Integration of Wind Power into the German High Voltage Transmission Grid

Transcription

1 Integration of Wind Power into the erman High Voltage Transmission rid I. Erlich, enior Member, IEEE, and H. Brakelmann, Member, CIRE Abstract This aer deals with the integration of wind ower into the erman high voltage grid. In ermany the installed wind ower caacity already exceeded 9 W and exected to reach 5 W by the year 22. For the connection of large offshore wind farms to the onshore AC grid DC transmission technologies can be sed for which sitable sbmarine cables are necessary. erman tilities defined grid code reqirements on wind trbines selling ot the necessary gridconform behavior. The most imortant obligations on wind generation lant oerators concern faltride throgh and reactive ower sly in steady state and dring grid falts. The athors discss the main isses contained in the revised erman rid Code released in 26. For incororating wind trbines and wind farms into ower system stability stdies, sitable models are resented. The behavior of a tyical wind farm dring three hase grid falt based on simlation reslts is shown. Index Terms Wind ower, control system, ower system stability, doblyfed indction machine T I. INTRODUCTION HE crrent erman energy olicy is focsed on the largescale tilization of renewable energy sorces. Of the available alternative energy technologies, wind ower seems to be the most romising one. By Agst 26 the installed wind ower in ermany srassed the9w mark (Fig. ). W till 993 Onshore Reowering Offshore 5 W by the year 22 Agst 26 9,36.4 MW installed WT Year Fig.. Wind ower tilization in ermany I. Erlich and H. Brakelmann are with the University DisbrgEssen, 4757 Disbrg, ermany ( erlich@nidisbrg.de and brakelmann@ets.nide.de) Most of the increase is exected to come from offshore lants bt some growth is also redicted onshore as a reslt of the sbstittion of older wind trbines by larger and more efficient nits. Today the nominal ower of a single wind trbine has already reached 5 MW. The challenge now is to make wind trbines more robst for offshore oeration and to meet grid reqirements in steady state as well as in grid level emergency sitation. Meanwhile wind trbine manfactrers contine to work on the develoment of even larger machines ranging to nit sizes of 78 MW. With the increasing wind ower tilization erman tilities are faced with considerable changes in steady state as well as dynamic behavior of the ower system. To identify ftre reqirements and exectations major erman tilities lanched a joint comrehensive stdy called denastdy [], which was comleted in 25. One of the concerns emanates from the inherent ncertainties associated with wind ower generation. Desite some imrovements, there is still a considerable error robability in wind seed forecasts reqiring a significant increase in the short term reserve ower caacity. Also, slementary conventional ower lants are needed for bridging eriods with low wind ower exectation. Frthermore, the existing high voltage grid mst be extended for the integration of the coming wind ower lants which are concentrated in the northern art of the contry. Fig. 2 shows, the 4kVlines that need to be bilt in the next few years to meet the wind ower transmission reqirements. The denastdy highlighted also the need for involving wind trbines into the overall reactive ower generation and control rocess. All in all, it became obvios from the stdy that large wind farms have to behave retty mch like conventional ower lants. In steady state they mst be able to sly reactive ower over a wide oerating range deending on the reqirements of the grid. Moreover, wind trbines have to stay connected to the grid dring voltage dis and to rovide, to the extent ossible, voltage sort. As a conseqence tilities decided to revise the existing grid codes to accont for the increasing share of wind ower generation and to consider new wind trbine technologies. E.on blished the new grid code in Aril 26 [2]. To assess the degree to which the wind trbines will affect the dynamic behavior of the ower system, there is a need for sitable simlation models and tools. Utilities already reqire detailed /7/$ IEEE.

2 2 dynamic simlation stdies before the connection to the grid of several offshore wind farms, which are crrently in the lanning stage. enerally, simlified dynamic models mst be made available to tilities to enable them carry ot stability stdies inclding the wind farms as art of whole system. North ea 8,64 MW Onshore 25, MW Baltic ea,7 MW technology the technically feasible caacity limit for AC offshore transmission is abot 37 MVA for 3core cables and abot MVA for three singlecore 4kVcables [3]. Higher caacities at this oint in time inevitably reqire more arallel cable systems. A new concet of biolar transmission systems has been roosed in [4], where two XLPE singlecore cables for each hase of the AC system are sed. This singlecore design ermits the se of relatively high transmission voltages (for examle, 4 kv) and maximm cross sections of to 2 mm 2, ths enabling a transmission caacity of to 2 MVA. The transmission range seems to be limited to abot 5 km economically. Alternatively, DC concets have been a toic of discssion already for several years. Conventional, crrent sorce converters (CC) based on thyristor switches have already been sed for long distance, blk electricity transort to high voltage levels. Concets adated to offshore wind farms have been roosed in [4] bt for a nmber of technical and economical reasons the rosects of these concets asserting themselves are qestionable. new lines 4 kv 22 kv Fig. 2. Exected wind ower in ermany by the year 22 and 4 kv grid extension needed for wind ower transmission II. LINK OF LARE OFFHORE WIND FARM TO THE RID Crrently, in the erman territorial waters of North and Baltic eas, offshore wind arks are nder develoment. In the initial ilot hase, tyical caacities are 4 MW, bt in the sbseqent stages caacities of to 5 MW er wind farm are envisaged. The distance to the onshore oint of connection to the transmission network ranges to 5 km. Concerning the transmission technology, different aroaches are being discssed. Threehase AC cables and the reqisite axiliary comonents (comensation) are roven to be available commercially. However, transort reqirements of three hase XLPE AC cables are rohibitive, most of all as a conseqence of restrictions on their hysical dimensions. Fig. 3 shows the crosssection of a tyical 5/7 kv cable. Key data redicating the technical feasibility are limited to a maximm coer cross section of abot 2 mm 2, cable diameter of more than 23 mm and a secific weight of abot kg/m. This reslts in a cable caacity of abot 25 MVA, deending on the transmission range. For tyical wind farms crrently nder develoment in Eroe, this necessitates the se of two cables laid in two searate trenches. The crosssection of the 245kV coer cable, crrently being introdced to the market, is restricted to abot 8mm 2, reslting in a maximm caacity of abot 35 MVA. In smmary, with the crrent state of the Fig. 3. Tyical three core AC XLPE sbmarine cable for 5/7 kv, 3**2 mm2; left ABB (FXBTV), right: Nexans (TKFA) The caacity of commercially available IBT based voltage sorce (VC) converters has increased significantly dring the ast years. Converter caacities of to MW reached the market [5], albeit from a limited nmber of sliers (HVDC light, HVDC ls). Using stateoftheart singlecore XLPE DC cables (Fig. 4) with a rated voltage of +5 kv and a crosssection of 2 mm 2. The maximm caacity er circit is abot 4 MW. This wold be sfficient, for examle, for most ilot rojects in ermany. Both DC cables can be laid together in the same trench. Limitations of this technology are the sbstantial costs for the ower converters, the necessary sace reqirements, in articlar offshore, and the conversion losses amonting to abot 2 3% er converter. Fig. 4. +/5kV XLPEDC cable (sorce: ABB)

3 3 The nmber of otential rojects nder develoment in the North ea oints to sbstantial sace reqirements for cable rotes. This is critically imortant as the coastal areas are sensitive and to a large extent declared natral arks and as sch more or less rotected. Conseqently, lanning and licensing of offshore rotes is a comlex and time consming rocess with an ncertain reslt. III. WIND TURBINE TECHNOLOIE To extract the maximm ower, wind trbines need variable seed generators. The otimm rotating seed of the blades deends on the wind seed which varies over a wide range. On the other side the grid freqency to which the wind trbines are connected is nearly constant. Two basic rinciles of wind trbines have established themselves in mltimegawatt level wind ower generation. The first and the most olar technology is based on the doblyfed indction generators (DFI). The rotor of the DFI is eqied with three hase windings, which are slied by a voltage sorce converter (VC) of variable freqency and amlitde (Fig. 5). l o tr n o c d e e w o P d n a h itc P Pref B CR RC control DFI Fig. 5. trctre of the DFI based wind trbine eed control Pitch and Power Pref P Q, cosφ, UT UDC Q RC Thr fast crrent control UT MC control DCLink CH RC LC CR CH B LC control LC rotor side converter line side converter rotor crowbar DClink choer earbox LC control P UT UDC Q, cosφ, UN needed for wind trbines is sally restricted to ±3 % arond the synchronos seed. The rotor ower and ths the size of the converter is always below 3% of the wind trbine nominal ower which is the main advantage of the DFI based aroach. Variable seed wind generators can be bilt also with synchronos machines (M) where decoling to the fixed grid freqency is achieved by a fll size VC laced between the stator of the machine and the grid to which it is connected (Fig. 6). If the M is eqied with an excitation winding and if the corresonding control kees the terminal voltage at the nominal level, simle diode rectifiers can be sed on the machine side. However, for wind trbines tilizing ermanent magnets for excitation generation of the reactive ower reqired by the M. resoses a controlled converter. The gear box, which is one of the most stressed comonents in wind trbines, adats the rotating seed of the trbine blades to the mch higher generator seed. However, it is also ossible to bild wind trbines withot gearbox, bt this calls for a mch larger nmber of oles and ths necessitates a greater machine size. Wind trbine control consists of two arts, one for itching the rotor blades when the nominal wind seed is exceeded and the other for fast electrical converter control. The converter control rovides the freedom for indeendent control of active and reactive crrents and ths the corresonding ower. Active ower exchange throgh the DClink is ensred throgh the control of the DCvoltage. The IBT converters can also be sed to generate reactive ower needed for the generator s own consmtion bt also for sly into the grid. Reactive ower generation in the DFI is more sited sing the rotor side converter on accont of the fact that the crrent trns ratio between stator and rotor is abot 3:. However, it is also ossible to se the line side converter as long as the active ower can be flly assed trogh the converter. Also, wind trbines with fllsize converter rovide excellent reactive ower control caabilities on the grid side. The reactive ower, the ower factor or the terminal voltage can be sed as alternative control variables, whereby the last otion reresents direct voltage control. B M Thr fast crrent control MC DCLink MC LC CH B Fig. 6. trctre of the Mbased wind trbine with fll size converter UT The rotor seed varies in relation to the seed difference between the stator and rotor magnetic fields. The variability CH LC UN machine side converter line side converter DClink choer earbox (my be absent) IV. RID CODE REQUIREMENT In the following sections the erman grid code will be discssed with focs on wind farm oeration in steady state and dring grid falts. A. teady tate Oeration The generated wind ower has always transit riority. o the variable generation in the system has to be garanteed by conventional generators. Restrictions on wind ower infeed are only allowed if it leads to transmission line overloading. As regards the necessary reactive ower sly, tilities defined the reqired range in terms of ower factor in relation to the voltage at the oint of common connection (PCC) with the grid. Figre 7 shows the strctre of a tyical wind farm connected to the grid throgh an AC cable inclding

4 4 alternatives for reactive ower generation. Long AC cables always reqire comensation by shnt reactors on both sides. To avoid eriods withot inadvertent comensation, fixed connection of reactors in both ends of the cable is recommended so that the cable and reactors will always be switched together. The cable itself reresents a reactive ower sorce that injects reactive ower into the grid. Offshor AC Cable e g lta o V Reqired oerating range cos φ PCC rid No triing Possibly TI TI: hort Term Interrtion No triing 4 tewise triing by system atomatic afegard II after.5 2.4s TI resynchronisation before rimary control,, Fig. 7. Alternatives for reactive ower generation VC, TCR TATCOM With slightly ndercomensated cables it is ossible to rovide a considerable contribtion to the overall var generation withot facing any technical roblems. In some cases additional comensation devices connected to the PCC and to the offshore latform might be needed. To rovide flexibility with minimm sace reqirements, it is also ossible to se shnt reactors eqied with tachangers. Wind trbines can sly/absorb reactive ower within a range of ± (.9.95) ower factors even when the maximm active ower is generated. With redced active ower ott the var caability of wind trbines will obviosly increase frther. Investigations have shown that reactive ower transmission from the wind farm to the grid may be desirable desite the accomanying losses. FACT may be an interesting alternative esecially nder ower system dynamic asects. However, ntil now FACT devices remain the last resort for reactive ower generation on gronds of cost. B. Behavior of Wind Trbines dring rid Falts In the ast wind trbines were disconnected from the grid following grid falts. However, as of now, this will no longer be ermitted as the searation of wind trbines each time the voltage dis below 8% of the nominal voltage wold lead to an intolerable loss of generation. Therefore, tilities reqire faltride throgh (FRT) caabilities that is secified in Fig. 8. Wind trbine mst stay connected even when the PCC voltage is zero. The 5 ms acconts for tyical oerating time of rotection relays. The red solid line in Fig. 8 marks the lower voltage bondary rather than any characteristic voltage behavior. T C A F Fig. 8. Faltride throgh reqirements According to the new E.on grid code of 26 [2] short term interrtion (TI) is allowed nder secific circmstances. TI in area 3 (see Fig 8) reqires resynchronization within 2 s and ower increase rates of at least % of the nominal ower er second. In area 2 the interrtion time allowed is mch less, amonting to jst a few hndred milliseconds. Besides, forced reactive ower sly is reqired dring this eriod. DFI based wind trbine can, for examle, flfill these aarently conflicting reqirements by controlling the grid side converter for maximm reactive ower generation while the stator remains disconnected. According to the erman grid code wind trbines mst rovide, as a mandatory reqirement, voltage sort dring voltage dis. The corresonding voltage control characteristics are smmarized in Fig 9. According to this stilation wind trbines have to sly at least... reactive crrent already when the voltage falls below 5%. A dead band of % is introdced to avoid ndesirable control actions. However, for wind farms connected to the high voltage grid continos voltage control withot dead band is also nder consideration. Dead band arond reference voltage 5% Voltage sort (overexcited mode) Additional reactive crrent I Q /I N % % % Voltage limitation (nderexcited mode) 2% Fig. 9. Characteristic of wind trbine voltage control Within dead band, e.g. const. ower factor control Activation of voltage control by exceeding dead band Contination of voltage control after retrn into dead zone at least abot 5 ms Voltage U/U N Control characteristics Reactive_crrent/voltage gain: k= I Q / U 2... Rise time < 2 ms Maximm available reactive crrent I Q_max = I N The latest E.on grid code has also defined conseqences for noncomliant behavior by wind farms. As shown in Fig. 9 when the voltage remains below 85 % and the wind farm still

5 5 doesn t sly the reactive ower reqired for voltage sort, safegard I imlemented in PCC will tri the wind farm after.5 s. afegard II at the wind trbine level is imlemented as system rotection acting after.5 s and incldes the stewise triing of wind trbines. Connection Point WF WT NET Fig.. Definition of safegard fnctions rid reqirements Falt Ride Throgh Contribtion to voltage maintenance Contribtion to freqency maintenance afegard at connection oint Triing of WT after.5 s Conditions: Reactive ower flow directed to wind farm Voltage di below 85% afegard at WT terminal nodes tewise triing of WT after,5 2,4 Condition: voltage di below 8% Triing of WT within ms Conditions: voltage abot 2% or freqency exceeds limits 47.5 Hz 5.5 Hz Power redction abot 5.2 Hz V. MODEL OF THE DFI BAED WIND TURBINE With the increasing tilization of wind ower it became obvios that stability kind of simlation stdies have to be extended to inclde wind trbines and their imact on ower systems. However, simlation models sed by manfactrers for design and verification roses are too comlex to be sed in large scale ower system simlations. Therefore, in the last few years intensive research has reslted in simlified models allowing the simlation of hndreds of wind trbines together with a large nmber of conventional ower lants [6][]. In this chater as an examle, models of the DFI based wind trbines will be introdced. A. DFI Model The derivation of the DFI model is shown in detail in []. The basic simlification comrises neglecting the transient comonents in the stator voltage eqation. This ste reslts in a comlex algebraic eqation for the stator circit that can be incororated into the grid eqations. The remaining three differential eqations describe the rotor flx in direct (d) and qadratre (q) comonents and the rotor seed so that the model is of 3rd order. Fig. shows the strctre of the model and the colings to the grid, converter and itch/seed control arts. The DFI is coled to the algebraic grid eqations throgh its Thévenin eqivalent where the voltage sorce (EMF) is a fnction of the rotor flx comonents that are state variables. B. Rotor ide Converter Model The rotor side converter (RC) slies the three hase rotor circits of the indction generator throgh sli rings. The RC model is shown in Fig. 2. Detailed descrition can be fond in []. It comrises two control channels, one for active and another for reactive ower. The inner control loos are realized as crrent control. DFI state eqations dψ dt r Rd R = ψ Rd + ψ lr dψ r = ψ dt l Fig.. DFI model ( ωr ω ) + krrri d + Rd R ( ωr ω ) ψrd + kr rr iq + =jω k R (ψ Rd + jψ ) Voltage orce coling variable to grid Fig. 2. Rotor side converter model R Rotor Flx WT _ ref _ ref + T x h q WT _ ref + T 2 LC q LC q _ ref x x ref Fast voltage controller h F VC ( ) x Rotor Voltage from converter control model [ k ( ψ i i ) + t ] dωr = R Rd q ψ dt T d The magnitde of the rotor crrent reference is limited by the IBT rating. However, following grid falts the actal rotor crrent may increase above the limit so that it is necessary to sto the RC. Nevertheless, the crrent contines to flow throgh freewheeling diodes into the DC link. bseqently the DC voltage will increase frther. Limitation of the DC voltage is accomlished by a DC choer and/or by the rotor crowbar. The choer is a resistance that is switched on when the voltage exceeds a certain limit. ometimes the choer is not effective enogh to limit the DC voltage. In this case the RC is searated from the rotor and the rotor crowbar is fired. After a redefined time the crowbar is switched off and the RC is started again. This FRT rocedre is essential to meet grid code reqirements. A more detailed descrition of FRT with DFI wind trbines can be fond in [2]. Int variables of the RC control as shown in Fig. 2 are the wind trbine active and reactive ower. The active ower reference is rovided by the seed/itchangle control that adats the generator seed and blade itchangle to varying wind seed conditions so that always the maximm ower is extracted. The reactive ower reference can be freely chosen within some limits. z' m i l a in r m e T I F D Part of algebraic grid eqations x h magnitde limiter ir_max i Rd _ ref i Rd i _ ref s i K + I TI s σ i Rd xr x h s i xr x σ K + I TI Trbine Torqe from wind ower conversion model rid m Magnitde limiter R_max Rd

6 6 C. LC Model The line or grid side converter (LC) controls the DC voltage and ths the ower flow throgh the DClink. A simlified model is shown in Fig. 3. The reactive ower control loo can be sed as long as the LC crrent limitation is not reached. However, active ower transfer throgh the LC has always riority over the reactive ower. Rd i Rd Different strategies for activating reactive ower q C _ ref Fig. 3. Line side converter model i Rotor ower i + Rd Rd + T C C _rid Injected ower at grid side converter terminal node D. eed/itch angle control and model of the wind ower conversion The seed control has to adat the wind trbine seed to the actal wind seed in sch a way that always the maximm ower is extracted from the wind. The rotating generator seed can be controlled throgh the reference ower assed to the converter control and throgh the itch angle. Below the nominal wind seed the blades are fixed at the osition of maximm ower generation. The seed is controlled along a characteristic ower/shaftseed diagram. When the wind seed exceeds the nominal vale the blades are itched for keeing the generated ower constant at the nominal ower. A simlified control diagram develoed for a 5MW wind trbine is shown in Fig. 4. WT ower WT ω R_nom V + T Rotor seed ω R TH max eed control thr Pitch TD + T V eed control thr Torqe t max ω R_ref V + T TT ref Pitch comensator & max T A & min Fig. 4. eed/itch angle control and model of the wind ower conversion i min Pitch actator max q C _ rid Wind seed v W + T W ω R ρ ArotCP ( λ, ) v 2 3 w WT ref. ower m Power conversion ω R WT_ref Trbine ower and torqe t m To verify the model wind rams from m/s to 4 m/s and back to m/s after frther 25 s have been calclated with the simlified model and also with a detailed model of the manfactrer sed for design roses. The reslts are shown in Fig. 5. In the range from 4 m/s the nominal ower generation and ths the nominal seed are reached. Frther increase of the seed mst be limited by itching the rotor blades. This change between the oerating modes reresents a challenge for the controller alied. As can be seen from the diagrams the simlified model rovides an excellent accracy from the ractical oint of view jstifying its se in ower system dynamic stdies. 5 wind seed [m/s] shaft seed [..] detailed model simlified model WT electrical ower /.. detailed model simlified model Fig. 5. Resonse of the wind trbine model to wind rams VI. WIND FARM IMULATION EXAMPLE In ftre wind farms with hndreds of megawatts caacity are likely to be bilt offshore. Many of these wind farms crrently nder investigation are located more than km away from the PCC. The oerational erformance of the ndergrond/sbmarine cables to be sed to transmit the wind ower to the grid and the oeration of the wind farms themselves reresent a technical challenge for which one has no exerience to fall back on. ecial interest is focsed on the dynamic resonse of wind farms to grid falts. The examle shown in this chater demonstrates the behavior of DFIbased WT following a threehase short circit in the 38kVgrid that reslts in a voltage di below % in the connection oint. Fig. 6 shows the strctre of the wind farm simlated. Reslts are comiled in Fig. 7. The length of the two 5 kv transmission cables is abot 44 km, which is already close to the ossible technical limit. hnt reactors are connected to both ends of the cable for comensation roses.

7 7 M 36 kv Wind farm total 4 MW Fig. 6. imlated offshore wind farm M rid and WT terminal voltages [..] grid falt WT grid WT rotor crrent [..] WT active and reactive ower [..] P Q rid 5 kv cable 38 kv 44 km The voltage di exerienced by the WT is mch less than that in the grid. This is de to the considerable imedance between both nodes. Moreover, the WT control is extended to rovide a strong voltage sort. In articlar, this is realized by the rotor side converter as long as it remains connected to the rotor circit. In addition, the line side converter is controlled for forced reactive crrent generation considering a temorary overload of the converter bridge. As a reslt the WT terminal voltage remains at abot 65%, which may not lead to comlsory crowbar firing. However, in this simlation the crowbar has been activated. Dring the falt the active ower slied to the grid is small as the reslt of the low voltage rofile. However, active ower infeed is not needed by the ower system in this stage becase of the fact that the load is also small, bt reactive crrent is needed to sort the voltage. In the initial hase extending to abot 2 ms, the WT discharges the magnetic energy so that the reactive crrent achieves its first eak of abot... Bt this crrent decays qickly. De to the fact that the crowbar is activated, the DFI becomes also a reactive ower consmer. However, since the line side converter also slies reactive crrent the WT as a whole remains a reactive crrent generator. On the grid side a considerable reactive crrent is observed first when the WT active ower infeed goes to zero. It shold be noted that the active crrent transmission adversely affects the grid voltage sort dring falts. Dring the time eriod when the rotor and converter are disconnected and the crowbar is switched off the ower generation becomes zero for hysical reasons. As can be seen from the simlation reslts the wind farm is able to sly abot 5% reactive crrent dring this stage. After voltage recovery the WT control brings back the voltage very fast near the reference vale withot considerable oscillations WT active and reactive crrents [..] i active i reactive Active and reactive ower in grid connection oint [..] P Q Active and reactive crrents in grid connection oint [..] i active i reactive VII. CONCLUION Wind ower is one of the major emerging technologies in the ower area. In the last decades, the size of wind trbines and wind farms increased raidly. Therefore, the integration of wind ower into the ower system needs articlar attention to ensre gridconform behavior. FRT and reactive ower generation are the major concerns that have to be addressed by the manfactrer before grid connection is aroved by the tilities. Modern wind trbines tilize voltage sorce converter technologies for adating rotor shaft seed to the wind seed. However, de to the ower electronic comonents wind trbines are sensitive to overloads. In the ftre frther research is needed to identify new or imroved aroaches for enhancing the interactive behavior of wind trbines and ower systems. Wind trbines differ from conventional generators, bt they also oen some new ossibilities that can hel rn ftre ower system more reliably and safely Fig. 7. Dynamic resonse of the offshore wind farm to threehase grid short circit

8 8 VIII. REFERENCE [] dena, Planning of the grid integration of wind energy in ermany onshore and offshore to the year 22 (dena grid stdy), available: htt:// [2] E.on Netz, rid Code, High and extra high voltage, Aril, 26 [3].E. Balog e.a, Energy transmission on long three core/three foil XLPE ower cables, Jicable 3 [4] H. Brakelmann, K. Brges, M. Jensen and T. chütte, Biolar transmission systems with XLPE HVAC sbmarine cables, in Proc. 6. Int. Worksho on Large cale Integration of Wind Power and Transmission Networks for Offshore Windfarms, Aril 26, Delft, [5] D. Wensky, leichstromnetzanbindng großer OffshoreWindarks, ew (25) H. 9,. 664 [6] ABB: Technical descrition of HVDC Light technology, rev. 2, Febrar 26, htt:// [7] M.A. Pöller, DoblyFed Indction Machine Models for tability Assessment of Wind Farms, in Proc. IEEE PowerTech, Bologna, Italy, Jne, 23, BPT345 [8] A. Mllane, M. O Malley, The Inertial Resonse of IndctionMachine Based Wind Trbines, IEEE Trans. Power ystems, vol. 2, , Ag. 25. [9] P. Ledesma, J. Usaola, Dobly Fed Indction enerator Model for Transient tability Analysis", IEEE Trans. on Energy Conversion, vol. 2, , Jn. 25. [] J. B. Ekanayake, L. Holdsworth, X.. W, N. Jenkins, Dynamic Modeling of Dobly Fed Indction enerator Wind Trbines, IEEE Trans. on Power ystems vol. 8,. 8389, May. 23. [] Y. Lei, A. Mllane,. Lightbody, R. Yacamini, Modeling of the Wind Trbine With a Dobly Fed Indction enerator for rid Integration tdies, IEEE Trans. on Energy Conversion, vol. 2, , March. 26. [2] F. hewarega, F., I. Erlich, Modeling of Wind Trbines Eqied with DoblyFed Indction Machines for Power ystem tability tdies, IEEE PE eneral Meeting, November 26 Atlanta, eorgia [3] Kretschmann, H.Wrede,. MellerEngelhardt, I. Erlich Enhanced Redced Order Model of Wind Trbines with DFI for Power ystem tability tdies, PECon Kala Lmr, 2829 November 26 IX. BIORAPHIE Istvan Erlich (953) received his Dil.Ing. degree in electrical engineering from the University of Dresden/ermany in 976. After his stdies, he worked in Hngary in the field of electrical distribtion networks. From 979 to 99, he joined the Deartment of Electrical Power ystems of the University of Dresden again, where he received his PhD degree in 983. In the eriod of 99 to 998, he worked with the conslting comany EAB in Berlin and the Franhofer Institte IITB Dresden resectively. Dring this time, he also had a teaching assignment at the University of Dresden. ince 998, he is Professor and head of the Institte of Electrical Power ystems at the University of Disbrg Essen/ermany. His major scientific interest is focsed on ower system stability and control, modelling and simlation of ower system dynamics inclding intelligent system alications. He is a member of VDE and senior member of IEEE. Heiner Brakelmann stdied electrical engineering at the Technical University of Aachen (diloma in 97), where he wrote his doctoral thesis abot circitbreakers in 973. In the following years, he was with the Felten & illeame Energietechnik A in Cologne as leader of an R&Dgro, engaged in ower cable roblems. In 977 he started as chief engineer at the University of Disbrg, got his habilitation in 985 and became a fll rofessor in 994. He is member of CIRE C 2 (Cables) and of CIREW B5 ( Transient effecting long cables ). He is the athor of more than 3 technical blications and of three bookblications in the field of cable techniqe as well as highvoltage and highcrrent roblems.