drawback of this scheme isthat the DFIG consumes huge reactive power during grid faultand further aggravates the voltage dip. Another configurationthat uses series dynamic resistors is proposed in [10] and [11].This can maintain the stator voltage or rotor currents within thelimits but not appropriate if the DFIG is being controlled tosupply the reactive power to the grid. Furthermore, above men-tioned fault ride through techniques fail to maintain the pre-faultvoltage across the stator windings which causes undesired elec-trical and mechanical transients in the system.

To obtain transient-free FRT performance, it is necessary tokeep the pre-fault voltage across the stator windings during thegrid faults. To achieve this, the dynamic voltage restorer (DVR)

arrangement using an additional VSC with an output filter and series transformer with bypass switches is investigated in[12]–[14]. To provide the protection against dead short circuit atDFIG terminals, the DVR should be rated for 100% of the fullDFIG rating. Moreover, this technique involves an operationaldelay of auxiliary semiconductor switches (used to bypassthe series transformer during normal condition) and hence theelectrical transients are unavoidable. Furthermore, this solutionis very costly as it involves many auxiliary components. Alter-nately, the use of parallel grid side rectifier and series inverter atthe Y-point of the stator windings is proposed and investigatedduring balanced faults in [15]. This scheme effectively reducesthe number of passive and active components used to achievefault-tolerant operation of DFIG. However, the major drawback of this scheme is that the stator windings need to carry the slip

power during super-synchronous speeds and inability to ridethrough unbalanced faults.

With the aim of achieving seamless FRT operation in linewith recent grid codes using minimum additional components,

a novel fault-tolerant confi

guration of DFIG using a nine-switchconverter is proposed in this paper. In the proposed configura-tion, a traditional six-switch GSC of DFIG is replaced with a re-cently proposed nine-switch converter [16]–[18] to provide twoindependent three-phase outputs. One of these three-phase out-

puts is connected to grid via an interfacing inductor to realizenormal GSC operation, while the second output is connectedto the neutral side of the stator windings to offer series voltagecompensation capability to DFIG for riding through any kindof grid faults. An appropriate control algorithm for the controlof a nine-switch converter is developed to achieve the seamlessFRT operation of DFIG. Moreover, to provide reactive currentsupport in line with recent grid code requirements during the

grid fault conditions, a coordinated reactive power controller isdeveloped to share reactive current between GSC and RSC. Itis worth noting that the proposed fault-tolerant DFIG configu-ration uses only three extra switches to achieve transient-freeoperation during any kind of grid fault.

II. PROPOSED DFIG CONFIGURATION AND MODELING

A. System Description

The schematic of the proposed fault-tolerant DFIG WT con-figuration to achieve seamless FRT operation is shown in Fig. 2.Similar to a conventional DFIG, the stator windings are di-rectly connected to grid and the rotor windings are connectedto the RSC (switches R1–R6) through slip rings. The RSC inthe proposed configuration shares the dc link capacitor (C) with


3/10


the dual-output nine-switch converter (switches G1–G9) insteadof conventional six-switch converter. The upper output of thenine-switch converter is connected to grid via interfacing in-ductor while, the lower output is connected to the neutralside of the stator windings of theDFIG. During the normal oper-ation of the DFIG the lower three switches (G3, G6, and G9) of the nine switch converter are short circuited to form the Y-point

of thestator windingswhile, theupper six switches (G1, G2, G4,G5, G7, and G8) are controlled as a conventional GSC to reg-ulate the dc-link voltage. During the voltage dip resulting fromthe grid faults, the lower three switches (G3, G6, and G9) alsostart switching to generate the compensating voltages on theneutral side of the stator winding to maintain pre-fault voltageacross it. This operation of generating the compensating volt-ages on the neutral side of the stator winding is termed here asneutral side converter (NSC) operation. During grid faults the

NSC will absorb part of the active power generated by DFIGand pumps it to the dc link, hence, dc-link voltage tends to riseif any preventive measures are not taken. To protect the dc-link

capacitor from over voltage during the grid faults, a dynamic braking resistor (DBR) is connected across the dc-link capac-itor as shown in Fig. 2.

B. Modeling of the DFIG

To analyze the transient and steady-state performance of thewound rotor induction machine with the six-terminal stator, it ismodeled in d-q reference frame rotating at synchronous speedfollowing the procedure given in [20]. The voltage and fluxequations of the induction machine with the six-terminal stator in d-q reference frame can be written as follows:

(1)

(2)

where suf fix and represents the -axis and -axis compo-nents of respective variables, and represent the voltagesavailable at the grid side of the stator terminals, andrepresent the voltages available at the neutral side of the stator terminals, and and represent the rotor terminal voltages.The variables and represent the stator currents, while

and represent the rotor currents. and representthe stator and rotor resistances referred to stator while , ,and represent the stator self-inductance, rotor self-induc-tance, and mutual inductances referred to stator, respectively.is supply angular frequency while is the rotor angular fre-quency in electrical radians per second.

By aligning the -axis of the reference frame to the air-gapflux, the simplified expressions for the electromagnetic torque

developed and stator reactive power in terms of rotor variablescan be written as

(3)

(4)

From (3) and (4), it can be seen that, by aligning the -axisof the reference frame to the air-gap flux, the electromagnetictorque developed is directly proportional to the -axis rotor cur-rent while the stator reactive power is directly proportional tothe -axis rotor current. Hence, by employing the decoupledcontrol of and , the electromagnetic torque (or active

power) and the stator reactive power can be controlled indepen-dently by RSC.

III. CONTROL OF THE DFIG WIND TURBINE

The control of the RSC and nine-switch converter (GSC and NSC) in the proposed DFIG configuration has two modes of

operation: 1) normal mode and 2) fault mode. The details of thecontrol strategies for both the converters during these modes of operation are discussed here.

A. Normal Mode Operation

In normal mode of operation,the DFIG is controlled to supplymaximum available active power by the WT and user-definedreactive power.

1) Rotor-Side Converter Control During Normal Mode: Tocontrol the electromagnetic torque and the reactive power pro-duced by DFIG independently, in normal mode of operationRSC is controlled in a synchronously rotating d-q referenceframe with d-axis aligned to stator flux vector. The expressions

for the electromagnetic torque and reactive power developed by DFIG with d-axis oriented to stator flux are given in (3)and (4), respectively. The electromagnetic torque and hence ac-tive power produced by DFIG is proportional to and can

be regulated by controlling . On the other hand, reactive power produced by DFIG is proportional to and can beregulated by controlling . The detailed control diagram of RSC is shown in Fig. 3. The reference -axis rotor currentis calculated from the reference torque command gener-ated using a PI controller over the rotor speed. The referencerotor speed command can be generated using an appropriatemaximum power point (MPPT) algorithm. The reference -axisrotor current is generated using a PI controller over reac-tive power supplied by the DFIG. In the normal mode of oper-ation, is allowed to take any value within the limits in order to extract the maximum power from WT while is limited as

per

(5)

to ensure RSC current is within the safe limit .The reference rotor currents and are tracked using de-

coupled PI current control by regulating and as per (2).The gate signals (R1 to R6) for RSC are then issued by com-

paring the reference RSC voltage with a triangular car-rier wave using a sinusoidal PWM technique.

2) Nine-Switch Converter Control During Normal Mode:

The control of a nine-switch converter consists of two parts:


4/10


Fig. 3. RSC control.

Fig. 4. Nine-switch converter control.

1) GSC control and 2) NSC control. The detailed schematic of nine-switch converter control is shown in Fig. 4.

The objective of the GSC is to regulate the dc-link voltageto its reference value irrespective of direction of rotor power flow. The voltage balance equation across interfacing inductor

with internal resistance in synchronous reference framecan be written as

(6)

By aligning the -axis of the reference frame of the GSC con-trol with the grid voltage, active and reactive power supplied or absorbed by the GSC can be written as

(7)

The active power and hence the dc-link voltage is propor-tional to , and can be regulated by controlling while,the reactive power is proportional to and can be regulated


5/10


by controlling . In normal mode of operation, the refer-ence -axis GSC current is generated using PI controller over average dc-link voltage and the reference -axis GSC cur-rent is set equal to zero. The reference voltage thathas to be generated by GSC in order to control GSC current toits reference value is computed by employing decoupled currentcontrol as per (6).

In normal steady state operation, the voltage to be injected by NSC at neutral side of the stator winding is zero. Hence, inthe normal mode of operation, the NSC controller is inactiveand the reference voltage to be generated by the NSC iskept equal to zero.

Both the GSC and NSC operation has to be carried out bya single nine-switch converter in the proposed fault-tolerantDFIG configuration. Like most reduced component converter topologies, the nine-switch converter faces limitations imposedon its allowable switching states. Both of the output terminalsof the same phase in the nine-switch converter can only connectto either [for phase-a: -ON, -ON, and -OFF] or

0 V [for phase-a: -OFF, -ON, and -ON], or its upper output terminal to and lower output terminal to 0 V [for phase-a: -ON, -OFF, and -ON]. The combinationwhere the upper output terminal needs to be connected to 0 Vand lower to is not allowed [for phase-a: -ON, -ON,and -ON] as this short-circuits the dc link. To resolve thislimitation, two modulating references of the same phase shouldshare the modulation space without intersecting each other [16]. This can be achieved by placing the reference for upper terminal always above that of the lower terminal by addingoffsets to both the references. The adjustment of three-phasemodulating reference signals in modulation space by addingthe same offset to all the three phases does not show any impact

on the output (line voltages) of the converter (for example,third-harmonic injection PWM method) [19]. The modulatingreference signal adjustment method for nine-switch converter

based on 120 -discontinuous modulation [19] is derived in[18]. The offset reference voltage signals for GSC and NSC as

per [18] can be written as

(8)

The offset reference signals and are fed to theindividual three-phase PWM generators (PWM-I and PWM-II)

which generate the two different sets of six PWM signals. If theGSC and NSC operations are to be achieved using two sepa-rate six-switch inverters, the PWM signals generated by PWM-Iand PWM-II can be directly given to corresponding inverters.However, in a nine-switch inverter, the middle three switchesare shared by GSC and NSC, so their gate pulses are generated

by logical OŔ operation of PWM signals corresponding to lower three switches by PWM-I and upper three switches by PWM-IIas shownin Fig. 4. ThePWM signals corresponding to the upper three switches by PWM-I and lower three switches by PWM-IIare directly issued to upper three and lower three switches, re-spectively, of the nine-switch converter. Additional informationon the nine-switch inverter operation can be found in [16]–[18].

It is important to note that during normal mode of operationthe reference voltages for NSC are zero and hence

offset reference signals are minus one. This meansthat the PWMsignals for lower three switches of the nine-switchconverter are always at logic high and corresponding switchesare ON. This effectively short circuit the stator windings on neu-tral side and form the Y-point.

B. Fault Mode Operation

During the fault mode of operation, the NSC is controlledto keep a pre-fault voltage across the stator winding while theRSC and GSC control is switched to supply reactive current inline with the requirement of the recent grid codes [Fig. 1(b)]. Toachieve transient-free operation, it is necessary to detect the gridfault with least possible delay. The fault in the system can bedetected almost instantaneously by measuring the absolute error

between reference grid voltage magnitude (1 p.u.) and actualgrid voltage magnitude (in ) as follows:

(9)

The grid fault is detected whenever exceeds the

threshold (typically 0.1 p.u.). Once the grid fault is detected,the signal in Fig. 3 and 4 changes its logic from low tohigh. As will oscillate above and below the thresholdduring the unsymmetrical faults, the fault removal is detectedwhen the one cycle average of reduces below thethreshold. The details of the actions taken by the controller of different converters during the grid fault condition arediscussed below.

1) RSC Control During Fault Mode: The rotor side converter has the capability to provide full reactive support required by therecent grid codes during the grid fault condition provided thatthe NSC keeps the pre-fault voltage across the stator winding.

However, if the RSC control is switched to supply full reactivecurrent, the active current supplied by the stator has to be re-duced to keep RSC switch currents below the safe limit. Thisleads to the over speeding of the rotor and, hence, increasedmechanical stress due to the storage of the energy produced bythe WT as kinetic energy. To reduce the negative impact of in-creased mechanical stress on WT’s construction, it is advisableto maximize active power extraction along with reactive cur-rent supply without exceeding the rating of any component inthe system. To achieve this, the reactive currents required to ful-fill the grid code requirements is shared by the GSC and RSCduring grid faults in proportion to their ratings. The duty of sup-

plying two third of the reactive current (maximum 0.6 p.u.) is

assigned to stator and hence RSC, while, one third (maximum0.3 p.u.) is assigned to the GSC. To supply the reactive currentassigned to the stator during fault condition, the is computedusing the PI controller over reference stator reactive current( -axis current is proportional to reactive power as the reference

-axis is oriented to stator flux) as shown in Fig. 3. The iscalculated using

(10)

to support two thirds of the reactive current required by gridcode as per Fig. 1(b).

In (10), is the positive sequence grid voltage magnitudeand can be calculated within 20 ms to fulfill the grid code re-quirement. In the fault mode of operation, the priority is given


6/10


to to strictly support reactive current and the limit on iscalculated as

(11)

2) Nine-Switch Converter Control During Fault Mode: Asdiscussed before, the DBR comes into picture during fault con-

dition to keep the dc-link voltage below the safe limit whichcomforts the GSC from the duty of keeping the dc-link voltageconstant. Therefore, during the grid fault condition to assist theRSC, the GSC controller is switched to supply one third of thereactive current required to fulfill the grid codes requirement.The reference reactive current for the GSC during faultmode of operation is computed as per

if if

(12)

and the GSC current is limited to by putting limits onactive current as per

(13)

The objective of the NSC control during fault condition isto keep the pre-fault voltage across the stator winding for the

proper functioning of the RSC control. The logic high on thesignal during fault condition activates the NSC controller

asshown inFig. 4.Thecompensatingvoltage thathas to be injected by the NSC on the neutral side of the stator windingduring fault condition can be computed as

(14)

In (14), and are the present grid voltages and the

pre-fault grid voltages computed using pre-fault grid voltagevector angle . The pre-fault grid voltage vector angleis obtained by saturating the phase-locked loop (PLL) over thegrid voltage using signal as shown in Fig. 4. The pre-faultgrid voltages are computed by holding the samples of twocycle mean value of when fault is detected.

The compensating voltages can be directly used as thereference voltages to control NSC in open-loop by convertingthem to stationary reference frame. The open-loop control canwork satisfactorily for the NSC due to the absence of outputfilter and series injection transformer. However, there will beseries voltage drop across the switches of the NSC which mayaffect the compensation. To achieve perfect compensation of

the voltage dip, the switching voltage drop is added to byestimating it using PI controller which maintains the stator flux

at its pre-fault value as shown in Fig. 4. Thed-q axis stator flux components in Fig. 4 are estimated using (1)and (2) while, the pre-fault stator flux components are obtained

by holding the samples of two cycle mean value of whenfault is detected.

IV. SIMULATED SYSTEM AND PERFORMANCE EVALUATION

To verify the effectiveness of the proposed DFIG windturbine configuration and its control to achieve fault tolerantoperation in line with the recent grid codes, an extensive simu-lation study is carried out in MATLAB. The detailed model of DFIG with six-terminal stator and the power electronics con-verters are developed using SIMULINK and SimPowerSystems

TABLE ISYSTEM SPECIFICATIONS

tool-boxes. To represent the actual operating conditions, it isassumed that the DFIG wind turbine is connected to a mediumvoltage network (120 kV) through a step-up transformer (575 V-Yg/25 kV ), a transmission line (25 kV–50 km),

and another step-up transformer (25 kV /120 kV-Yg) asshown in Fig. 2. The wind turbine specifications and machine parameters used are tabulated in Table I.

The performance of the proposed fault-tolerant DFIG config-uration is evaluated under LLLG, LLG and LG faults at 120 kV

bus and the corresponding results are shown in Figs. 5–7. To prove the compatibility of the proposed configuration with re-cent grid codes for the effective fault ride through, faults areemulated for the duration of 150 ms (nine cycles of supply). Allof the results are represented in per unit (p.u.) for a wind speedof 15 m/s.

The detailed response of the proposed fault-tolerant DFIGconfiguration during the LLLG fault is shown in Fig. 5. The

system is under steady state before the simulation time of 6 s.A LLLG fault lasting for 150 ms is applied on 120 kV bus at6 s, which can be observed from the dip in the grid voltages

from 1 to 0.1 p.u. The moment fault is detected, fault modeoperation is enabled by a logic high of the signal. Thisenables the NSC which injects compensating voltage (PWMswitching voltage switched between on the neutral sideof the stator winding to maintain pre-fault voltage across it.For better visualization of NSC functionality in maintaining

pre-fault voltage across the stator winding, in Fig. 5 an average(over a switching period) compensating voltage is showninstead of the actual PWM switching voltage. Note that the av-erage voltage across the stator winding does not experi-ence any change due to the instantaneous voltage compensa-tion by the NSC (without any delay as no transfer switches are


7/10


Fig. 5. Different system variables to study the behavior of proposed fault-tolerant DFIG configuration during LLLG fault at 120-kV bus.

involved). The effectiveness of the NSC in keeping pre-faultvoltage across the stator winding is also evident from the traceof the stator flux vector Sye as it is maintained constantthroughout the fault duration.

As discussed in the control section, during the grid fault con-dition, the RSC control is switched to supply reactive current

by giving priority to . With priority to , the reference cur-rent (responsible for maintaining WT speed and hence ac-tive power extraction) is limited to (11) with set equal to1.1 p.u. (with rated rotor current as base current). The evolutionof variables and during fault mode operation is depictedin Fig. 5. To track these reference currents, the RSC injects an

average rotor voltage, across the rotor terminals which forcesthe rotor current, to flow in rotor winding and correspondingstator current, in stator winding. Note that the sinusoidalstator current is achieved with permissible switching rip-

ples even though the voltage injected by NSC on the neutral sideof the stator winding is the PWM switching voltage. The suf fi-ciently high stator self-inductance served the purpose of filter inductance. This shows that there is no need of an output filter with NSC similar to RSC.

The GSC also switched to reactive current control modeduring fault condition (Fig. 4). The resulted GSC current dueto this control switch over is depicted in Fig. 5 as . Theinjection of the reactive current assigned to GSC as per (12)can be noticed from the phase jump and increase in magnitudeof GSC current.

To illustrate the effectiveness of reactive currentcontrolmode precisely, the active and reactive components of thestator and GSC currents during LLLG fault are presented inFig. 5. As the positive sequence grid voltage falls to 0.1 p.u.(well below 0.5 p.u.) during the fault condition, the DFIG in-

jects a total of 0.9 p.u. reactive current as per Fig. 1(b). Note that this total reactive current is shared by stator and GSCin 2:1 ratio. Also note that, due to the increase in stator andGSC reactive currents, corresponding active currents arereduced for the safe operation of the converters.

To better understand the functionality of different converters,the active and reactive powers absorbed by the different com-

ponents of the DFIG system are also plotted in Fig. 5. Wherein, positive sign indicates power absorption while negative sign in-dicates power generation. From the active power plot, it can benoticed that during steady state (before 6 s) the active power ab-sorbed by the grid is equal to active power generated bythe stator plus GSC . Note that the active power pro-duced by the rotor is exactly equal to as the objectiveof the GSC is to transfer into the grid via a dc link duringthe normal mode of operation. The power absorbed by the NSCcan be observed to be zero during steady state as the NSC is notactive in normal mode of operation. Whereas during the gridfault, as discussed in the control section, with the reduction in

active component of stator and rotor currents to accommodatethe necessary reactive current, a little dip in the and can


8/10


Fig. 6. Important system variables to study the behavior of proposed fault-tolerant DFIG configuration during LL fault at 120-kV bus.

Fig. 7. Important system variables to study the behavior of proposed fault-tolerant DFIG configuration during LG fault at 120-kV bus.

be observed although the stator voltage is maintained at its pre-fault value by the NSC. The active power injected into thegrid by the GSC is zero as the GSC is injecting reactivecurrent up to its limit during the grid fault. With the reduction in

grid voltage during grid fault, there is a proportionate reductionin the active power delivered to the grid . Therefore, thereis a net amount of excess stator active power equal to deference

between absolute values of and . This excess power is ab-sorbed by the NSC and dissipated in the DBR at dc link.The active power produced by the rotor is also dissipatedin the DBR as the power absorbed by the GSC from thedc link is zero. A similar analysis can also be drawn for the re-active power of different components from the plot of reactive

powers.Because of the reduction in active power of the stator to ac-

commodate the required reactive current component during gridfault,alittleriseinrotorspeed(w)canbeobservedduetotheac-cumulation of kinetic energy. Moreover, there is a net amount of excess power pumped into the dc link by NSC and RSC during

fault condition; the DBR comes into the picture to prevent thedc link from overvoltage (not more than 1250 V). As soon asfault is removed from the system, the active current of thestator again increases within 20 ms to feed the full active power

produced by the WT which effectively fulfi

lls the recent gridcode requirements.The performance of proposed fault-tolerant DFIG configu-

ration is also evaluated for the unsymmetrical faults. The re-sponses of the DFIG during LL and LG faults are shown inFigs. 6 and 7, respectively. It can be seen that the DFIG seam-lessly ride through both the faults without noticeable transients.It can also be noticed that the currents injected by the DFIG tothe grid are balanced although the faults are unsymmetrical innature. Moreover, the DFIG effectively injects the reactive cur-rent corresponding to positive sequence grid voltage accordingto Fig. 1(b).

After evaluating the performance of proposed fault-tolerantDFIG during different fault conditions, the superiorities and

benefits associated with it are summarized here.


9/10


• The proposed DFIG configuration can effectively keep the pre-fault voltage across the stator winding of the DFIGduring any type of fault condition to achieve transient-freeFRT.

• It is an economic solution to achieve series voltage compensation as only three full-rated extra switches are usedin the proposed solution compared with the DVR solution

where a full-rated six-switch converter is required.• With the proposed reactive current sharingcontrol, thepro-

posed fault-tolerant DFIG is able to fulfill all of the require-ments of the recent grid codes.

• In the proposed DFIG configuration, as the compensationof grid voltage variations is achieved from the neutral sideof the stator winding, there is no need for a full-rated seriesinjection transformer as in case of the DVR solution.

• There isno needfor anoutput filter for the injected voltagesas the stator self-inductance will be suf ficiently high.

• The proposed fault-tolerantDFIGseamlessly rides throughany kind of grid fault as it does not involve any transfer switch (required to short circuit series injection trans-former during steady-state operation in the case of DVR solution) operation during FRT.

V. CONCLUSION

A novel fault-tolerant DFIG configuration with minimumadditional components (only three extra switches) is proposedfor seamless operation during grid faults. Furthermore, thedetailed control strategies for the RSC and nine-switch con-verters during steady-state and fault conditions are developed tofulfill recent grid codes requirements. The effectiveness of the

proposed fault-tolerant DFIG in riding through different types

of grid faults (LLLG, LL and LG) is evaluated through detaileddigital simulation studies. The simulation studies show that the

proposed fault-tolerant DFIG seamlessly rides through all of the faults while fulfilling grid codes requirements. The advan-tages and benefits associated with the proposed configurationover other FRT techniques can be summarized as seamlessFRT, strict satisfaction of grid code requirements, economicalsolution with only three additional switches, no need of outputfilter, series injection transformer, and its bypass switches.

R EFERENCES

[1] W. Christiansen and D. R. Johnsen, “Analysis of requirements in se-lected grid codes, section of electric power engineering,” Tech. Univ.of Denmark, 2006, Tech. Rep..

[2] M. Tsili and S. Papathanassiou, “A review of grid code technical re-quirements for wind farms,” IET Renew. Power Gener., vol. 3, no. 3, pp. 308–332, Mar. 2009.

[3] E.ON Netz GmbH., Bayreuth, Germany, “Grid code—High and extrahigh voltage,” Apr. 2006.

[4] EirGrid Grid Code. ver. 3.5, EirGrid plc, Ireland, Mar. 2011.[5] R. Pena, J. Clare, and G. Asher, “Doubly fed induction generator using

back-to-back converters and its application to variable-speed wind-en-ergy generation,” Proc. Inst. Electr. Eng.—Elect. Power Appl., vol.143, no. 3, pp. 231–241, May 1996.

[6] S. Muller, M. Deicke, and R. W. De Doncker, “Doubly fed inductiongenerator systems for wind turbine,” IEEE Ind. Appl. Mag., vol. 8, no.3, pp. 26–33, May–Jun. 2002.

[7] J. Lopez, P. Sanchis, X. Roboam, and L. Marroyo, “Dynamic behavior of the doubly fed induction generator during three-phase voltage dips,”

IEEE Trans. Energy Conver s., vol. 22, no. 3, pp. 709–717, Sep. 2007.

[8] S.Seman, J. Niiranen, and A. Arkkio,“Ride-throughanalysis of doublyfed induction wind-power generator under unsymmetrical network dis-turbance,” IEEE Trans. Power Syst., vol. 21, no. 4, pp. 1782–1789, Nov. 2006.

[9] J. Morren and S. W. H. de Haan, “Ride through of wind turbines withdoubly-fed induction generator during a voltage dip,” IEEE Trans. En-ergy Convers., vol. 20, no. 2, pp. 435–441, Jun. 2005.

[10] A. Causebrook, D. J. Atkinson, and A. G. Jack, “Fault ride-through of large wind farms using series dynamic braking resistors,” IEEE Trans.

Power Syst., vol. 22, no. 3, pp. 966–975, Aug. 2007.[11] J. Yang, J. E. Fletcher, and J. O’Reilly, “A series-dynamic-resistor- based converter protection scheme for doubly-fed induction generator during various fault conditions,” IEEE Trans. Energy Convers., vol.25, no. 2, pp. 422–432, Jun. 2010.

[12] A. O. Ibrahim, T. H. Nguyen, D.-C. Lee, and S.-C. Kim, “A faultridethrough technique of DFIG wind turbine systems using dynamicvoltage restorers,” IEEE Trans. Energy Convers., vol. 12, no. 3, pp.871–882, Sep. 2011.

[13] C. Wessels, F. Gebhardt, and F. W. Fuchs, “Fault ride-through of aDFIG wind turbine using a dynamic voltage restorer during symmet-rical and asymmetrical grid faults,” IEEE Trans. Power Electron., vol.26, no. 3, pp. 807–815, Mar. 2011.

[14] D. Ramirez, S. Martinez, C. A. Platero, F. Blazquez, and R. M. deCastro, “Low-voltage ride-through capability for wind generators based on dynamic voltage restorers,” IEEE Trans. Energy Convers.,vol. 26, no. 1, pp. 195–203, Mar. 2011.

[15] P. Flannery and G. Venkataramanan, “A fault tolerant doubly fed in-duction generator wind turbine using a parallel grid-side rectifier andseries grid-side converter,” IEEE Trans. Power Electron., vol. 23, no.3, pp. 1126–1135, May 2008.

[16] C. Liu, B. Wu, N. R. Zargari, D. Xu, and J. Wang, “A novel three- phase three-leg ac/ac converter using nine IGBTs,” IEEE Trans. Power Electron., vol. 24, no. 5, pp. 1151–1160, May 2009.

[17] T. Kominami and Y. Fujimoto, “Inverter with reduced switching-de-vice count for independent ac motor control,” in Proc. IEEE-IECON ,2007, pp. 1559–1564.

[18] L. Zhang, P. C. Loh, and F. Gao, “An integrated nine-switch power conditioner for power quality enhancement and voltage sag mitiga-tion,” IEEE Trans. Power Electron.,vol.27,no.3,pp.1177–1190,Mar.2011.

[19] A. M. Hava, R. Kerkman, and T. A. Lipo, “Carrier-based PWM-VSIovermodulation strategies: Analysis, comparison, design,” IEEE Trans. Power Electron., vol. 13, no. 4, pp. 674–689, Jul. 1998.

[20] P. C. Krause, O. Wasynczuk, and S. D. Sudhoff , Analysis of Elec-tric Machines and Drive Systems, 2nd ed. New York, NY, USA:Wiley–IEEE, 2002.

Parag Kanjiya received the B.Eng. degree in elec-trical engineering from the B.V.M. Engineering Col-lege, Sardar Patel University, V.V. Nagar, India, in2009, and the M.Tech. degree in power systems fromthe Indian Institute of Technology Delhi (IITD), NewDelhi, India, in 2011.

Since October 2011, he has been a ResearchEngineer with the Masdar Institute of Science andTechnology, Abu Dhabi, United Arab Emirates.His research interests include applications of power electronics in distribution systems, power quality

enhancement, renewable energy, FACTS, and power system optimization.Mr. Kanjiya was the recipient of the K.S. Prakasa Rao Memorial Award for

getting the highest C.G.P.A at IITD in August 2011.

Bharath Babu Ambati received the B.E. degreein electrical and electronics engineering from Sir C. R. Reddy College of Engineering (af filiatedwith Andhra University), Eluru, India, in 2009, andthe M.Tech degree in power electronics, electricalmachines and drives from the Indian Institute of Technology Delhi (IITD), New Delhi, India, in 2011.He is currently working toward the Ph.D. degree atMasdar Institute of Science and Technology, AbuDhabi, United Arab Emirates.

From July 2011 to June 2012, he was withSchneider Electric India Private Ltd. as a Product Expert of Motion & Drives.His current research interests include power electronics, electrical machines,renewable energy generation, and power quality improvement.


10/10


Vinod Khadkikar (S’06–M’09) received the B.E.degree from the Government College of Engi-neering, Dr. Babasaheb Ambedkar MarathwadaUniversity, Aurangabad, India, in 2000, the M.Tech.degree from the Indian Institute of Technology Delhi(IITD), New Delhi, India, in 2002, and the Ph.D.degree from the École de Technologie Supérieure(E.T.S.), Montréal, QC, Canada, in 2008, all inelectrical engineering.

From December 2008 to March 2010, he was aPostdoctoral Fellow with the University of Western

Ontario, London, ON, Canada. Since April 2010, he has been an Assistant Pro-fessor with the Masdar Institute of Science and Technology, Abu Dhabi, UnitedArab Emirates. From April 2010 to December 2010, he was a Visiting Fac-ulty Member with the Massachusetts Institute of Technology, Cambridge, MA,USA. His research interests include applications of power electronics in dis-tribution systems and renewable energy resources, grid interconnection issues, power quality enhancement, ac tive power filters, and electric vehicles.

Documents

A Novel Fault-Tolerant DFIG-Based Wind Energy