Extended Summary 本文は pp.396–401

Dynamic Behaviour of DFIG-Based Wind Turbines During Grid Faults

Istvan Erlich Non-member (University Duisburg-Essen, istvan.erlich@uni-due.de)

Holger Wrede Non-member (SEG GmbH & Co. KG, h.wrede@newage-avkseg.de)

Christian Feltes Non-member (University Duisburg-Essen, christian.feltes@uni-due.de)

Keywords: wind turbine, doubly-fed induction generator, fault-ride through

According to grid codes issued by utilities, tripping of wind tur-bines following grid faults is not allowed. Besides, to provide volt-age support to the grid mandatory reactive current supply is nec-essary. To enable wind turbines ride-through low voltage periodsspecial protection measures have to be implemented. In this paperthe behavior of DFIG based wind turbines during grid faults is dis-cussed and elucidated using simulation results. It is shown that withproperly designed crowbar and DC-link chopper even zero voltageride-through is possible.

In the past wind turbines were separated from the grid follow-ing grid faults. However, as of now, separation of wind turbines forvoltage values below 80% of the nominal voltage would lead to lossof an undesirable portion of power generation. Therefore, utilitiesnow require an FRT capability as specified in Fig. 1.

According to the German grid code wind turbines have to pro-vide a mandatory voltage support during voltage dips. The corre-sponding voltage control characteristics are summarized in Fig. 2.Accordingly wind turbines have to supply at least 1.0 p.u. reactivecurrent already when the voltage falls below 50%. A dead band of10% is introduced to avoid undesirable control actions.

Fig. 1. Fault-ride through requirements

Riding through grid faults means higher stress for the convertersand requires a better protection system. In this paper a DFIG systemfor improved fault ride-through capability is introduced. It containstwo protection circuits, a DC-chopper and an AC-crowbar to avoidDC-link over-voltages during grid faults. The chopper module is notessential for fault ride-through operation but it increases the normalrange of DFIG operation by smoothing the DC-link voltage duringheavy imbalances of active power on the rotor-side and line-sideconverter.

The simulation results presented in the paper show that an ex-tended protection system is necessary for fault-ride-through. Thisis done by an example showing the effects of a short-circuit with-out any countermeasures on the converter and generator side. Inthis case the grid fault can lead to considerable overcurrents andover-voltages putting the whole facility under stress. Further simu-lation results prove that wind turbines equipped with rotor crowbarand DC-link chopper are able to stay on grid and limit currents andvoltages below the threshold values. Additionally, the grid code re-quirements can be fulfilled, while a crowbar firing on voltage returnis prevented by appropriate chopper design.

Fig. 2. Characteristic of wind turbine voltage control

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Paper

Dynamic Behaviour of DFIG-Based Wind Turbines During Grid Faults

Istvan Erlich∗ Non-member

Holger Wrede∗∗ Non-member

Christian Feltes∗∗∗ Non-member

According to grid codes issued by utilities, tripping of wind turbines following grid faults is not allowed. Besides,to provide voltage support to the grid mandatory reactive current supply is necessary. To enable wind turbines ride-through low voltage periods special protection measures have to be implemented. In this paper the behavior of DFIGbased wind turbines during grid faults is discussed and elucidated using simulation results. It is shown that withproperly designed crowbar and DC-link chopper even zero voltage ride-through is possible.

Keywords: wind turbine, doubly-fed induction generator, fault-ride through

1. Introduction

In many countries in the world energy policies are focusedon the increased utilization of wind energy due to the factthat wind power can provide a considerable input to electric-ity production. Besides, wind represents a natural resourcewhich is renewable, environmentally benign and is availablenearly in every country. In Germany the installed wind tur-bine capacity already reached 19 GW. By the year 2020 a to-tal wind power capacity of nearly 50 GW is expected, whichis more than 50% of the German peak load.

It is obvious that for secure power system operation windturbines have to meet grid requirements. For guaranteeinggrid compliance utilities established Grid Codes (1) (2) that rep-resent fundamental guidelines for wind turbine manufactur-ers and wind farm planners. The basic requirements com-prise fault ride-through (FRT) capability and contribution togrid voltage control during emergency situations.

In this paper the authors will discuss the dynamic be-haviour of wind turbines based on doubly fed induction gen-erators (DFIG) which is currently the most popular technol-ogy in this field. The main advantage of using DFIG resultsfrom the fact that only 30% of the power passes through theconverter that is, therefore, smaller and cheaper comparedwith a full-size one.

During grid faults the DFIG experiences overcurrentswhich lead also to increasing DC-voltage on the converterside. To avoid damage some special measures are neces-sary which however should not contravene grid requirements.Therefore, understanding between power engineers and de-velopers of wind turbine converters is one of the prerequi-sites for mutually acceptable technical solutions. The paperis organized as follows: First the fundamental Grid Code re-quirements will be explained briefly. Then, the behaviour of

Based on “Dynamic Behaviour of DFIG-Based Wind Turbines Dur-ing Grid Faults” by Istvan Erlich, Holger Wrede and Christian Felteswhich appeared in the proceedings of the 2007 Power ConversionConference—Nagoya, c©2007 IEEE.∗ University Duisburg-Essen, istvan.erlich@uni-due.de∗∗ SEG GmbH & Co. KG, h.wrede@newage-avkseg.de∗∗∗ University Duisburg-Essen, christian.feltes@uni-due.de

the DFIG wind turbines following grid faults is explained.The objective is to demonstrate the expected stress on thewind turbine and the converter in the absence of any counter-measures. After that FRT measures currently under consid-eration in large multi-megawatt wind turbines will be intro-duced, and their effectiveness demonstrated on the basis ofsimulation results.

2. Grid Requirements

In the past wind turbines were separated from the grid fol-lowing grid faults. However, as of now, separation of windturbines for voltage values below 80% of the nominal voltagewould lead to loss of an undesirable portion of power gener-ation. Therefore, utilities now require an FRT capability asspecified in Fig. 1. Wind turbine must stay connected evenwhen the voltage at the point of common connection (PCC)with the grid drops to zero. The 150-ms delay shown in thefigure accounts for the normal operating time of protectionrelays. The red solid line in Fig. 1 marks the lower voltageboundary rather than any characteristic voltage behavior.

According to the new E.on (one of the major utilities inGermany) grid code of 2006 (2) short term interruption (STI)is allowed under specific circumstances. STI in area 3 re-quires resynchronization within 2 s and a power increase rate

Fig. 1. Fault-ride through requirements

c© 2008 The Institute of Electrical Engineers of Japan. 396

Dynamic Behaviour of DFIG WT During Grid Faults

Fig. 2. Characteristic of wind turbine voltage control

of at least 10% of the nominal power per second. In area 2the interruption time allowed is much less, just a few hundredmilliseconds. However, the main focus of this paper is set onthe protection measures and the FRT capability of the WTfrequency converter. Since the electromagnetical transientsdecay in a very short time, only short term fault cases withsevere voltage drops are of interest in this study. During fault-ride through, reactive power supply by wind turbines is a re-quirement. According to the German grid code wind turbineshave to provide a mandatory voltage support during voltagedips. The corresponding voltage control characteristics aresummarized in Fig. 2. Accordingly wind turbines have tosupply at least 1.0 p.u. reactive current already when the volt-age falls below 50%. A dead band of 10% is introduced toavoid undesirable control actions. However, for wind farmsconnected to the high voltage grid continuous voltage controlwithout dead band is also under consideration.

3. Effects of Grid Faults on DFIG

3.1 Configuration of DFIG-based Wind TurbinesThe rotor of the DFIG is equipped with three phase windings,which are supplied via the rotor side slip rings by a voltagesource converter (VSC) of variable frequency and magnitude(Fig. 3). Speed variability is ensured by the bi-directionaltransfer of slip power via the frequency converter.• In the sub-synchronous operating mode (partial load

range) the generator stator feeds all generated electri-cal power to the grid, and additionally makes slip poweravailable which is fed from the frequency converter tothe rotor.• In the super-synchronous operating mode (nominal load

range) on the other hand total power consists of the com-ponent from the generator stator plus slip power, whichis fed from the rotor to the grid via the frequency con-verter. Active power, which is fed to the grid via theconverter, amounts to roughly 25% of total power.

The inverter rating is typically 25–30% of the total systempower, while the speed range of the DFIG is +/−30% aroundthe synchronous speed. Lower inverter rating results in lowerlosses and thus higher overall efficiency, and lower costs forthe inverter as well as for filters compared to a full-sized con-verter system.

The control of the DFIG wind turbine embodies threeparts:

Fig. 3. Structure of the DFIG based wind turbine

Fig. 4. Configuration of the simulated test system

• Speed control by controlling the electrical power refer-ence provided to the converter as well as by the pitchangle.• Rotor side converter (RSC) control directed at the con-

trol of active and reactive power on the stator side.• Line side converter (LSC) control that keeps the DC-link

voltage constant and provides the additional opportunityto supply reactive power into the grid.

The RSC and LSC control is usually implemented to-gether in the same converter control software whereas thespeed/pitch control is realized as a separate unit.

The converter enables decoupled active and reactive powercontrol of the generator (3)–(10). Moreover, reactive power con-trol can be implemented at lower cost since the DFIG sys-tem basically operates in a similar manner to that of a syn-chronous generator as the excitation energy is provided bythe converter. The control of active and reactive power is ac-complished by the fast-dynamic control of the magnitude aswell as the phase angle of the EMF supplied to the rotor. Thesuperior dynamic performance of the DFIG results from thefrequency converter which typically operates with samplingand switching frequencies of above 2 kHz. A detailed de-scription of the DFIG configuration and control can be foundin Ref. (11).

3.2 Effect of Three Phase Grid Short Circuits Todemonstrate the effect of a grid short-circuit a DFIG basedwind turbine together with a small grid has been simulatedusing a detailed time domain model. The configuration ofthe simulated grid is shown in Fig. 4. The system parametersare summarized in Appendix 1.

The three phase short circuit of the DFIG and the corre-sponding transient stator and rotor current components havealready been derived in Ref. (12). The simulation examplepresented here is aimed to show the effects of a short-circuit

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without any countermeasures on the converter and generatorside. As can be seen in Fig. 5 the grid fault can lead to con-siderable overcurrents and over-voltages putting the wholefacility under stress. Conclusions deduced from the simula-tion results are summarized as follows:•As a consequence of the sudden short circuit the sta-

tor currents contain DC-components. On the rotor sidethese currents appear as AC, superposing the muchslower steady-state rotor currents injected by the con-verter. Thus the nominal rotor current is exceeded morethan 2–3 times, which obviously is not acceptable.•Higher rotor currents lead also to rising DC-link voltage.

The DC-capacitor is usually not able to reduce this effectconsiderably so that the DC-voltage, without counter-measures, would reach values of about 2–3 UDC nominal,which are far away from capacitor specifications due toconverter design.•Due to the common control, the LSC tries to stabilize

the DC-voltage. This causes the LSC current to increaseup to 50% of the wind turbine nominal current, whichrepresents a significant overload of the LSC. Despitethe action of the LSC, the DC-voltage will reach values

Fig. 5. Behavior of DFIG wind turbine during a threephase grid short circuit without FRT measures (theoreti-cal case)

mentioned above.• Immediately after the short circuit clearance, the wind

turbine shaft will experience oscillating torque, the firstpeak of which reaching 2–3 tnominal leading to severestressing of the turbine shaft.

As a general conclusion one can easily realize that with-out countermeasures grid short circuits would easily resultin deterioration of the converter system. On the other hand,fast separation from the grid is not a favorable option sinceutilities expect voltage support during the fault and in its af-termath. Besides, the active power in-feed should not be in-terrupted for a longer period of time. In the next chapter FRTcontrol will be discussed in detail.

4. Fault-Ride Through with DFIM

Figure 6 shows a DFIG system for improved fault ride-through capability (13). It contains two protection circuits,a DC-chopper and an AC-crowbar to avoid DC-link over-voltages during grid faults. The chopper module is not es-sential for fault ride-through operation but it increases thenormal range of DFIG operation by smoothing the DC-linkvoltage during heavy imbalances of active power on the rotor-side and line-side converter. Theoretically converter andchopper could be designed to withstand even short-circuitsat the stator terminals but economic considerations normallylimit to a lower rating.

Fig. 6. DFIG base system extended by FRT protectiondevices

Fig. 7. Operating modes of the DFIG rotor side con-verter

398 IEEJ Trans. IA, Vol.128, No.4, 2008

Dynamic Behaviour of DFIG WT During Grid Faults

Fig. 8. FRT example with only one crowbar period byusing both crowbar and DC-link chopper as well

Then, for deep voltage sags the crowbar short-circuits therotor to protect the converter and the system goes to an asyn-chronous machine operating mode, in which the DFIG is notcontrolled by the machine-side converter. The crowbar firingis triggered by the DC-voltage which rises due to the first ro-tor current peak. The IGBT’s are usually stopped by the pro-tection but the current and thus the energy continues to flowinto the DC-link through the freewheeling diodes leading to avery fast voltage increase. When the crowbar is switched on,the converter is separated from the rotor circuits. The char-acteristic operating modes of the rotor side converter and the

Fig. 9. FRT example with a second crowbar period aftervoltage recovery (no chopper is implemented)

crowbar during FRT are shown in Fig. 7.Following the short circuit the system goes first from nor-

mal mode 1 into mode 4. Then, mode 2 follows which isalso called crowbar mode. After a short delay of about 60–120 ms during which the flux transients in the machine diedown, the converter resynchronizes and the system goes backinto controlled DFIG operation. Because the crowbar thyris-tor switches used in many applications will not interrupt thecurrent before their zero-crossing, the exact interruption timeis not predictable. Therefore, between crowbar interruptionand converter resynchronisation is a possible time slot withopen rotor circuits characterised as mode 3 in Fig. 7. When

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Fig. 10. Zero voltage ride through by using both crow-bar and DC-link chopper as well

the converter is started again the DFIG can provide reactivepower support to the grid and thus help stabilizing the gridvoltage. To avoid overload, active power will be reduced au-tomatically when the converter current reaches its rated level.Additionally reactive power is dynamically provided by theline-side converter even during the period of crowbar firing.

In Fig. 8 the characteristic behaviour of the DFIG is shownfor the same fault as simulated for Fig. 5 but by consideringcrowbar and also the DC-link chopper. Immediately after theshort circuit the DC-voltage rises to about the threshold valueof 1.1 p.u., despite the intervention by the chopper. Then thecrowbar is switched on and the RSC is separated from theDFIG. After 60 ms the RSC is re-synchronized and the windturbine starts to supply active and reactive currents to the gridalthough the grid fault is still not cleared. To avoid overload,

the rotor current limiter is active during this period. Follow-ing voltage recovery the system experiences a similar distur-bance as at the beginning of the fault. However, in this casethe DC-link voltage is kept within limit by the chopper sothat the crowbar is not needed for a second time. As can beseen from the results the RSC has been successfully protectedagainst the overload during the grid fault. On the other hand,the short circuit currents in the stator and rotor circuits arenot limited.

Thus, the first peak in the electromagnetic torque is stillhigh putting stress on the machine shaft. The whole systemgoes back to normal operation even if the grid voltage re-mains low. This operation mode requires also pitching ofthe rotor blades to adapt the generated power to the reducedactive power supply that is now lower due to the current lim-itation. It is a matter of proper chopper design to avoid asecond crowbar firing. Otherwise the second crowbar periodwould result in much higher reactive power consumption bythe DFIG that is not desirable from the grid point of view.This case is shown in Fig. 9 for the same system fault as ap-plied in the previous simulation examples.

To demonstrate the effect of the second crowbar firing nochopper has been considered. As can be seen from the figurethe reactive power consumption during this period reaches1 p.u. that leads to considerable voltage excursion. In theworst case scenario, it may result in grid voltage collapse.Therefore, a second crowbar firing should be avoided in allpossible cases.

At even lower voltages down to 0% the IGBTs areswitched off and the system remains in a standby mode,where the controller is still in operation and observes the gridvoltages. Simulation results are shown in Fig. 10.

If the voltages are above a certain threshold value, theDFIG system is very quickly synchronized and is back in op-eration again, otherwise the system shuts down after a presettime. This sequence of events leads to the conclusion that aminimum, physically reasonable voltage level should be de-fined in the grid code, above which reactive power support isdemanded and below the standby mode prefered to expediteimmediate system operation after voltage recovery.

5. Conclusions

DFIG based wind turbine systems are heavily stressed dur-ing grid faults. To guarantee grid connectivity even at zerogrid voltage in accordance with utility requirements specialFRT measures are necessary. Wind turbines equipped withrotor crowbar and DC-link chopper are able to stay on gridand limit currents and voltages below the threshold values.Since during the crowbar period the DFIG is not controllable,reactive current support to the grid is provided by the LSC,which may result in a temporary overload. However, the LSChas also to control the active power flow through the con-verter by controlling the DC-link voltage that always has tobe ensured by the LSC control. A second crowbar period af-ter voltage recovery represents the worst case scenario for thegrid because of the high reactive power demand. However,with a well-designed DC-chopper the crowbar firing at themoment of voltage recovery can be precluded, which catersnot only for the needs of the transmission network, but alsofor secure, safe operation of the mechanical shaft system of

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the wind turbine.(Manuscript received April 24, 2007,

revised Sep. 12, 2007)

References

( 1 ) I. Erlich and U. Bachmann: “Grid code requirements concerning connec-tion and operation of wind turbines in Germany”, Power Engineering SocietyGeneral Meeting, 2005, IEEE, June 12–16, pp.2230–2234 (2005)

( 2 ) I. Erlich, W. Winter, and A. Dittrich: “Advanced Grid Requirements for theIntegration of Wind Turbines into the German Transmission System”, IEEE-PES General Meeting Montreal, panel paper 06GM0837 (2006)

( 3 ) T. Burton, D. Sharpe, N. Jenkins, and E. Bossanvi: “Wind Energy Hand-book”, John Wiley & Sons, Ltd (2001)

( 4 ) S. Mler, M. Deicke, and R. De Doncker: “Adjustable speed generators forwind turbines based on doubly-fed induction machines and 4-quadrant IGBTconverters linked to the rotor”, Records of the IEEE IAS Conferernce, Rome,CD (2000)

( 5 ) R. Datta and V.T. Ranganathan: “Decoupled control of active and re-active power for a grid-connected doublyfed wound rotor induction ma-chine without position sensors”, Conference Record of the 1999 IEEE In-dustry Applications Conference, Thirty-Fourth IAS Annual Meeting (Cat.No.99CH36370), pp.2623–2628

( 6 ) A. Geniusz and Z. Krzeminski: “Control system based on the modified multi-scalar model for the Double Fed Machine”, Records of the PCIM Conference,Nnberg (2005)

( 7 ) Z. Krzeminski: “Sensorless Multiscalar Control of Double Fed Machine forWind Power Generators”, Osaka (2002)

( 8 ) W. Leonhard: “Control of Electrical Drives”, Springer-Verlag, 2nd Edition(1996)

( 9 ) S. Peresada, A. Tilli, and A. Tonielli: “Power control of a doubly fed in-duction machine via output feedback”, Control Engineering Practice, Vol.12,pp.41–57 (2004)

(10) A. Petersson: “Analysis, Modeling and Control of Doubly-Fed InductionGenerators for Wind Turbines”, Thesis for the degree of licentiate of engi-neering, Department of Electric Power Engineering, Chalmers University ofTechnology Goteborg, Sweden (2003)

(11) I. Erlich, J. Kretschmann, J. Fortmann, S. Mueller-Engelhardt, and H. Wrede:“Modeling of Wind Turbines based on Doubly-Fed Induction Generatorsfor Power System Stability Studies”, IEEE Transactions on Power Systems,Vol.22, No.3, August (2007)

(12) M.S. Vicatos and J.A. Tegopoulos: “Transient State Analysis of a Doubly-Fed Induction Generator under Three Phase Short Circuit”, IEEE Transac-tions on Energy Conversion, Vol.6, No.1, March (1991)

(13) A. Geniusz and S. Engelhardt: “Riding through Grid Faults with Modi-fied Multiscalar Control of Doubly Fed Asynchronous Generators for WindPower Systems Records of the PCIM Conference”, Nnberg (2006)

Appendix

1. System Parameters

app. Table 1. Machine parameters (related to statorside)

Rated power 3 MWStator rated voltage 750 VStator rated frequency 50 HzStator resistance 2.2 mΩStator leakage reactance 19.8 mΩRotor resistance 1.6 mΩRotor leakage reactance 51.6 mΩMain reactance 1.02ΩRotor/Stator voltage turns ratio 2.5

app. Table 2. Converter parameters

DC voltage 1300 VDC capacitance 15 mFChopper resistance 0.5ΩCrowbar resistance 0.4Ω

app. Table 3. Transformer parameters

Primary voltage 36 kVSecondary voltage 750 VApparent power 4 MVARelative short circuit voltage 9 %Resistance 0.6 %

app. Table 4. MV grid parameters

Voltage 36 kVShort circuit power 150 MVAX/R ratio 5Fault impedance ZK fault 1..3 2ΩFault impedance ZK fault 4 0Ω

Istvan Erlich (Non-member) (1953) received his Dipl.-Ing. degreein electrical engineering from the University of Dres-den/Germany in 1976. After his studies, he workedin Hungary in the field of electrical distribution net-works. From 1979 to 1991, he joined the Depart-ment of Electrical Power Systems of the University ofDresden again, where he received his PhD degree in1983. In the period of 1991 to 1998, he worked withthe consulting company EAB in Berlin and the Fraun-hofer Institute IITB Dresden respectively. During this

time, he also had a teaching assignment at the University of Dresden. Since1998, he is Professor and head of the Institute of Electrical Power Systemsat the University of Duisburg-Essen/Germany. His major scientific interestis focused on power system stability and control, modelling and simulationof power system dynamics including intelligent system applications. He is amember of VDE and senior member of IEEE.

Holger Wrede (Non-member) (1971) received his Dipl.-Ing. degreein electrical engineering from the Technical Univer-sity Braunschweig, Germany, in 1998. From 1998 to2004 he joined the Institute for Electrical Power Engi-neering and Power Electronics of the Ruhr-UniversityBochum, Germany, where he worked on FACTS de-vices, power quality as well as compensation strate-gies and received his PhD degree in 2004. Since 2004he is with SEG GmbH & Co. KG, Kempen/Germany,presently manager of the group Innovation/Converter

Technology and responsible for system designs and simulations, controlstrategies and patents. He is a member of VDI and IEEE.

Christian Feltes (Non-member) (1979) received his Dipl.-Ing. de-gree in electrical engineering from University ofDuisburg-Essen/Germany in 2005. Since January2006 he is doing his Ph.D. studies in the Departmentof Electrical Power Systems at the same University.His research interests are focused on wind energygeneration, control, integration and dynamic interac-tion with electrical grid.

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