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476 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 2, FEBRUARY 2010

Extending the Life of Gear Box in Wind Generatorsby Smoothing Transient Torque With STATCOM

Marta Molinas, Member, IEEE, Jon Are Suul, and Tore Undeland, Fellow, IEEE

Abstract—Gearboxes for wind turbines must ensure high reli-ability over a period of 20 years, withstanding cumulative andtransient loads. One main challenge to this is represented byelectromagnetic torque transients caused by grid faults and dis-turbances, which will result in significant stresses and fatigueof the gearbox. Possibilities for limiting the torque transients infixed-speed wind generators have not been previously reported.This paper presents a technique by which the transient torquesduring recovery after a grid fault can be smoothed in a wind farmwith induction generators directly connected to the grid. A model-based control technique using the quasi-stationary equivalent cir-cuit of the system is suggested for controlling the torque with astatic synchronous compensator (STATCOM). The basis of theapproach consists of controlling the induction generator terminalvoltage by the injection/absorption of reactive current using theSTATCOM. By controlling the terminal voltage as a function of thegenerator speed during the recovery process, the electromagnetictorque of the generator is indirectly controlled, in order to reducethe drive train mechanical stresses caused by the characteristics ofthe induction machine when decelerating through the maximumtorque region. The control concept is shown by time-domainsimulations, where the smoothing effect of the proposed techniqueon a wind turbine is seen during the recovery after a three-phase-to-ground-fault condition. The influence of the shaft stiffness in amultimass drive train model is discussed, and the performance ofthe control concept in the case of parallel connection of severalturbines is investigated to discuss the applicability in a wind farm.

Index Terms—Static synchronous compensator (STATCOM),voltage source converter, wind energy.

I. INTRODUCTION

A LLEVIATION OF mechanical stresses in wind gen-eration systems has traditionally been associated with

variable-speed wind turbine topologies by performing directcontrol of the electromagnetic torque through power electron-ics. By partial or complete decoupling of the mechanical tran-sients of the wind turbine from the electrical transients of thepower system, smoothing of the transient torques on the turbinedrive train is thus possible by controlling the electromagnetictorque directly [1]–[5]. However, in fixed-speed wind genera-tion systems based on induction generators directly connectedto the grid, there has been no obvious way to control the torquefor alleviating mechanical stresses in the drive train. This isconfirmed by the lack of literature on such a possible approach.Still, many installations today are using this type of generation

Manuscript received December 1, 2008; revised October 15, 2009. Firstpublished November 6, 2009; current version published January 13, 2010.

The authors are with the Department of Electric Power Engineering,Norwegian University of Science and Technology, 7491 Trondheim, Norway.

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

Digital Object Identifier 10.1109/TIE.2009.2035464

system, and more are being planned for the future. Therefore,having the possibility to control torque transients related to gridfaults could be beneficial for reducing the stresses in the drivetrain of fixed-speed generators.

Fixed-speed generators are vulnerable to contingencies suchas faults and disturbances that might appear in the nearby gridbecause they are locked to the grid voltage and frequency.In these cases, the stresses on the generator and drive trainwill be determined by the input mechanical torque and theelectrical connection to the grid. However, if, as proposedin [6], an indirect control of the torque is implemented byinjecting/absorbing reactive current with a static synchronouscompensator (STATCOM), transient torques during recoveryafter a grid fault can be smoothed. This way, it could be possibleto extend the life of the drive train by reducing cumulative andtransient loads related to grid faults [7].

Several studies of low-voltage ride through (LVRT) haveconfirmed how reactive compensation can increase the torquecapability and, by that, the stability limit of induction genera-tors [8]–[10]. As shown in [11] and [12], this will also increasethe maximum torque that occurs during the recovery processwhen standard control strategies are implemented. This paperinvestigates the implementation of indirect torque control (ITC)by a STATCOM to wind turbines in a wind farm. The basicassumption is to utilize a STATCOM originally installed forproviding LVRT capability [8], [12] for controlling the torqueof the machine only during the recovery process.

The STATCOM-based torque control is implemented asa model-based control approach and added to the standardSTATCOM control structure as a new mode of operation. Thismode of operation was first presented in [6] and is labeled asITC since it is based on using the STATCOM to control thevoltage at the generator terminals and, by that, indirectly con-trolling the torque during the recovery process after a grid fault.

In [6], the concept of ITC was presented and applied asa technique for torque transient alleviation in a single windturbine unit. In this paper, a more elaborated explanation ofthe proposed concept is presented, and the properties of theITC are further investigated. Based on the results in [6],the control concept is analyzed both for critical faults close tothe stability limit of the system and for shorter faults where theoperation of the STATCOM is not needed to ensure stability ofthe induction generator. The torque control concept is furtherextended to a wind farm composed of paralleled turbines, witha STATCOM at the terminal of each generator. All STATCOMsare using the proposed technique in its control structure as anadditional mode of operation that is activated only during therecovery process after a grid fault. The parallel operation of

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MOLINAS et al.: EXTENDING LIFE OF GEAR BOX IN WIND GENERATORS 477

Fig. 1. Schematic configuration of the wind farm composed of two windgeneration systems in parallel, each with a STATCOM.

the STATCOM has raised new operational issues related to thereactive power exchange between STATCOMs and the grid,which are discussed in this paper.

II. STATCOM-BASED TORQUE CONTROL CONCEPT

Fig. 1 shows the schematic configuration of the wind farmin which the ITC technique is implemented. The wind farminvestigated in this paper is modeled in PSCAD®™ by twowind generation units in parallel, each with a STATCOM at theterminals. One wind generation unit consists of an inductiongenerator driven by a wind turbine through a gear box, con-verting the low speed of the turbine shaft into a high speedthat matches the rotational speed of the induction generator.The induction generator is connected to the grid through atransformer, and a STATCOM is connected at the generator ter-minals to control the voltage level at the generator by injectionof reactive current [6], [13]–[15]. As reported in [8] and [12],the STATCOM was originally installed to improve the transientstability and the critical clearing time of the wind generator and,by that, to increase the LVRT capability. High levels of reactivecompensation to improve the fault ride-through capability ofthe system will, however, increase the maximum torque of thegenerator during the recovery process [8], [11].

In order to temporarily avoid the high level of reactivecompensation during recovery, the control structure of theSTATCOM is expanded, as shown in the block diagram ofFig. 2. The introduction of the ITC block, in addition to the nor-mal STATCOM control, allows for torque transient alleviationduring the recovery process after a grid fault. This is possibleto achieve by reducing the voltage reference of the STATCOMcontrol system and, by that, the reactive compensation whenstability is ensured after fault clearing but before the gridvoltage and the speed of the generator have returned to theprefault values. In this way, the STATCOM can improve thetorque capability of the induction generator when this is neededto keep the system stable, and once stability is ensured, itcan reduce the maximum torque during recovery. The strainon the drive train can thereby be reduced. This is particularlyrelevant in the context of LVRT where wind turbines cannotjust disconnect from the grid to protect the installation from riskof mechanical damage that might be caused by the cumulativestress of repeated peak torque transients [7].

Fig. 2. Block diagram of the control system, including ITC and normalSTATCOM.

Fig. 3. Quasi-stationary equivalent circuit for the system under study, consist-ing of the traditional induction machine equivalent, the STATCOM modeled asa current source, and a grid equivalent.

At the clearing instant of a fault and afterward, transientsof the electromagnetic torque will result in significant stressesfor the wind turbine mechanical system and can have harmfuleffects on the lifetime of drive-train-sensitive components suchas the gearbox [16]–[18]. Gearbox fatigue is caused by stressingof the gearbox teeth in response to torque overloads. For aninput torque in excess of the gearbox rating, the fatigue damageincreases in the extent to which the rating is exceeded andalso as the length of the time that the overload persists [19].In addition to that, lifetime of the gearbox is reported to beinfluenced by the load–duration distribution. The accumulatedduration of torque levels significantly influences the fatigueload on the gearbox and, therefore, its lifetime [7]. Taking thisinto account, not only the high transient torques will representstresses on the gearbox but also the cumulative torque stressesunder normal operation by adding up to the high transienttorques during recovery after a fault.

The short circuit initial torque transients of the inductiongenerator are not the target of the proposed ITC concept.These transients will contribute to the cumulative stresses ofthe system but cannot be influenced by the STATCOM whenthe grid voltage is close to zero. This paper focuses on therecovery process after fault clearing and the mitigation ofrelated transient torques observed in induction machines. Athree-phase grid failure is used as an example to put in evidencethe torque transients that appear after a grid fault is cleared.However, the recovery process of the induction generator isalmost independent of the type of fault, which will have no sig-nificant influence on the performance of the proposed approach.



III. MODEL-BASED TORQUE CONTROL:ANALYTICAL DERIVATION

The equivalent circuit of the system in Fig. 3 is used toderive the equations that will serve as the analytical basis for thedesign of the model-based control. The analytical results willthen be used to generate a speed-dependent reference voltagefor the STATCOM control system under the ITC mode ofoperation. For investigation of the concept, the analytical equa-tions are implemented in PSCAD® and used to calculate thespeed-dependent voltage reference that is temporarily allowedto override the normal voltage reference value when the ITC isactive, as shown in Fig. 2.

The process of deriving and applying the proposed conceptis explained hereafter starting from (1) which gives the relationbetween the quasi-stationary electromagnetic torque and therotor current of the generator. From this equation, a referencevalue for the quasi-stationary rotor current can be obtained asa function of the slip of the machine, by specifying the valueof the electromagnetic torque. Introducing the rotor current i2found from (1) into (2) will give the stator current i1 as afunction of the slip of the machine. By using the results from(1) and (2), the corresponding value of the terminal voltage ofthe generator can be found from (3). By using this equation asa speed-dependent voltage reference value for the STATCOM,indirect control of the generator torque can be obtained duringthe recovery process

τem,i =r2,i

si|i2,i|2 (1)

i1,i =r2,i

si+ j(x2,i + xm,i)

jxm,ii2,i (2)

v1,i =vSTATCOMi,ref

= i1,i (r1,i + req,r,i + j(x1,i + xeq,r,i)) . (3)

The core concept of the ITC is to limit the electromagnetictorque during the recovery process after a grid fault. Thereference value for the torque must be set at a value thatlimits the torque transients but, at the same time, ensures stabledeceleration of the generator until the system is recovered fromthe influence of the fault. The validity of implementing thesimplified quasi-static model of the induction generator for cal-culation of critical speed and LVRT capability was discussed in[12]. When using the same model to develop the ITC concept,the validity of the quasi-stationary approach can be consideredmore reasonable since the electromagnetic torque is controlledto a value close to the mechanical torque.

As understood from this description, the concept of ITC isderived from a rather simple set of equations. It is also seenthat this control concept, and the quasi-stationary model onwhich it is based, gains its validity by limiting the torque ofthe induction generator during the recovery process after a fault.The control structure sketched in Fig. 2 also shows how the con-cept can be implemented only by temporarily modifying the re-ference value for the normal voltage control objective of theSTATCOM. The control is therefore presented as an additionalcontrol feature of a STATCOM intended for voltage control andimprovement of LVRT capability of wind turbines. Althoughvery simple in the basic concept, there are no previously re-

ported interpretations of the induction machine quasi-stationaryequations for torque control purposes as presented in this paper.

As can be seen from the equations and also from Fig. 2, thepresented ITC concept will be dependent on the speed or theslip of the induction machine. One drawback of the methodis therefore that practical implementation will require speedsensors. The other main issue related to this concept is that,since the decelerating torque of the generator is limited, therecovery process of the system will be longer than for thecase of normal control of the STATCOM with a fixed voltagereference value. The longer recovery time will also result in alonger time with high reactive power flow in the system.

The ITC control concept as presented in this paper is inde-pendent of the inner current control structure of the STATCOM.Therefore, this concept can be applied in combination withany kind of established current control strategy and modu-lation techniques [20], [21]. In this paper, voltage-orientedvector current control and carrier-based pulsewidth modulation(PWM) with third harmonic injection are applied [22], [23].For simulating the system in PSCAD®/EMTDC™, an averagemodel of the PWM is implemented to increase the simulationspeed, since switching transients are not of main importance tothe proposed concept [24].

IV. TORQUE SMOOTHING CONCEPT ON

SINGLE GENERATION UNIT

To illustrate the main response of an induction generator toa grid fault when the ITC is implemented, simulations of onewind generation unit with a lumped mass model are carriedout with PSCAD®/EMTDC™. The investigated case is faultclearing after a three-phase-to-ground fault condition at thegenerator terminals (point A) in Fig. 1. The blue solid line andthe red dashed line in Fig. 4(a) show, respectively, the STAT-COM current with the normal STATCOM control and the ITC,for a critical fault that brings the system close to the stabilitylimit. It can be noticed that, some time after the clearing ofthe fault, during the recovery process, the STATCOM currentwith ITC goes from injection to absorption of reactive currentto reduce the torque. With the normal STATCOM control, therewill be only capacitive operation of the STATCOM, and thespeed of recovery of the system is the fastest possible. The re-covery process with the ITC is longer than that with the normalSTATCOM control, because the decelerating torque is limited.The corresponding curves in Fig. 4(b) show the differencebetween the terminal voltages with the normal STATCOMcontrol and with ITC. The terminal voltage remains below ratedvalue for longer time due to the slower deceleration introducedby the ITC control. The value of the remaining voltage dependson the system parameters and the reference torque selectedwhen implementing the ITC. In the simulations presented inthis paper, the torque reference is chosen to be 1.15 pu, whichis very close to the mechanical torque of 1 pu.

In Fig. 5(a), the torque trajectories for both normalSTATCOM operation and for operation with ITC show how theITC is limiting the peak torques during the recovery process.Fig. 5(b) shows the reactive current trajectory of the STATCOMas a function of the generator speed. It can be seen that, with



Fig. 4. Time responses of STATCOM reactive current with normal STATCOM control and ITC, and time responses of terminal voltage with normal STATCOMcontrol and ITC for two different fault durations.

Fig. 5. Torque and current trajectories during recovery process with normal STATCOM control and with ITC control for two different fault durations.

the normal STATCOM control, the current is kept around themaximum value during the fault and the recovery process untilthe speed of the system is back to the initial value. With the ITC,the current during the recovery process is instead a functionof the speed as given by (4), shown at the bottom of the page[6]. The same equation is obtained in [8] as an estimate ofthe required compensation current as a function of mechanicaltorque and generator speed.

As expected by the torque reference setting, the maximumtorque amplitude with ITC control is 1.15 pu at the beginningof the recovery process. The torque is slightly reduced as thesystem is getting closer to the rated speed and rated voltagebecause of the dynamics of the machine when decelerating asdescribed in [12]. As a result of the almost constant torque ofthe ITC, there is a characteristic linear change of speed of thegenerator as can be seen in Fig. 6. With the normal STATCOM

Fig. 6. Time response of generator speed for two different faults with andwithout ITC.

control and for parameters used in this paper, the maximumtorque amplitude that appears during the recovery process isabout 43% above the rated torque. The propagation of these

|iSTATCOM|2 +2 · i2(req,i2 · req,STATCOM + xeq,i2 · xeq,STATCOM)

r2eq,STATCOM + x2

eq,STATCOM

|iSTATCOM| + (req,i2 · i2)2 + (xeq,i2 · i2)2 − |vg|r2eq,STATCOM + x2

eq,STATCOM

= 0

(4)



torque transients in the drive train is determined by its torsionalcharacteristics, which is investigated in Section IV with a two-mass model.

Figs. 4–6 also show results corresponding to a shorter faultof 150 ms, which is well within the stability limit of the system.For this fault, the system will be stable without compensationby the STATCOM. The black dotted lines in the figures showthe response with normal control of the STATCOM, while thegreen dashed curves show the response with the ITC concept.As shown in Fig. 5(a), the 150-ms fault is cleared at a speedwhere the torque capability of the induction generator is closeto the maximum value. Because the remagnetization of themachine occurs at a speed where the torque capability is highand, therefore, also at a higher voltage, the flux transientsare contributing more to the torque, and the maximum torqueduring the recovery process is even higher than that in the caseof a fault close to the stability limit. In this case, the limitingeffect of the ITC on the torque is increased, since the torqueis brought into the same trajectory as the one described for thecase of the critical fault. As can be seen from both Figs. 4(a) and5(b), this is achieved by bringing the STATCOM current frommaximum capacitive compensation during the fault to inductiveoperation as soon as the fault is cleared. The influence on thegrid voltage is shown in Fig. 4(b), while the generator speed isshown in Fig. 6.

These results indicate the general capability of the ITC inlimiting the torque during recovery processes after any fault thatis within the stability limit of the system. This is obtained with-out interfering with the primary objective of the STATCOM forimproving the LVRT capability. In order to further investigatethe effect of the ITC on the electromagnetic torque of thegenerator when the torsional torque is taken into consideration,a two-mass model of the wind generation system is introducedin the next section [25].

V. RESPONSE OF REAL WIND GENERATION SYSTEM

For application in a real wind farm, there are several fac-tors that need to be taken into account when considering theimplementation of a control technique as the ITC. One of themain issues will be the influence of the real drive train torsionalbehavior of a wind turbine with a large gear ratio. Another issuewill be the control performance in a wind farm, where therewill be several units operating in parallel. This might lead tointeractions between the different units when the ITC controlis in operation, due to the control nature of the ITC that makesthe operation to depend on the effective Thévenin impedance asseen from each of the units.

A. Influence of Two-Mass Model on ITC

When the wind turbine and the wind generator are mod-eled as a single-mass lumped model with a combined inertiaconstant, stability analysis may give significant error whencompared to a multimass model [26], [27]. The effect of in-ertia constants, shaft stiffness, self-damping of the individualmasses, and mutual damping of the adjacent masses must betaken into consideration when investigating the performanceof the ITC in real wind turbine generation systems. A two-

Fig. 7. Two-mass model of the wind energy generation system.

TABLE IMACHINE AND GRID PARAMETERS USED IN SIMULATIONS

mass drive train model, as shown in Fig. 7, is simulated inPSCAD®/EMTDC™ with the parameters given in Table Iunder a fault at the point of common coupling (PCC) in Fig. 1.

The simulation results shown in Figs. 8 and 9 illustratethe influence of shaft stiffness and mutual damping on theperformance of the ITC control. In Fig. 8, the time responsesof turbine and generator speeds show how the initial responseafter fault clearing is influenced by the shaft stiffness. Withthe ITC, it is seen that the generator speed is allowed toincrease again after the initial torsional transient, but then,the generator goes into the characteristic linear decrease ofspeed. The speeds of the generator and the turbine are afterwardreduced simultaneously in a smooth way, and the oscillationsof the turbine speed around the generator speed, as shown inthe case with normal STATCOM control, are avoided. Thisis confirmed by the results presented in Fig. 9 which showsthe time responses of the torque on the turbine shaft and thegenerator. The effect of the ITC can be seen by the smoothedtorque compared to the normal STATCOM control as a result ofthe torque limitation imposed by the ITC controller. This givesan indication of the reduced torque stresses for the gearboxcompared to the stresses with normal STATCOM control. Itshould be noted that the reduced stresses in the turbine shaftare at the cost of a slower recovery process and lower terminalvoltage after fault clearing. The STATCOM current during thefault and recovery process is shown in Fig. 10. During the fault,both cases result in maximum amount of reactive current, butafter the fault clearing instant, the system with ITC respondsto the voltage reference calculated according to the proposedalgorithm, and the STATCOM goes from capacitive to inductiveoperation depending on that voltage reference value.



Fig. 8. Time responses of turbine and generator speeds with normalSTATCOM and ITC.

Fig. 9. Time responses of turbine shaft and generator torque with normalSTATCOM and ITC.

Fig. 10. Time responses of STATCOM currents with and without ITC.

The influence of the mutual damping has also been investi-gated in [6] by reducing the value of the damping coefficient.As the damping coefficient was reduced, the responses becamemore oscillatory as a result of the typical speed oscillationsoriginated by the torsional shaft of the two-mass model. The

results with reduced damping coefficient indicated that the windgeneration system can become oscillatory unstable and the ITCdoes not perform as expected if the system has a shaft withlow damping. To improve the performance for low-dampingfactors, the two-mass model of the drive train system should betaken into account when designing the ITC control. This mightrequire design of a control structure operating with closed-loop control and not only in a feedforward manner as thepresented model-based ITC concept. Then, it could be neces-sary to include feedbacks that would require either additionalmeasurements or design of suitable observers. More advancedcontrol methods could also be relevant for adding dampingand suppressing torsional oscillations while still obtaining theobjective of controlling the torque [28]–[33].

B. Operation of ITC in Parallel-ConnectedWind Generation Units

For application in a wind farm, several identical turbinesoperating in parallel should be considered. The main impedanceseparating the different turbines will, in this case, be given bythe transformer impedances, and then, all turbines will see acommon substation transformer as part of the impedance of themain grid.

Since most of the internal grid in a wind park is usuallyby cable connection, local short circuit faults at the individualturbines are more unlikely to happen than faults in the maingrid that will influence the entire wind farm by causing severevoltage drops. Therefore, a situation with two turbines in paral-lel, as shown in Fig. 1, is simulated, and for simplicity, a gridfault is applied at the PCC. When the turbines are operating atthe same conditions, they will, in general, behave equally, andthe ITC algorithm in the different turbines will have the sameeffect. Therefore, almost exactly the same curves as shown inFigs. 8–10 result from the simulation of each turbine.

In cases where each turbine in the farm has different oper-ating condition, the ITC controllers of the different turbinesmight interact. An example of this is shown in Figs. 11–14,where the system is simulated for the same fault as before,but where one turbine is operating with a constant mechanicaltorque of 1.0 pu, while the other turbine is operating witha torque of 0.9 pu. As can be seen from the simulations ofthe system with normal STATCOM control, the two turbinesbehave quite similarly with the main difference being a lowermaximum speed and lower maximum torque on the generatorwith the lowest mechanical torque. In the case with ITC, itcan be seen that the generator and shaft torques are limitedto nearly the same maximum values regardless of the dif-ferent operating conditions, which is clearly observed by theshaft torque trajectory in Fig. 13. In Fig. 14, an interestingbehavior of the compensating current is observed. Generator 2with lower operating torque has a faster recovery processdue to a larger braking torque compared to Generator 1,since both ITC controllers are designed with the same torquethreshold. This brings as a consequence that Generator 2 ITCwill shift to normal STATCOM control faster than Generator 1ITC. As a result of this, there is a transfer of reactive currentbetween the two STATCOMS. This is partly seen in the upper



Fig. 11. Time responses of turbine and generator speeds with normalSTATCOM and ITC with two turbines in parallel at two different operatingtorque conditions.

Fig. 12. Time responses of turbine and generator torques with normalSTATCOM and ITC with two turbines in parallel operating at two differenttorque conditions.

plot of Fig. 14 when the STATCOM at Generator 2 runs intothe capacitive limit while the STATCOM at Generator 1 runsinto the inductive current limit. Since the ITC depends onthe equivalent grid impedance and the voltage drop caused bythe flow of reactive power, this problem cannot be completelyavoided and will be most significant when the impedancebetween two units is small. The influence of this problem canhowever be reduced if each ITC could be designed with torquethresholds that are sensitive to their actual operating conditions.If the torque reference of the different units is selected suchthat the decelerating torque is the same, this will reduce thedifference in recovery time and therefore limit the period whenthe controllers run into saturation.

The current that will be injected/absorbed by the STATCOMunder the ITC operation mode is given by (4), and as shownfrom this equation, it will depend upon the parameters of the

Fig. 13. Torque trajectories of each turbine in the farm with and without theITC for two different operating torque conditions.

Fig. 14. STATCOM current and reactive power profile at the PCC bus withand without the ITC when two turbines operate in parallel at two differenttorque conditions.

equivalent Thévenin impedance of the rest of the farm andthe grid combined, and the corresponding Thévenin-equivalentgrid voltage vg . This equation determines, at the same time,the required rating of the STATCOM to operate under the ITCmodality.

The reactive current interchange at the PCC bus is shownin the lower plot in Fig. 14. This interchange will dependon the voltage reference set by the ITC and according to the



Fig. 15. Generator terminal voltages, PCC voltage, and grid power with andwithout ITC for two turbines operating in parallel at two different torqueconditions.

impedance values of the Thévenin equivalent circuit of thewind farm and grid combined. The resulting influence onthe voltage at the generator terminals can be observed fromthe upper curves of Fig. 15. In the same figure, the overalleffect of the ITC on the PCC voltage is shown, indicating aremaining voltage well within today’s grid code requirements.The last plot in Fig. 15 shows the power flow into the grid forboth the normal STATCOM and the ITC. It is interesting tonote that, with the ITC, the power flow to the grid could bekept under control regardless of the direct connection of thegenerator to the grid. This is a feature that can be considered asemidecoupling effect that is similar to the one obtained with aconverter-controlled grid interface.

The results observed in the last figures can also be relevantwhen interconnecting wind farms with different sizes. Thepossible interaction between units with ITC control and nearbywind farms or other units with voltage regulation will alsorequire similar considerations.

VI. CONCLUSION

A technique has been developed to alleviate torque transientsin fixed-speed wind generation systems with asynchronousgenerators during the recovery process after a grid failure.The main objective of the proposed controller is to reducestresses in the gearbox and other mechanical components ofthe drive train. The new proposed technique labeled as ITC

has been first implemented by extending the standard controlcapabilities of a STATCOM compensating a single generationunit. Simulation results indicate how the STATCOM can allowfor the implementation of such a control strategy to reduce themechanical stresses on the drive train of a wind turbine duringrecovery after a fault, but at the expense of a reduced voltagelevel and a longer recovery time. Results from a critical faultand a generic shorter fault indicate the general applicability ofthe technique to adapt the STATCOM control to the needs ofthe system during recovery after any kind of contingencies.

The ITC technique is additionally implemented and verifiedby simulating a two-mass model and two turbines operatingin parallel for investigating the performance of the controllerin a wind farm. The limitations of the presented concept inthe case of shafts with low damping have been commentedand indicated the need for further work on more sophisticatedconsiderations in such conditions. Simulation results from op-eration of the ITC on two turbines in parallel indicate that someinteraction between the two STATCOM control systems andthe grid can occur if the different turbines are operating withdifferent mechanical torques. This might result in unintendedinterchange of reactive current among units in the case of non-identical operating conditions, and saturation of the STATCOMcurrent references might make the ITC incapable of limitingthe torque to the specified value. This problem will, however,not compromise the stability of the generator and only lead to amaximum torque that can be slightly higher than specified.

As one of the drawbacks of the ITC could be the slowrecovery process with low remaining voltage, a tradeoff be-tween minimum required voltage level in the grid allowed flowof reactive power depending on grid code requirements, andsmoothed transient torque should be attempted when imple-menting the ITC control in practice. Influence of low dampingin the mechanical drive train, controller parameter sensitivity,and dependence on speed measurement are other factors whichwill affect the performance in practical implementation of thetechnique. In a further step, an experimental validation of thetechnique in a reduced-scale laboratory model should be carriedout in order to investigate and propose a practical solution forthese.

REFERENCES

[1] F. D. Kanellos, S. A. Papathanassiou, and N. D. Hatziargyriou, “Dy-namic analysis of a variable speed wind turbine equipped with a voltagesource AC/DC/AC converter interface and a reactive current controlloop,” in Proc. 10th Mediterranean Electrotech. Conf., May 2000, vol. 3,pp. 986–989.

[2] S. A. Papathanassiou and M. P. Papadopoulos, “Dynamic behavior ofvariable speed wind turbines under stochastic wind,” IEEE Trans. EnergyConvers., vol. 14, no. 4, pp. 1617–1623, Dec. 1999.

[3] J. M. Carrasco, L. G. Franquelo, L. T. Bialasiewicz, E. Galván,P. C. P. Guisado, M. Á. M. Prats, J. I. León, and N. Moreno-Alfonso,“Power-electronic systems for the grid integration of renewable energysources: A survey,” IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1002–1016, Aug. 2006.

[4] F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overviewof control and grid synchronization for distributed power generationsystems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1398–1409,Oct. 2006.

[5] J. A. Baroudi, V. Dinavahi, and A. M. Knight, “A review of power con-verter topologies for wind generators,” Renew. Energy, vol. 32, no. 14,pp. 2369–2385, Nov. 2007.



[6] M. Molinas, J. A. Suul, and T. Undeland, “Torque transient alleviationin fixed speed wind generators by indirect torque control withSTATCOM,” in Proc. 13th EPE-PEMC, Poznan, Poland, Sep. 1–3, 2008,pp. 2318–2324.

[7] B. Niederstucke, A. Anders, P. Dalhoff, and R. Grzybowski, “LoadData Analysis for Wind Turbine Gearboxes,” in “Final Report,ELA—Enhanced Life Analysis of Wind Power Systems,” (OriginalGerman title: Abschlussbericht, Forchungsvorhaben ELA—ErvweiterteLebendsdaueranalyse für Windenergieanlagen), Germanischer LloydWindEnergie GmbH, Hamburg, Germany, Rep. 71048, Appendix 10,4 pp. [Online]. Available: http://edok01.tib.uni-hannover.de/edoks/e01fb04/39616871X.pdf

[8] M. Molinas, J. A. Suul, and T. Undeland, “Low voltage ride through ofwind farms with cage generators: STATCOM versus SVC,” IEEE Trans.Power Electron., vol. 23, no. 3, pp. 1104–1117, May 2008.

[9] S. K. Salman and A. L. J. Teo, “Improvement of fault clearing timeof wind farm using reactive power compensation,” in Proc. IEEE PortoPower Tech Conf., Sep. 10–13, 2001, vol. 2, 6 pp.

[10] S. K. Salman and A. L. J. Teo, “Investigation into the estimation of thecritical clearing time of a grid connected wind power based embeddedgenerator,” in Proc. IEEE/PES Transm. Distrib. Conf. Exhib., Oct. 6–10,2002, vol. 2, pp. 975–980.

[11] M. Molinas, J. A. Suul, and T. Undeland, “Wind farms with increasedtransient stability margin provided by a STATCOM,” in Proc. CES/IEEE5th IPEMC, Aug. 13–16, 2006, vol. 1, pp. 63–69.

[12] M. Molinas, J. A. Suul, and T. Undeland, “A simple method for analyt-ical evaluation of LVRT in wind energy for induction generators withSTATCOM or SVC,” in Proc. 12th EPE, Aalborg, Denmark, Sep. 2007,pp. 1–10.

[13] G. Chicco, M. Molinas, T. Undeland, and G. Viglietti, “Study of thetransient stability margin in a wind system with STATCOM,” in Proc. 6thWorld Energy Syst. Conf., Torino, Italy, Jul. 10–12, 2006, pp. 371–376.

[14] L. Gertmar, L. Liljestrand, and H. Lendenmann, “Wind energy powers-that-be successor generation in globalization,” IEEE Trans. EnergyConvers., vol. 22, no. 1, pp. 13–28, Mar. 2007.

[15] S. Hartge and F. Fischer, “FACTS capabilities of wind energy converters,”in Proc. EWEC, Athens, Greece, Feb. 27–Mar. 2, 2006.

[16] M. Papadopoulos, P. Malatestas, and J. Tegopoulos, “Stresses of selfexcited induction generators during abnormal supply conditions,” in Proc.ICEM, 1992, vol. 3, pp. 1072–1076.

[17] J. Faiz, M. Ghaneei, and A. Keyhani, “Performance analysis of fast reclos-ing transients in induction motors,” IEEE Trans. Energy Convers., vol. 14,no. 1, pp. 101–107, Mar. 1999.

[18] S. Papathanassiou and M. Papadopoulos, “Mechanical stresses in fixed-speed wind turbines due to network disturbances,” IEEE Trans. EnergyConvers., vol. 16, no. 4, pp. 361–367, Dec. 2001.

[19] W. E. Leithead, S. de la Salle, and D. Reardon, “Role and objectives ofcontrol of wind turbines,” Proc. Inst. Elect. Eng.—C, vol. 138, no. 2,pp. 135–148, Mar. 1991.

[20] M. P. Kazmierkowski and L. Malesani, “Current control techniques forthree-phase voltage-source PWM converters: A survey,” IEEE Trans. Ind.Electron., vol. 45, no. 5, pp. 691–703, Oct. 1998.

[21] J. Holtz, “Pulsewidth modulation—A survey,” IEEE Trans. Ind. Electron.,vol. 35, no. 5, pp. 410–420, Dec. 1992.

[22] V. Blasko and V. Kaura, “A new mathematical model and control ofa three-phase AC-DC voltage source converter,” IEEE Trans. PowerElectron., vol. 12, no. 1, pp. 116–123, Jan. 1997.

[23] D. G. Holmes and T. A. Lipo, Pulse Width Modulation for PowerConverters, Principles and Practice. Piscataway, NJ: IEEE Press, 2003.

[24] D. Maksimoviæ, A. M. Stankoviæ, V. J. Thottuvelil, and G. C. Verghese,“Modeling and simulation of power electronic converters,” Proc. IEEE,vol. 89, no. 6, pp. 898–912, Jun. 2001.

[25] Y. Shima, R. Takahashi, T. Murata, J. Tamura, Y. Tomaki,S. Tominaga, and A. Sakahara, “Transient stability simulation ofwind generator expressed by two-mass model,” IEEJ Trans. Inst. Elect.Eng. Jpn. B, vol. 125-B, no. 9, pp. 855–864, 2005.

[26] S. M. Muyeen, M. Hasan Ali, R. Takahashi, T. Murata, J. Tamura,Y. Tomaki, A. Sakahara, and E. Sasano, “Comparative study on transientstability analysis of wind turbine generator system using different drivetrain models,” IET Renew. Power Gener., vol. 1, no. 2, pp. 131–141,Jun. 2007.

[27] S. M. Muyeen, M. Hasan Ali, R. Takahashi, T. Murata, J. Tamura,Y. Tomaki, A. Sakahara, and E. Sasano, “Transient stability analysis ofwind generator by using six-mass drive train model,” in Proc. ICEMS,Nagasaki, Japan, 2006.

[28] Y. Hori, H. Iseki, and K. Sugiura, “Basic consideration of vibration sup-pression and disturbance rejection control of multi-inertia system using

SFLAC (state feedback and load acceleration control),” IEEE Trans. Ind.Appl., vol. 30, no. 4, pp. 889–896, Jul./Aug. 1994.

[29] K. Sugiura and Y. Hori, “Vibration suppression in 2- and 3-mass systembased on the feedback of imperfect derivative of the estimated torsionaltorque,” IEEE Trans. Ind. Electron., vol. 43, no. 1, pp. 56–64, Feb. 1996.

[30] Y. Hori, H. Sawada, and Y. Chun, “Slow resonance ratio control forvibration suppression and disturbance rejection in torsional system,” IEEETrans. Ind. Electron., vol. 46, no. 1, pp. 162–168, Feb. 1999.

[31] W. Li and Y. Hori, “Vibration suppression using single neuron-based PIfuzzy controller and fractional-order disturbance observer,” IEEE Trans.Ind. Electron., vol. 54, no. 1, pp. 117–126, Feb. 2007.

[32] K. Szabat and T. Orlowska-Kowalska, “Vibration suppression ina two-mass drive system using PI-speed controller and additionalfeedbacks—Comparative study,” IEEE Trans. Ind. Electron., vol. 54,no. 2, pp. 1193–1206, Apr. 2007.

[33] M. Cychowski, K. Szabat, and T. Orlowska-Kowalska, “Constrainedmodel predictive control of the drive system with mechanical elasticity,”IEEE Trans. Ind. Electron., vol. 56, no. 6, pp. 1963–1973, Jun. 2009.

Marta Molinas (M’94) received the Diploma inelectro-mechanical engineering from the NationalUniversity of Asuncion, Asuncion, Paraguay, in1992, the M.Sc. degree from Ryukyu University,Okinawa, Japan, in 1997, and the Dr. Eng. degreefrom Tokyo Institute of Technology, Tokyo, Japan,in 2000.

In 1998, she was with the University of Padua,Padua, Italy, as a Guest Researcher. From 2004 to2007, she was a Postdoctoral Researcher with theNorwegian University of Science and Technology,

Trondheim, Norway, where she is currently a Professor with the Departmentof Electric Power Engineering. From 2008 to 2009, she was a JSPS ResearchFellow with the Energy Technology Research Institute, National Institute ofAdvanced Industrial Science and Technology, Tsukuba, Japan. Her researchinterests include wind/wave energy conversion systems, and power electronicsand electrical machines in distributed energy systems.

Prof. Molinas is actively engaged as a Reviewer for IEEE TRANSACTIONS

ON INDUSTRIAL ELECTRONICS and IEEE TRANSACTIONS ON POWER

ELECTRONICS. She is an AdCom member of the IEEE Power ElectronicsSociety.

Jon Are Suul received the M.Sc. degree from theNorwegian University of Science and Technology,Trondheim, Norway, in 2006, where he is currentlyworking toward the Ph.D. degree in the Departmentof Electric Power Engineering.

In 2008, he was a guest Ph.D. student with theEnergy Technology Research Institute, National In-stitute of Advanced Industrial Science and Technol-ogy, Tsukuba, Japan. From 2006 to 2007, he waswith Sintef Energy Research, working on simulationof power electronic systems. His research interests

include control of power electronics converters in power systems and forrenewable energy applications.

Tore Undeland (M’86–SM’92–F’00) received theM.Sc. and Ph.D. degrees from the Norwegian Uni-versity of Science and Technology, in 1970 and 1977,respectively.

He has been a Full Professor with the Departmentof Electric Power Engineering, Norwegian Univer-sity of Science and Technology, Trondheim, Norway,since 1984, an Adjunct Professor with ChalmersUniversity of Technology, Göteborg, Sweden, since2000, and a Scientific Advisor to Sintef EnergyResearch. He is the coauthor of the well-known book

Power Electronics: Converters, Applications, and Design. His research interestsare in the areas of power electronics and wind energy systems.

Prof. Undeland is the President of the European Power Electronics societyand a member of the Norwegian Academy of Technological Sciences.


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