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2014 Ninth International Conference on Ecological Vehicles and Renewable Energies (EVER)

Review of Control Strategies for DFIG-Based WindTurbines under Unsymmetrical Grid Faults

Pavlos Tourou, Constantinos Sourkounis

Institute for Power Systems Technology and Power Mechatronics, Ruhr-University BochumUniversittsstr. 150

44801 Bochum, GermanyEmail: [email protected], [email protected]

Abstract As the penetration of wind power and other

distributed generation sources increases, more demanding

grid code requirements are introduced by the network

operators in order to ensure the stability of the evolving

electrical networks. The most challenging requirement for

wind energy converter systems is the low voltage ride

through capability that expects generators to remainconnected during symmetrical and asymmetrical grid

faults and to contribute to the system recovery.

Asymmetrical voltage conditions and dips in the grid can

have significant negative effects on the performance of

doubly-fed induction generators. These effects can

decrease the lifetime of sensitive components in the wind

energy converter in the long term and in extreme cases

they can cause damages and tripping of the system, leading

to violation of the grid code requirements. Protective

measures must be taken so that the wind energy converters

remain connected and support the grid without putting the

reliability of the system at risk. Various control solutions

have been developed to deal with these challenges.Although the most common voltage dips caused by grid

faults are asymmetrical, the majority of the control

solutions developed so far consider only symmetrical faults

and they cannot mitigate the problems faced under

asymmetrical conditions. This paper provides a

comprehensive overview of different linear vector control

methods for wind energy converter systems with doubly-

fed induction generators which have been proposed in the

literature to deal with the challenge of operating during

voltage asymmetry and riding-through all types of

voltage dips, symmetrical and asymmetrical.

Keywords doubly-fed induction generator (DFIG),wind energy, grid integration, energy conversion, low

voltage ride through (LVRT), asymmetrical faults,

voltage unbalance, current control, power control,

voltage dips, control system, resonant controllers,

linear control, double synchronous reference frame

(DSRF), single synchronous reference frame (SSRF)

I. NOMECLATURE

, - stator and rotor voltage vectors , - induced rotor emf space vector in the static androtor reference frames respectively

, - stator and rotor currents space vectors, - stator and rotor flux space vectors, - stator and rotor resistances - rotating speed of the arbitrary reference frame - rotor electrical angular speed - magnetizing inductance, - stator and rotor self-inductances - moment of inertia of the rotor and shaft

- number of pole pairs in the generator

- mechanical torque applied at the generator shaft - electromechanical torque of the generator - time constant of the statorII. INTRODUCTION

Wind power has been proven both technologically andeconomically, as a reliable source for electricityproduction. Significant amounts of clean electricity areproduced by wind energy converter systems (WECS)

without burdening the environment with greenhouse gasesthat contribute to global warming and extreme weathereffects [1]. In 2011 WECS provided about 3.5% of theglobal electricity demand and this figure is expected to riseto 16% by 2030 under moderate scenarios [2]. In Europe itis estimated that in 2013 an installed wind energy capacityof 117 GW produced 257 TWh of electricity, enough tocover 8% of the EUs electricity consumption [3]. As thepenetration of wind power and other renewable energysources in electrical grids increases, WECS and windfarms are expected to behave more like conventional

978-1-4799-3787-5/14/$31.00 2014 IEEE


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power plants by fulfilling stricter grid code requirements.One of the most important requirements for the gridstability in the case of grid faults is the low voltage ridethrough (LVRT), by which the WECS must remain inoperation during symmetrical and asymmetrical voltagedips. Additionally they must support the voltage recovery

by supplying reactive current to the grid during the fault[4].

The LVRT requirement is particularly challenging forWECS with doubly-fed induction generators (DFIG) dueto the limited rating of the power electronic converters thatare used to manage the electrical power. Most of theWECS in the Megawatt range installed around the worldare based on the DFIG topology. Their popularity ismainly due to the partial rating of the power electronicconverters which allows for a variable speed range that issufficient for maximum power conversion, while theinvestment costs are significantly lower compared to

WECS with fully-rated power electronic converters.

Fig. 1: Typical configuration of a wind energy converter system witha doubly-fed induction generator

As the stator terminals of the DFIG are connecteddirectly or through a transformer to the grid, significantdisturbances in the grid voltage induce very highelectromotive forces and currents in the rotor circuit whichcan damage the power electronic converters if the system

remains connected to the grid. In the case of asymmetrical(also called unbalanced) grid voltages, the rotor-sideconverter (RSC) can saturate and significant power andtorque oscillations can arise. Therefore protectivemeasures must be taken so that the WECS remainsconnected to the grid and fulfills the LVRT requirements,without damaging sensitive components of the system.

Although a plethora of control systems have beenproposed in the literature, the majority of them deal onlywith symmetrical grid faults which are extremely rare inelectrical grids in comparison to asymmetrical faults. This

paper presents various control systems and fault-ridethough strategies that can be applied during asymmetricalas well as during symmetrical faults. The main objectiveof these solutions is mitigating the negative effects on theWECS caused by these abnormal grid conditions in orderto meet the various grid code requirements. Due to space

limitations, the current review considers only linear vectorcontrol methods.

III. GRID FAULTS AND ASYMMETRICAL VOLTAGES

Faults in electrical power systems mainly occur due tonatural phenomena such as storms and lightning and to asmaller extend due to animal behavior and technicalreasons [5], [6]. Four different types of short-circuit faultscan occur in three-phase power systems: single-phase-to-ground (1), phase-to-phase (), two-phase-to-ground(2) and three-phase-to-ground (3). Fault types 1, 2and result in asymmetrical voltage dips, where the

voltage magnitude of the three phases is not the sameand/or the phase difference between successive phases isnot 120 as it is the case under normal operation. The mostcommon short-circuit faults in power are single-phase-to-ground followed by two-phase faults [7].

Most of the voltage dips caused by such faults have avery short duration because the source of the fault is oftentemporary or due to the fast protection employed thatdisconnects any lines affected by longer faults. Thesevoltage dips last until automatic reclosing equipmentreconnect the tripped lines. Less severe but sustainedasymmetrical voltage conditions can be common in remote

rural networks where wind farms are connected to the gridthough long transmission lines with asymmetric electricalparameters [8] or in weak distribution grids which containhigh percentage of single phase loads [9], [10].

IV. NORMAL AND FAULT BEHAVIOUR OF THE DFIG

The behavior of the DFIG under normal operation andunbalanced operation is explained based on the approachdeveloped in [11] and [12].

A. Normal operation

The DFIG is modelled in a static reference frame using

the following dynamic equations: (1) (2)

The stator and rotor flux vectors can be expressed interms of the stator and rotor current vectors and themagnetizing and leakage inductances:

(3)


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(4)In the above equations all the rotor quantities are

referred to the stator based on the stator-to-rotor turnsratio. The rotor mechanical speed can be describeddynamically in terms of the external mechanical torque

from the wind rotor and the electromechanical torqueusing the equation of motion:

(5) 1.5 . 1.5 . (6)

The rotor flux can be written in terms of the stator fluxand rotor current using (3) and (4)

(7)where is the leakage factor

1 (8)From (2) and (5) the rotor voltage is obtained

(9)The rotor voltage consists of the first term that is the

voltage induced by the variation of stator flux and thesecond term that is the voltage drop across the rotorresistance and the rotor transient inductance. If the rotor isopen-circuited the rotor voltage will be equal to theinduced emf

(10)The induced emf depends on the slip and on the

variation of the stator flux. In the case of a grid-connectedDFIG the stator flux variation is imposed by the gridvoltage. As a result any change in the grid-voltage willaffect the stator flux and consequently the induced emf in

the rotor. Abrupt and deep voltage dips can induce verylarge voltages in the stator and rotor and they can lead as aconsequence to excessively large currents.

At normal operation the stator voltage is given by

(11)If the stator resistance is ignored the stator flux space

vector is

(12)

B.Behavior under dips in the grid voltage

The theory of symmetrical components is used toanalyze three-phase systems under symmetrical andasymmetrical conditions. According to this theory anyasymmetrical set of N phasors can be decomposed into alinear combination of N symmetrical sets of phasors [13].In a three-phase system the decomposition results in the socalled positive, negative and zero sequence components.

An asymmetrical grid voltage at the Point of CommonCoupling (PCC) can contain all three sequences. As thevoltage propagates through the intermediate transformersthe negative sequence can be eliminated before it reachesthe terminals of the WECS. The voltage dip at the WECSterminals, not only depends on the type of asymmetricalvoltage at the PCC but also on the winding connections ofthe transformers, as well as on the winding connection ofthe stator [14]. In most common wind farm configurationsthe zero-sequence voltages are blocked and only positive-sequence and negative-sequence voltages are experiencedby the WECS [15], [16]. Therefore zero sequencecomponents can be omitted in the subsequentmathematical analysis of the DFIG.

In asymmetrical grid voltage conditions the statorvoltage consists of two components, a positive andnegative sequence components [11]

(13)During steady-state the forced stator flux will be

made of two components corresponding to the positive andnegative sequences of the stator voltage. Although thevoltage can change instantaneously at the start and end ofthe dip, the stator flux is a continuous variable and anabrupt decrease in the flux is not physically possible. If thetotal stator flux immediately after the fault is not equal tothe stator flux immediately before the fault, then atransient flux exists, called natural flux. The initial valueof this transient flux depends on the type and timing of thefault. Its decay rate depends on the time constant of thestator in the rotor open-circuit case. Therefore the totalstator flux is made of two steady-state and one transientcomponent [12], [17]

. / (14)

Similarly, the total emf induced by the variation ofstator flux in in the rotor terminals can be expressed in therotor reference frame as

1. . 2 . 2. 2


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. (15)The first term of the equation rotates with the slip

frequency and it is proportional to the positive sequencevoltage and to the slip. This emf is also present during

normal operation. The RSC is normally designed so that itcan produce voltages large enough to counteract this emfand to regulate the rotor currents. The third term is atransient emf that appears at the start and end of the dipand it can occur in both symmetrical and asymmetricaldips. The second term appears only in the presences ofasymmetrical voltage. It is proportional to the negativesequence voltage and to (s-2). At severe unbalance thetotal emf can be larger than the rated RSC voltage, leadingto loss of current regulation.

C.Active power, reactive power and generator torque

The apparent power at the stator is given by

1.5 (16)The active and reactive powers are given by

1.5 ,2 ,2 (17) 1.5 ,2 ,2 (18)

The components of the powers expressed in positiveand negative rotating reference frames are

,,,,

(19)

Similarly, the electromagnetic toque is given by

1.5 ,2 ,2 (20)

,,

(21)

V. EFFECTS OF ASYMMETRIC CONDITIONS ON WECS

Asymmetric grid voltages can deteriorate theperformance, reliability and fault-ride through capabilityof the DFIG-based WECS. If no protecting measures aretaken, the WECS is subject to different negative effectsincluding:

- Significant stator current unbalance even with lowgrid voltage unbalance due to the low negativesequence impedance of the generator at normaloperating slip speeds as it can be observed inequation (15) [9]

-

Unequal magnetic distribution in the stator androtor; can cause unexpected magnetic saturation,thermal losses and excessive heating [8], [18]

- Electromagnetic torque oscillations; torsionalforces can reduce the lifetime of rotating parts onWECS drive-train (shaft, couplings, gearbox,blades) and create acoustic noise [19], [20]

- Reduction of the average torque and as aconsequence reduction of the maximum outputpower capability of the DFIG [18]

- High GSC current unbalance even at low grid

voltage unbalance- Voltage and current harmonics in the DC-link; can

trip the back-to-back power convert, shortenlifetime of DC-link capacitor and lead to capacitorfailure [8]

- 100 Hz oscillations in the instantaneous active andreactive power

VI. LINEAR VECTOR CONTROL

Vector control is the most common method used for

DFIG-based wind energy converters. Cascaded vectorcontrol systems implemented in a synchronous referenceframe (SRF) have been extensively used for the powerconverter control under normal operating conditions [21],[22], [23], [24], [25], [17]. Reference and feedbackvariables are transformed in a SRF (dq+-frame) thatrotates with the positive sequence space vector of thestator flux (stator-flux orientation) or with the positivesequence space vector of the grid voltage (grid-voltageorientation) where they appear as DC values at steady-state. Proportional-integral controllers (PI) are used in theinner and outer loops as they provide satisfactory

performance when acting upon DC signals [26]. Thecontrol system of the RSC includes fast inner control loopsregulating the rotor currents and slower outer loopscontrolling active and reactive power at the stator. TheGSC outer loops are used to regulate the DC-link voltageand the reactive power at grid side. This control structurehas been used extensively in combination with hardwareprotection (e.g. rotor crowbar, DC-link chopper) tomaintain operation during symmetrical grid faults [27],[28], [29].

During asymmetrical conditions, the presence ofnegative sequence components in the voltages, currents


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and fluxes deteriorates significantly the DFIG performanceas described in section V. To mitigate the negative effectson the WECS and to improve its LVRT capability thenegative sequence currents in the DFIG must be controlledsimultaneously with the positive sequence currents. Whentransformed in the positive sequence SRF, negative

sequence currents appear as double frequency signalssuperimposed on the DC signals of the positive sequencecurrents. PI controllers cannot regulate effectively thenegative sequence currents due to their limited closed-loopbandwidth. The bandwidth can be increased by modifyingthe gains of the PI controllers but this drives the currentloop to instability [30]. Various solutions have beenproposed in the literature which extend the vector controlconcept in order to be applied under asymmetricalconditions. These solutions mainly aim at a simultaneousregulation of the positive and negative sequencecomponents in the rotor and grid-side converter currents.

VII.

DSRFVECTOR CONTROL WITH PICONTROLLERS

In [31] a model of the DFIG in terms of positivesequence and negative sequence components is developedand expressions for the powers and generator torque in thecase of asymmetrical conditions are obtained. A doublesynchronous reference frame (DSRF) current control isapplied to the RSC of the DFIG. It uses a conventionalSRF aligned with the positive sequence stator flux androtating with grid angular frequency s to regulate thepositive sequence currents and an additional SRF rotatingwith the same speed but in the opposite direction toregulate the negative sequence currents as shown in Fig.2.The reference currents are calculated using equations (19)and (21) and four PI controllers regulate the four currentcomponents. One of the following targets can be achievedat one time: balanced stator current, constant stator activepower, constant torque, no rotor current oscillations. Allthese targets increase the oscillations in the DC-linkcurrent as a tradeoff and they can be fully achieved only atmoderate voltage unbalance. The voltage rating of theRSC must be increased in order to effectively control thenegative sequence currents in the presence of high gridvoltage unbalance. The main disadvantage of this methodis the need for extracting the sequence components of the

currents. This introduces significant time delays and errorsin the magnitude and phase of the feedback signals. As aresult, accurate decoupling during transients cannot beachieved and the stability of the system is limited [32].

The DSRF current control method is applied to bothpower converters in [33] and [20]. Independent control ofthe RSC and GSC is employed in [33] with the objectivesto limit torque pulsation and DC-link voltage ripplerespectively. A coordinated control between the two powerconverters is proposed in [20] by taking account of thegrid code requirements and the control capability of the

converters. Supplying the positive-sequence reactivecurrent required by the grid codes during the fault and thenmitigating the torsional oscillations are the prioritieschosen for the RSC control. The GSC control prioritizespositive-sequence current control to regulate the averageDC-link voltage and to contribute to the reactive current

requirement while the elimination of DC-link voltagefluctuations is given a lower priority.

Fig. 2: Typical reference frames used in DSRF (-stationary, r-rotor, dq+-positive sequence synchronous, dq--negative-sequencesynchronous) [31]

A similar approach is used in [34] where a wind farmmade of DFIGs is configured to inject negative sequencecurrents in the grid in order to decrease the asymmetries inthe PCC voltage. Four control strategies are proposed:negative sequence current injection by the RSC only,

negative sequence current injection by the GSC only,negative sequence current injection by both RSC and GSCwith priority given to the GSC and lastly minimization oftorque oscillation by the RSC while the GSC provides thenegative sequence voltage compensation. Simulationinvestigations showed that the third strategy can providemaximum grid voltage compensation with the tradeoff ofsignificantly increased torque oscillations while the fourthstrategy minimizes torque oscillations but it cannotremove completely the negative sequence in the gridvoltage.

A coordinated control strategy and improvements to

the DSRF method and are reported in [32]. A conventionalcurrent loop without any sequence extraction is employedfor the main controller in the positive SRF, thus avoidingunnecessary computation and errors during normaloperating conditions. Sequence separation is still needed inthe auxiliary current controller operating in the negativeSRF which is activated only when significant voltageunbalance is detected. The negative sequence currentreferences of the RSC control are calculated so as toremove the torque oscillations. The negative sequencecurrents of the GSC are controlled to compensate theactive power oscillations at the stator, and thus achieving


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constant total active power. If both control targets are metthe oscillations in the DC-link voltage are also eliminated.

VIII. SSRFVECTOR CONTROL WITH PROPORTIONAL-INTEGRAL PLUS RESONANT CONTROLLERS

A simple extension of the conventional single

synchronous reference frame (SSRF) PI-based vectorcontrol was proposed in [35]. A high-Q second-orderresonant filter tuned at 100 Hz is used as a feedbackcompensator in the current loop. This approach minimizedthe torque oscillations at moderate voltage unbalances.The current closed-loop gain bandwidth is increasedslightly by the addition of the compensator. As stated in[30], this type of current compensation can be effectiveonly if the current controller bandwidth is larger than 100Hz.

The current control proposed first in [32] is improvedin [36] by the addition of a resonant compensator parallelto the PI controller (PI+R) in the positive synchronousreference frame. The dc components in the synchronousreference frame are mainly regulated by the PI controllerwhile the double-frequency ac signals due to the negativesequence are fully controlled by the R compensator.Simultaneous regulation of both positive and negativesequences is achieved in the positive sequence referenceframe without the need for sequence extraction and for aseparate negative-sequence synchronous frame controller.The introduction of the compensator increases the closed-loop bandwidth of the current control slightly and thephase margin is reduced by 20 which decreases the

robustness of the system especially during transients.In [37] the PI+R concept is extended to a PI regulator

and a dual-frequency resonant compensator (DFR), tunedat twice and six times the grid frequency designed toregulate simultaneously the fundamental and fifth- andseventh-order harmonic components of the currents. In thisway the system performance can be improved in the caseof asymmetrical as well as harmonically distorted voltageconditions. A similar approach is adopted in [38] where amore complex current control structure with resonantcontrollers is used to compensate for asymmetricconditions and higher order harmonics.

IX. STATIONARY REFERENCE FRAME VECTOR CONTROL

An alternative current control method, implemented inthe stationary reference frame, is first proposed in [39] forapplication in DFIG-based WECS and it was furtherinvestigated in [40]. In this case proportional-resonantcontrollers (PR) are used, with two resonant componentstuned at frequencies of the positive and negativesequences. PR implementation is challenging for the RSCbecause the resonant frequencies vary with the mechanicalspeed of the generator and they have current controllermust be continually tuned. The frequency adaptive

resonant controllers presented in [40] exhibit the samedynamical response as the synchronous reference framecontrollers under normal operating conditions. Underasymmetrical conditions the suggested method provides asuperior performance compared to classical PI control inthe SSRF as it can control simultaneously the positive and

negative sequence components of the rotor current. It isalso claimed in [40] that this type of controllers have abetter transient performance than solutions described insections VII and VIII. This is because the FrequencyLocked Loop (FLL) required for tuning the resonancefrequency is faster and more robust than the stator fluxestimator or Phase Locked Loop (PLL) which is needed toprovide the phase angle for synchronization with the SRF[40].

Current control in the stationary reference frame is alsoinvestigated in [41]. The objective here is to limit thetransient rotor inrush currents of DFIG at fault occurrence

and to minimize the negative sequence effects duringsteady-state. The strategy presented consists of aproportional-resonant (PR) controllers and auxiliary PRcontrollers. The auxiliary controllers are activated only inthe case of grid faults in order compensate the outputvoltage of the RSC and to limit rotor fault currents withoutextracting dc and negative sequence components of thestator flux as in [42]. Accurate detection of the rotorangular frequency and very good tuning of controlparameters are required, otherwise the stability andperformance of the proposed control strategy deteriorates.It should be notes that stationary reference frame current

control with resonant controllers is more commonly usedfor grid-connected voltage source converters [43], [44],[45] since the grid frequency is constant under normalconditions and it can be a good solution for the GSC ofDFIG systems.

X. TRANSIENT FLUX DEMAGNETIZATION

The transient flux demagnetization method is firstproposed in [42] as an extension to the conventionalstator-flux-oriented vector control strategy to help theDIFG ride-through grid faults. The RSC is used tosynthesize a rotor current space vector with appropriate

amounts of dc and negative sequence components in orderto oppose the respective undesired components in thestator flux of equation (14) as shown in Fig.3. Theobjective is to limit the overvoltages and overcurrents inthe rotor and to avoid the activation of a crowbar. Quickdemagnetization of the natural flux is achieved within 50ms, which is faster than the uncontrolled case [46]. Over-modulation of the RSC voltage and a rotor over-current upto 2 p.u. are allowed during the fault. The DFIG can ridethrough severe single-phase short circuit faults without theneed to apply a crowbar. With other fault types theeffectiveness of this method decreases with increasing


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fault severity and increasing rotor speed. The converterrating must be increased in order to maintain control of thetransient rotor currents otherwise the use of the crowbarfor RSC protection cannot be avoided. Design of thegenerator with increased stator or rotor leakage inductanceis suggested for improving the effectiveness of the current

control and fault-ride through capability.

Fig. 3: Algorithm used in [42] to decompose the stator-flux vectorinto positive, negative, and zero sequence (dc) components

The demagnetization method is used also in [47] and[28] in coordination with the crowbar control. Thecrowbar is used at the start of severe grid faults while thedemagnetization current required for maintaining currentcontrol is estimated. As a soon as the estimated current iswithin the current and voltage capability of the RSC, thecrowbar is deactivated and current control is resumed bythe RSC, injecting reactive current into the grid andproviding at the same time the necessary demagnetizing

currents to keep total rotor voltage below the limit of theconverter. This allows a faster reactive current injectionthat meets the strictest grid code requirements [4].

Similar approaches with lower dependence on theDFIG parameters are adopted in [48] , [49] and [50] inorder to damp the stator flux transients but as with [47]and [28] only symmetrical voltages dips are considered.Stator flux estimation and feed-forward transientcompensation of the current loops for both symmetricaland asymmetrical dips is proposed in [51] but the methodis only tested under symmetrical grid voltage. Dc andnegative sequence components still have to be extracted

increasing computational time, slowing response speedand reducing effectiveness of the decoupling duringtransients.

The proportional-resonant (PR) current control strategyin a stationary reference frame proposed in [41] claims amuch better performance in terms of transient fluxdamping because no frame transformations are needed, nocoupling terms are affected by temperature and systemparameters and the compensation can be achieved withoutextracting dc and negative sequence components of thestator flux.

XI. CONCLUSION

The challenges faced by grid-connected wind energyconverter systems with doubly fed induction generatorsin the case of asymmetrical voltage dips have beenpresented. The fault behavior of the DFIG was briefly

analyzed and the impact of asymmetrical conditions onthe its operating performance and reliability werediscussed. Classical current control in a synchronousreference frame cannot provide the required performanceunder these conditions. An overview of state-of-the-artlinear vector control solutions was presented. Thedeveloped solutions improve significantly the fault ride-through performance of the DFIG-based WECS bymitigating the negative effects caused by transients andvoltage asymmetries and they can be extended tocompensate also for higher harmonics. At extreme faultsthe effectiveness of these control strategies decreaseswith increasing grid voltage unbalance at which the

physical boundaries become the limiting factor.

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