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Anaysis of Transien Performance for DFIGWind Turbines under he Open Swich Faus

K Da Nee EEE A ombo e ee EEE, S Chowu ee EEE

S P Chowhu ee EEE

tct The fast development of grid-integrated wind powerintroduces new requirements for the operation and control ofpower networs. In order to maintain the reliability of a hostpower grid, it is preferred that the grid-connected wind turbineshould restore its normal operation with minimized power lossesin events of grid fault. This paper presents the results for thetransient performance of a doubly-fed induction generator(DFIG), a type of variable-speed wind turbine. The paperconcentrates on transient performance of the said generatortechnology under open-switch grid faults. The simulation wasperformed using MA TLAB Simulink software. The results

obtained have shown that the control schemes employed for theDFIG wind turbines played an effective role in the restoration ofthe normal operation for the wind turbine in response to gridfaults. The results for both during and aer the grid fault will bediscussed in this paper.

n Te Crowbar, doubly-fed induction generator, openswitch, transient analysis, ride-through, wind turbine

V' V' V' Vi i ii

I. OMENCLTU

d-q axis machine voltages

dq axis machine currents

ds' s' d' Iqr d-qaxismachinefuxesstator ux and mechanical rotorpositionmechanical speedelectromagnetic torquemechanical torqueinertia and viscous iction

II. NTRODUCTION

THE DIG wind turbine technology has increasinglybecome the most widely used energy conversiontechnology in wind power generation, especially in large wind

farms. The main reason for the popuarity of the gridconncton o FIGs s thr ablty to supply powr atconstant voltage and equency, despite the fact that these

turbines operate at variable-speeds. The DIG technology alsoprovides a capability to control the overal system powerfactor.

M. K. Das is with dian Maritime University, Kolkata Camus, India(email: milton.dasrediffmail.com).

.I.Elombo is with the University of Stelenbosch, South frica (email:elombosabeyahoo.com)

S. P. Chowdhury is with Electrical Engg. Det., University of Cae Town,South frica (email: s.chowdhuryuct.c.za).

S. Chowdhury is with Electrical Engg. Det., University of Cae Town,South frica (email: sunetra.chowdhuryuct.c.za).

Over the recent years, most of the national network codesand standards did not take into cognizance the prospects of thecapabilities of network support for wind turbines during gridfaults. Due to the increasing popularity of grid-connected wind

turbines, newer network codes and standards have been issuedwith the recognition of the grid support capabilities of windgenerators during power disturbances on the national powernetworks. It is, therefore, necessary to carry out accurate

transient simulations in order to understand the impact of thehost power system disturbances on the connected windturbines.

The transient simulation anaysis is also a usel too or thedesign of the rotor over-current protection mechanisms. Theover-current protection circuit, the so-called crowbar, isneeded in order to protect the rotor side equency converterduring disturbances on the network [1]. The most commonapproach in dynamic modeing of DIG for wind turbines isusing a space vector theory based model of a slip-ringinduction machine. This method is quite effective, even whenvoltage dips occur on the network due to severe grid faults.The transient analyses of the DIG windturbine have beenstudied in [9], where the crowbar is realized by using six antiparallel thyristors and with an active crowbar [10].

This paper presents the transient simulation analysis of a 2-MW DIG for wind power application under open switchfaults occurring on both the machineside and ine-sideconverters. The eect of open switch faults on DFIG variableswil aso be explored. As explained above, the DFIG issimulated by means of space-vector theory based analyticalmodel of a slipring induction generator together withequency converter, transformer and contro using MATLAB Simulink. The simulation results have shown that the controlschemes employed could effectively restore the wind turbinenormal operation under openswitch faults.

III. ESCRTION O DFIG D URBINE

There are two basic types of wind energy conversionsystems, viz. xed speed and variablespeed turbinetechnologies. ixed-speed turine technologies operate atconstant speeds, regulated by using turbine pitch-controllerseven under varying wind speeds. In contrast, the aero-turbinerotational speed can be allowed to follow the wind speedvariations to maintain a constant and optimum tip speed ratio.However, variablespeed turbines aso require active pitchcontrol mechanisms to allow for optimal operation of the

turbine during excessive wind speeds [11]. Variable-speedinduction generators are considered more attractive due to

their exible rotor speed characteristics in contrast to theconstant speed characteristics of synchronous generators.

DFIG conguration is best suited for variale speedgeneration since it can be controled om rotor side as well as

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stator side. This is possible since the rotor circuit is capable ofbi-directional power ow. The doubly-fed machine can beoperated in generating mode in both sub-synchronous andsuper-synchronous modes [11].

A DIG consists of a wound-rotor induction machine, bac

to-bac PWM converters, a three-phase lter and a threephase transformer. As shown in ig. 1, the stator is connecteddirectly to the grid whereas the rotor is connected to the gridvia the bac-to-bac PWM converters, lter and transformer.When compared with other the system components, the bac

to-bac converters are the least reliable of the generator shownin ig. 1.

t , " '

Wi Tubi FG l Aym '

Fig. 1: DFIG with contrl and rtection

IV. TEDY TTE ODEL OF DIGAs explained in section V, DIG is widely used to extract

wind energy in WCS due is conrollailiy from boh hestator and the rotor sides. or the purposes of simulating theoverall WCS, a mathematical model has been developed tosimulate the transient behavior of the DIG. It is important tonote that a wound-rotor type was used for the DIG machineused in this study, and that this rotor type needs to be fed omboth the stator and rotor sides, hence the name oubly-fedNormally, the stator is directly connected to the grid, whereasthe rotor side is interfaced through a variable frequency powerconverter. In order to cover a wide operation range om subsynchronous to super-synchronous speeds, the power converterplaced on the rotor side was made to operate in a bi-directionalpower ow mode. The DIG can be regarded as a traditionalinduction generator with a non zero rotor votage [1].Neglecting the stator transients, the per-unit electricalequations of DIG can be written in phasor form as folows:

IdOds O

and _

d

Oqs

O

Vs =rs dS+- }q. Vqs-rs1qs+-+ slds

IF r and qvr r r r qr Vq = q +t t LJ Mr and qs = LJqs + MI =L + and =L M s q q qs

dn and Tm T, + J d+In T, = -PT(lqJd, -IdJqr)

= vss vqslqs and Qs = Vqs1ds -Vds1qs

V. DIG YSTEM OTROL

A DIG system is a wound rotor induction generator withslip rings, with the stator directly connected to the grid and

with the rotor interfaced through a bac-to-bac partial-scalepower converter. The DIG is doubly fed by means ofsupplying a voltage on the stator om the grid and also bysupplying a voltage on the rotor as induced by the powerconverter [3, 4]. The converter consists of two conventional

voltage source converters (rotor-side converter, RSC and gridside converter, GSC) and a common dc-bus, as ilustrated inig..

contro in norma operationThe DIG control strucure, illustrated in ig , contains

the electrical control of the power converters, which isessential for the DIG wind turbine behaviour both in normaloperation and during fault conditions. Power converters areusualy controled utilizing vector control techniques [6],which allow de-coupled control of both active and reactivepower. The aim of the RSC is to independently control theactive and reactive power on the grid, while the GSC is

responsible of eeping the dc-lin capacitor voltage at a preset value regardless of the magnitude and the direction of therotor power. As shown on ig. , both RSC and GSC arecontrolled by a two stage controller. The rst stage consists ofvery fast current controllers, regulating the rotor currents to

the reference values that are specied by a slower powercontroller (second stage).

The active and reactive power set-point signals for thesecond stage controllers of the converters in ig. , aredependent on the wind turbine operational mode (normal orfaul operaion) signals. o eample, in nomal operaion

The active power set-point pd for the rotor-sideconverter is dened by the maximum power tracing

point (MPT) loo-up table. The reactive power set-point Q for the rotor-sideconverter can be set to a certain value or to zeroaccording to whether or not the DIG is required tocontribute reactive power.

The grid-side converter is reactive neutral(i.e. Qc 0) in normal operation. This means that,in normal operation, the GSC and the grid exchangeactive power only, and therefore the transmission ofreactive power om DIG to the grid is done through

the stator only.The dc-voltage set-point signal U

de is set to a constant value,

independent on the wind turbine operation mode. On the onehand the use of the partial-scale converter to the generatorrotor maes the DIG concept quite attractive om aneconomical point of view. However, on the other hand, thisconverer arrangement requires advanced protection system, asit is very sensitive to disturbances on the host power grid.

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2nd tag(powr)

P v

Q GSC

Fig. 2: DFIG control n normal oeration

B. contro uner grifaut

The active power setpoint P or the RS control, innormal operation, is dened by the maximum power tracingpoint (MPT) as a nction o the optimal generator speed, asshown in ig 3. In events o grid aults, the generator speedvariation is not due to the wind speed change but due to

electrical torque reduction This means that, during grid aultsthe active power setpoint has to be dierently dened,ie as the output o a damping controller Such a controller has

tas to dampen the torsional excitations, which are excited inthe drive train owing to the grid ault

'

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resembles the stator crrent. In order to compensate for theincreasing rotor crrent, the RC the rotor voltage referenceincreases, which implies a shO of power om the rotorterminals throgh the converer. As the grid voltage dropsimmediately aer the falt, the GC becomes incapable of

transferring the whole power om the rotor throgh theconverter rther to the grid. The GC's control of the dcvoltage ths qickly reaches its limitation. As a reslt, theadditional energy goes into charging the dcbs capacitor andthe dcvoltage rises rapidly, as it can be seen in Fig.. Theprotection system is activated whenever the limit of the rotorcrrent or that of the dcvoltage is exceeded. This shortcircits the generator rotor by triggering the crowbar. Furtherthe RC is blocked and its capability of the rotor crrentscontrol, therefore, becomes disabled. While the crowbar is inits triggered state, the dcbs capacitor starts to discharge.Also, the GC begins to track the dclink voltage back to itsreference. It shold be noticed that, as long as the crowbar is

in its triggered state, the generator behaves as a conventionalsqirrelcage indction generator (CIG), for which theconverter rotor voltage otpt is set to zero.

1.50 - - - - -Fau-- - FaU- - -cowb-- - I

l2O l . = 1= d - - rwua l : I o .0l- - -- : _r _ _ _ _ _ _ _ _

6 ----- :------i +----- t

1

0.000 sec 0.125 0.250

t200_0 -- - - - - - - - - -: - - - - - - - II50 0. - - - - - - -i- - - - - - - I :OOO - - - - - - - - - -i - l - - - - - t50.0 - - - - - - - - - - - i - '

0.00 0.5 ': .-- 0:.25012500t 7500

sec= = = = 1 f = = T 25.00

.Oor - - - - : : - " -75.00 J25.00 00o -s -e c O1 25 O;.25 0200

}.60

,-: ; : - ! ! o

t-9-

- -

=c r,_- 040

i

000

0.401.12

O --1.08 -

1.06

104

0.000 sec 0.125 0.250

,: Il l: I,

0.000 0250Fig. 4: Wind f teinl voltge, ctive ower, rective ower, electricl

torque, mechnicl torque nd seed

B. Behaviour aerfaut cearance

Fig. 4 shows that the stator voltage is restored and theelectromagnetic torqe an actve power begin to increase,immediately aer the falt is cleared. With reference to Fig. it can be seen that the demagnetized stator and rotor act tooppose the increase in the grid voltage and x, leading to acorresponding increase in the rotor and stator crrents. It hasbeen observed that the voltage does not completely recoverinstantaneosly aer the falt clearance. Rather, the voltagerecovers to a level lower than its nominal vale immediatelyaer the falt clearance, whereas it rther recovers to thenominal level once the crowbar has been removed. The reasonfor this is that the generator contines to behave as CIG jst

aer falt clearance, which reslts in the absorption of reactivepower for the magnetization. ince the RC is disabled ntil

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the crowbar is removed, this means that the RSC cannot beable to provide the reactive power necessary for themagnetization of the generator. As a result, the generatordraws reactive power from the grid and this kind of operationhampers the recovery process of the grid voltage.

The crowbar is only removed once the grid voltage hasrecovered to the desired level. Thereaer, the generatorcurrents and voltages start to converge to their prefault valuesand the RSC starts to actively control the active and reactivepower.

C Tranient behaiour

From the transient stability viewpoint, the drive train of thewind turbine needs to be represented by a twomass model inorder to be able to simulate the torsional oscillations excited inthe drive train system during grid faults. A damping controllertherefore needs to be implemented and tuned to activelydampen these oscillations, which otherwise might lead to self

excitation and high mechanical stress of the drive train system.The converter control and protection is essential for the faultridethrough capability of the DFIG wind trbine. Theprotection system of the converter (i.e. crowbar resistance) istriggered when high transient currents and voltages occur inthe generator and converter, otherwise the power converterdevice would be damaged during grid faults. When thecrowbar is triggered, DFIG behaves as a conventional SCIG,and therefore its controllability is temporarily lost. Thecrowbar resistance, which value is strictly dependent on thegenerator data sheet, has inuence on the rotor current and onthe reactive power demands of the generator during grid faults.

2.00 1.50 1.0u 0.50 O.Of

___ Yl

---------- ; - ;----- 1-O.50'

040JO:0.20o

0000 sec 0125 0250 : I" ,= = = = = ; = ='= = = = = I

O.IO_ ,----- - -- 0.00 ,0.10 L__________ ____

1.70

; 1.6;1.5> 140

0000 sec 0125 0250-----

:

-- -- I- - - - - I = == = = = = I

L0 0000 sec 0125 0250Fig. 5. Rotor currents and voltages

VIII. ONCLUSIONS

This paper deals with the analysis of the performance of aDFIGbased wind generator and its interaction with the mainsin case of grid failure. The voltage drop related problemsduring a grid fault can be split up into two pars in general.The rst part is the drop of stator (grid) voltage and the second

part is the switchin process of the crowbar resistor whichconverts the DFIG conguration into an ordinary squirelcage

induction generator with increased rotor resistance. Firstly, theswitchingin of the crowbar resistor at ll grid and thus llstator voltage has been presented and analyzed. Thedependency of the rotor currents and rotor voltages as we asthe torque on the rotor resistance has been shown. This can be

sed as an introductory analysis of the DFIG behavior.Secondly, the decrease in stator voltages with additionalswitchingin of the crowbar resistor has been analyzed. Withreference to the simulations, the dependency on the crowbarresistance for the rotor currents, rotor voltage and torque hasbeen presented. The simulation results provide insightlunderstanding on the most signicant phenomena concerningthe behavior of DFIGbased wind turbines during grid faults.

IX. ApPED

Generator Parameters:Rated power = 2 MW, Rated voltage = 0.69 kV, Base angular

equency = 314.6rad/s, Stator/rotor tus ratio = 0.4333,ngular moment of inertia (J=2H) = 1.9914 p.u, Mechanicaldamping = 0.02 p.u, Stator resistance = 0.0175 p.u, Rotorresistance = 0.019 p.u, Stator leakage inductance = 0.2571 p.u,Rotor leakage inductance = 0.295 p.u, Mutual inductance =6.921 p.u.

X. ACKNOWLEDGEMENT

The athors gratelly acknowledge the support rendered byJadavpur University, India, University of Stellenbosch, SouthAica and University of Cape Town, South Aica for

carrying out the research work.

XI. EFERENCES

[1] J. B. Ekanayake, L. Holdsworh, X G. Wu, N. Jenkins,

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[10 J. Niiranen, oltage Di Ride rough of DFIG Equied with ctiveCrowbar, resented at the Nordic Wind Power Conference 2004, 12March 2004, Chalmers University of Technology, Goteborg, Sweden.

[11] Richard Gaon, Gilber Sybille, Serge Beard, Daniel Pare, odelingand RealTme Simulation of a DFIG driven by a wind turbineQ

Proceedgs of teational Conferences on ower systems transients(ST@5) in Montreal, Canada, June 1923.2005, . No.ST0512.

XII. IOGRPHS

M K Das received his BEE and ME (Contrl System Engg.) in 2002 and2005 resectively. He is a member of the T (UK). He is resently Lecturer

of Indian Maritime University, Kolkata Camus, (Govt. of India, Mistry ofShiing) India and acting as the Chairman of the Y Section of Kokata

Network of the T(UK).Email: milton das@redifail comA I Elombo received his B.Sc. deee in electrical engineering, which wasawarded with rst class honors in 2009 from the University of Cae Town inSouth frica. He is currently working toward his M.Sc. deee in electrical

engineering at the University of Stellenbosch. He is also serving as anassistant engineer for Namibia Power Cororation (Pty) Ltd, NamPower.

Email: elombosabehoo comS. Chowdhury received her BEE and PhD in 1991 and 1998 resectively.She was connected to MS M.N.Dastur & Co. Ltd as Electrical Engineer from1991 to 199. She served Women Polyechnic, Kolkata, dia as SeniorLecturer frm 1998 to 200. She is currently a Senior Lecturer theElectrical Engineering Dearment of The University of Cae Town, Southfrica. She became member of EE n 2003. She visited Brne University,UK and The University of Manchester, UK several times on collaborativeresearch rome. She has ublished two books and over 55 aers mainlyin ower systems. She is a Member of the T (UK) and () and Member ofEE(US).

Email: sunetra chowdhct ac za

S. P. Chowdhuryreceived his BEE, MEE and PhD n 1987, 1989 and 1992

resectively. 1993, he joined E.E.Dett. of Jadavur University, Kolkata,India as Lecturer and served till 2008 in the caacity of Professor. He is

cuently ssociate Professor in Electrical Engineering Dearment of theUniversity of Cae Town, South frica. He became EE member in 2003.He visited Brne University, UK and The University of Manchester, UKseveral times on collaborative research roamme. He has ublished twobooks and over 110 aers mainly in ower systems and renewable energy.He is a fellow of the T (UK) with C.Eng. (I) and the TE () andMember of EE (US). He is a member of Knowledge Management Boardand Council of the T (UK).Email: sp chowdhuuct ac za