Improvement of DFIG Wind Turbine Power Performance During Grid Voltage Sag

Transcription

1 3-E-PQA-98 mprovement o DFG Wind Turbine Power Perormane During Grid oltage Sag M. Sadeghi Gougheri, M. Parastegari, F.Azam Department o Eletrial Engineering, Faulty o Engineering University o sahan sahan, ran mojtaba.38@gmail.om, parastegari@eng.ui.a.ir, arzadazam@gmail.om Abstrat n this paper a new protetion tehnique is presented or use in doubly ed indution generars o wind turbines. The method enhanes the overall system perormane by proteting the generar against grid voltage sag. For this purpose, an inverter and three single phase transormers are plaed in series with the generar. The series inverter enters the iruit upon ault events. Fuzzy and posiast ontrollers are used in the struture o the proposed method keep the star and ror urrents within an aeptable limit and thereore prevent generar-network disonneting due overloads. Using omputer simulations the perormane o the proposed approah is evaluated. Simulation results onirm the eetiveness o the proposed method in DFG over urrent protetion at sag onditions. t is shown that the system is apable o keeping the generar output voltage at p.u. during ault events. Keywords Doubly Fed ndution Generar, oltage Sag, Fuzzy Controller, Posiast Controller.. NTRODUCTON n reent years, the utilization o distributed energies is inreasingly used in eletrial power systems. They play an important role in power prodution. Reproduibility, inexpensiveness and leanness are some harateristis whih makes renewable energy soures attrative in eletrial power systems. The utilization o wind power has a rapid inrease. The extensive development o wind turbines has led reliability improvement and ost redution. Most wind power plants inorporate doubly ed indution generars (DFGs) or power onversion. The DFG is apable supply power at onstant voltage and requeny while the wind and ror speed varies [,]. They an also ontrol ative and reative power o the generar in all our operational regions [3]. oltage sag is one o the main reasons or power quality problems and happens due the temporal and unpredited power interrupts in the network. Lightning, sudden short iruits, network interrupts/disonnets are other resoures o the voltage sag in the power system [4]. One o the main disadvantages o DFG is its undesired operation in the presene o voltage sag aults. Sag in the grid voltage inreases the ror urrent whih an damage the power eletroni onverter iruitry [5]. n order protet wind generars against over urrents, these generars were usually disonneted rom the grid prevent iruit damage [3]. Crowbar is one o the reent methods whih an be used bypass the ror urrent in the sag ondition [5, 6, 7]. Some studies use modiied ontrol approahes ontrol the DFG in sag onditions [8, 9,, and ]. Reerene [8] proposes a ontrol method based on dynami programming power ontrol (DPPC) protet the ror and onverter. n this method, in order protet wind generars against over urrents, star and ror urrents are ontrolled in a way that their values are not exeed their nominal values. Thereore, subsequently, the ative and reative powers are not remain at their nominal value. Furthermore, this method requires omplex alulations, these disadvantages leads use other methods protet wind generars against over urrents. n [9] a method based on analysis o positive and negative urrent - voltage omponents is used ontrol the DFG in the presene o unbalaned sag onditions. n this reerene, a Crow bar is used prevent overvoltage in the DC bus due the exess urrent produed rom the DGG ror side the network. A non-linear ontrol method is presented in [] whih redues the ror urrent, the rque and the DC link voltage lutuations o DFG. But the method only redues the transient osillations during ault events; it ould not work properly in the sag onditions. Reerene [] develops a ontrol method inrease the probability o suessul grid ault ride-through using voltage and urrent o ror side onverter. n this method, the ror urrent is kept at a level near zero in the event o aults. This proedure will protet the generar over urrent, but the star and ror urrents and the rque will not stay at their nominal values. n [], passive resisrs are plaed in the star iruit protet the DFG during sags. n this work, the value o the rque is not kept at its nominal value during sags. Furthermore, the value o the rque exeeds its nominal value at the start and end o voltage sag. n [3] a ontrol method is proposed ensure that DFG wind turbine operation during grid aults. The rest o the paper is organized as ollows: Setion explains grid voltage sag eets. n setion, wind energy power stations and their existing ontrol methods are briely explained. Setion presents the proposed ontrol method used enhane the DFG s perormane in the presene o grid sag. Simulation results are given in setion and inally some onluding remarks are presented in setion.

2 mprovement o DFG Wind Turbine Power Perormane During Grid oltage Sag 8 th Power System Conerene - 3 Tehran, ran. OLTAGE SAG N POWER NETWORK oltage sag is one o the most ommon aults in eletri power systems. Short iruits in the power system or initial energizing o big loads are the main reasons o voltage sag. not proteted, voltage sag an damage the DFG o wind turbines. The perormane o DFG under voltage sag has been investigated in several papers [4, 5]. During sag, the ror urrent inreases onsiderably. This inrease is most signiiant in the initial and inal steps o the sag. During this period, the eletromagneti rque and DC link voltage also experiene high lutuations [4]. n order protet the ror and power eletroni iruitry against over urrents, the magnitude o ror and star urrents should be limited and they should remain at their nominal values while the eletromagneti rque and DC link voltage lutuations should be damped.. WND ENERGY POWER SYSTEM Figure () represents the basi diagram o a typial wind energy power system with DFG. n this struture, the turbine is oupled with the generar via a gearbox. The star is diretly onneted the network while the ror winding is onneted through bak bak onverters whih inlude the grid side onverter (GSC) and ror side onverter (RSC). The two onverters are onneted gether via a DC link apair. This oniguration allows the power be transerred in both diretions; thereore both the ative and reative power an be ontrolled separately. Furthermore with this oniguration, only % 3% o the generar power passes through the eletroni power onverters whih results in lower onverter power dissipation, urrent rating and ost [6]. n the bak bak onverters, the GSC is used ontrol the DC link voltage while the RSC is used ontrol ative and reative power o the generar. With the GSC ontroller, the quadrati omponent o ror urrent and DC link voltage are used ontrol the q and d omponents o voltage, respetively. While with the RSC ontroller, the ative and reative power omponents are used ontrol the q and d omponents o ror voltage, respetively. To ahieve unity power ar, the reerene reative power is assumed be zero. A sheme or ontrolling the bak bak onverter is presented in [7].. PROPOSED CONTROL METHOD The proposed method used enhane the perormane o the DFG is presented in Fig.. n this method, a series inverter (S) is used protet the DFG during voltage sag in the grid. The inverter is used Gear box DFG RSC P s, Q s GSC Fig.. Overall struture o the DFG wind power station system grid produe the bakup voltage [4]. The S is supplied rom the DC link. To redue power dissipation, the S only enters the network during sag. During sag the inverter is plaed in the iruit using the swith and when the sag is reovered the inverter will disonneted. The bypass swith mehanism is the same as the one presented in [8]. The opening and losing o this swith is based on detetion o voltage sag whih is shown in Fig.. n the ase o ault events, the third inverter keeps the generar voltage between p.u. and thus the ror and star urrents remain at their nominal values. n these onditions, it will not be required disonnet the turbine rom the network. The S is onneted grid via a transormer. The apaity o the series transormer is equal the nominal apaity o the DFG. This is beause in the event o a short iruit, the series inverter ould keep the urrent and voltage at their nominal values and supply sensitive loads at their terminal through the transormer. Usually the primary transormer winding nominal voltage should not be more than 5% when the voltage ompensation level does not exeed over 5% [9]; but in the presented method the ompensation o 5% exess voltage should be possible. To simpliy the ollowing o relations, it is onsidered that the transormer winding ratio is :. The struture o the proposed subsystems will be explained in the ollowing subsetions. A. S reerene voltage As shown in Fig., the DFG bus voltage is the sum o series transormer voltage ( t ) and grid voltage ( ). Furthermore, L, C and R onstrut a ilter redue swithing ripples []. Thus, by using Kirhho voltage law, the S output voltage and apair voltage an be alulated as ollows: = t in = d = C = t R L d n the stationary reerene rame (α-β), the ollowing equation an be obtained: in = t = d = C R L d n order ontrol the DFG terminal voltage at its nominal value even in the presene o network aults, the voltage o the series inverter should be ontrolled. Based on the equations () and (), the ontrol o the inverters voltage is possible through the ontrolling o the voltage o the C. () ()

3 mprovement o DFG Wind Turbine Power Perormane During Grid oltage Sag 8 th Power System Conerene - 3 Tehran, ran Gear box DFG RSC GSC Fig.. Proposed method struture L R S By Pass t C in : grid PWM zero Control Ciruit Sag & liker detetion By Pass Swith ON/OFF The ontrol o the voltage o the C is possible by ontrolling the apair s urrent. So, the urrent o the apair is the main parameter that should be determined ontrol the DFG terminal bus voltage. The struture o the proposed ontrolling method is presented in Fig. 3. As shown in this igure, at irst the apair reerene urrent is alulated using the desired and the measured value o the DFG bus voltage. For this purpose, the voltage o DFG terminal ( ) is irst transerred the stationary reerene rame (α-β). This voltage is used as the input o the uzzy ontroller blok. The output o the uzzy ontroller blok is used determine the apair reerene urrent ( i re ). By using the reerene and measured value o the apair urrent, and equation d = C, the apair voltage is determined. t should be noted that the oeiient K is determined by p K p = / Cω. When the apair voltage is determined, the inverter voltage an be alulated onsidering equations () and (). The apair urrent error ( ire i ) is multiplied by a onstant oeiient and added the grid voltage error (the dierene between measured grid side voltage ( ) and its reerene value ( re )) onstrut the inverter reerene voltage. B. Fuzzy ontroller blok n this paper a uzzy ontroller is used instead o the onventional approah, beause it an produe the required satisary response or dierent sag events. The struture o re uzzy logi i re ontroller i re Fig. 3. Proposed Controlling method k p Posiast PWM inv uzzy ontroller blok is presented in Fig. 4. As shown in this igure, the atual and reerene voltage o the grid is transerred the synhronous reerene rame (d-q) using the park transorm ( determine ). Then is ompared with and the resulting error will be used as the input o re the uzzy logi ontroller (FLC). t should be noted that, the amplitude o the reerene voltage is equal the nominal value DFG bus voltage. The phase angle o the reerene voltage is obtained rom the PLL. The PLL s input voltage is the network voltage on the DFG side. Thereore, the output o PLL is the phase angle o the voltage o the network. The output o the FLC is transerred the stationary reerene rame onstrut the output o the FLC blok. The under studied uzzy ontroller has two inputs; the error (e) and error deviation (de). The rule sets o the uzzy ontroller is determined in a way that overshoot, settling time, rise time and steady state error o the response o the system is minimized. The membership untions are onsidered be a triangular untion between 3 and -3. t should be noted that, the ontroller oeiients are adjusted suh that the input and output o the ontroller resides within this range. The membership untions inlude 7 untions; NB (Negative Big), NM (Negative Medium), NS (Negative Small), Z (Zero), PB (Positive Big), PM (Positive Medium) and PS (Positive Small). The uzzy rules used in this paper are listed in Table. TABLE. Fuzzy ontrol rules Units de PB PM PS Z NS NM NB e PB Z Z Z PB PM Z Z PM Z Z Z PM Z Z Z PS Z Z Z PS Z Z NM Z PB PM PS Z NS NM NB NS PM Z Z NS Z Z Z NM Z Z Z NM Z Z Z NB Z Z NM NB Z Z Z re PLL θ Fig.4: Fuzzy logi ontroller blok diagram uzzy logi ontroller θ 3

4 mprovement o DFG Wind Turbine Power Perormane During Grid oltage Sag 8 th Power System Conerene - 3 Tehran, ran main grid MA X / R =. S base base = 3.8K = MA.5 ( 3.5km) j.96% 69K 3.8K Feeder3 Feeder 69/3.8 5MA.667 j5.33% Feeder PCC.43 j.54% ( 89m) Bus j.5% ( 4.83km).56 j.33 ( 36m) 3.8/ MA.44 j4.8% j.66% ( 976m) Bus.5MAR 3.8 /.575 5MA 5MA DFG Bus.73 j.95% ( 4m) j5.7% (.6km).4 j.35% ( 48m) 3.8/.48.5MA 6.48 j38.3% 3.8/.48.5MA 5.6 j48.% L 3 3.MW.9MAR 3.8/.4.5MA 3.9 j.3% 3.8/.48 MA 8. j57.5% L 5.9MW L 4.9MW.6MAR L.5MW.MAR L.8MW.47MAR. SMULATON RESULTS A. Power network system Fig. 5 represents the blok diagram o the system under study []. The system inludes three distribution eeders, whih are all onneted a 3.8k bus. This 3.8k bus is onsidered as the point ommon oupling (PCC). The PCC is onneted the main network through a 69/3.8 K, 5MA transormer. The main network is shown by a 69K voltage soure with MA nominal power and X/R=.. The PCC is supplied by a 5Mar apair bank. L L5 are loal linear loads. The wind arm is onneted bus through a 3.8/.575K transormer. t onsists o three DFG with tally power o 5MA. The speiiations o the wind turbine power station are shown in the appendix C. B. Proposed method perormane during sag n this subsetion, the perormane o the DFG is investigated or.7 p.u. voltage sag. Then, in the next subsetion, in order evaluate the advantages o the proposed method its results are ompared with the results o the methods presented in [5] and []. n this study, it is onsidered that the sag happens during a.5se -.5se interval. n this period, the sag deter plaes the series inverter in the system. The struture o the sag deter is given in Appendix A. n Fig. 6 and Fig. 7 the results related the system with and without the S stage are shown or the.7 p.u. voltage sag. Fig. 6 orresponds the ase where no S exists in the iruit and Fig. 7 orresponds the ase where the S is used. The DFG s bus voltage is shown in Fig. 6. As shown in this igure, without using the S stage, the voltage suers rom Fig. 5. Under study power system diagram signiiant hanges during sag. As it an be seen in Fig. 7, the voltage remains at its nominal value in the ase where the S exists. The star urrent during.7 p.u sag is presented in Fig. 6. As shown in this igure, during sag, the magnitude o the star urrent inreases. This inrease is most signiiant in the initial and inal states o the sag. By using the proposed method the magnitude o star urrent is limited its nominal value (Fig. 7). Fig. 6() represented the ror urrent. This urrent inreases in proportion the star urrent. This inrease is most signiiant in the initial and inal states o the voltage sag whih an damage the power eletroni iruitry in the ror side o the DFG. By using the proposed method the magnitude o ror urrent is limited its nominal value (Fig. 7()). The presented method has also enhaned the eletromagneti rque and DC link voltage harateristis o the DFG. Without the S blok, the magnitude o eletromagneti rque in the initial and inal states o the voltage sag lutuates by p.u (Fig. 6(d)) while by using the proposed method, the inrease in voltage magnitude is damped (Fig. 7(d)). As shown in Fig. 6(e) the magnitude o DC link voltage an reah up.4k whih ould damage the link apair, while in the presented method the DC link voltage overshoot remains relatively onstant (Fig.7 (e)). C. Comparison between the perormane o the proposed method and methods presented in [5] and [] n this study, in order evaluate the perormane o the proposed method, its results are ompared with the results o the methods presented in [5] and []. n [5], the Crowbar resistane is used ontrol the wind turbine's urrent during sag. n [], the passive series resistane in the star iruit is used prevent generar damages during ault events. 4

5 mprovement o DFG Wind Turbine Power Perormane During Grid oltage Sag 8 th Power System Conerene - 3 Tehran, ran (pu) (pu) - - Star Current (A) Ror Current (A) () (d).5 Star Current (A) Ror Current (A) () (d) DC Link oltage () (e) Fig. 6. DFG perormane in the presene o.7 p.u. network voltage sags without the presene o S. DFG s bus voltage star urrent () ror urrent (d) eletromagneti rque (e) DC link voltage DC Link oltage () (e) Fig. 7. DFG perormane in the presene o.7 p.u. network voltage sags with the presene o S. DFG s bus voltage star urrent () ror urrent (d) eletromagneti rque (e) DC link voltage 5

6 mprovement o DFG Wind Turbine Power Perormane During Grid oltage Sag 8 th Power System Conerene - 3 Tehran, ran D Link oltage () Star Current (A) () Figure 8. Evaluation o generar perormane in the rowbar method Star urrent rque () DC link voltage Fig. 8 and 9 show the star urrent, rque and DC link voltage in the two "rowbar" [5] and "series star iruit passive resistane" [] approahes or the understudied generar during sag. As it an be seen in Fig. 8, in the rowbar method, the star urrent is kept at low values. t is also evident rom Fig. 8 that the rque does not remain at its nominal value during sag and also the DC link voltage dereases onsiderably. However in the proposed method these disadvantages are not enountered. The simulation results o the method presented in [] is shown in Fig. 9. t an be seen rom the results o this method that during sag the star urrent and DC link voltage remain at an aeptable value. But as it an be seen rom Fig. 9(), during sag the rque does not remain at its nominal value and it atually inreases. This rque inrease will eventually lead substantial mehanial stress. Thereore it an be onluded that the presented method has better perormane harateristis ompared with the methods presented in [5] and []. Star Current (A) DC Link oltage () () Figure 9. Evaluation o generar perormane in the passive resisr method Star urrent rque () DC link voltage. CONCLUSON n this paper, a new method was developed or the protetion o the DFG wind power system during voltage sag. During sag, the ror urrent inreases onsiderably. This inrease is most signiiant in the initial and inal steps o the sag. During this period, the eletromagneti rque and DC link voltage also experiene high lutuations. n order protet the ror and power eletroni iruitry against over urrents, the magnitude o ror and star urrents should be limited and they should remain at their nominal values while the eletromagneti rque and DC link voltage lutuations should be damped. However in the proposed method, an inverter used in series with the DFG, prevents urrent overloads. n addition DFG power iruit protetion, the proposed method keeps the DFG bus voltage between p.u. 6

7 mprovement o DFG Wind Turbine Power Perormane During Grid oltage Sag 8 th Power System Conerene - 3 Tehran, ran Appendix A. oltage sag deter A voltage sag deter is used identiy voltage sag aults in the network. The deter operates based on the balane between the voltage terminal and the star synhronous lux speed voltage. The voltage sag ondition is deined as ollows: st jω λ e st p A. Where st is the star voltage, ω e is the nominal requeny, λ st is the star lux and p is the maximum limit or unbalane onditions whih is assumed. in this paper. B. Generar parameters Pnom = 4.5MW,osϕ =.9, nom = 575v, P = 3, Rs =.76 pu, Lls =.7pu, Rr =.5pu Llr =.5pu, Lm =.9 pu, H = 5.4s, F =.pu D link parameters d link =, Cd link =. 3F Grid side parameters R =.3pu, L =. g g 3 Controller parameters Td = 5µ se, δ =, R C = 7µ F pu =, L =.3mH, REFERENCES [] A G. Abo-Khalil,.Synhronization o DFG output voltage utility grid in wind power system., Renew. Energy,, 44 : [] X. Yan, G. enkataramanan, P.S. Flannery, Y. Wang:.Low oltage Ride Through For DFG Wind Turbines Using Passive mpedane Networks. Pro. nt. Con. on Sustainable Power Generation and Supply 9, -9, 6-7, [3] L. Xu,. Wang:.Dynami Modeling and Control o DFG-Based Wind Turbines Under Unbalaned Network Conditions. EEE Trans. Power Syst. 7, ():34-33 [4] H. C. So, Y. S. Lee, M.H.L. Chow,:.Design o a -ka parallel-type AC voltage sag ompensar. ET Power Eletron.,, 5 (5): [5] J. Morren, W.H. de Haan:.Ridethrough o Wind Turbines with Doubly- Fed ndution Generar During a oltage Dip. EEE Trans. energy onvers. 5, (): [6] J. López, E. Gubía, E. Olea, J. Ruiz, L. Marroyo:.Ride Through o Wind Turbines With Doubly Fed ndution Generar Under Symmetrial oltage Dips., EEE Trans. ndustrial Eletro.. 9, 56 (): [7] A. D. Hansen, G. Mihalke,.Fault ride-through apability o DFG wind turbines. Renew. Energy. 7, 3 (9):594 6 [8] D.S. Martin, J.L. Rodriguez-Amenedo, S. Arnaltes:.Providing Ride- Through Capability a Doubly Fed ndution Generar Under Unbalaned oltage Dips. EEE Trans. Power Eletro. 9, 4 (7): [9] O. Gomis-Bellmunt, A. Junyent-Ferr e, A. Sumper, J. Bergas-Jan e,:.ride-through Control o a Doubly Fed ndution Generar Under Unbalaned oltage Sags. EEE Trans. Energy Convers. 8, 3 (4):36-45 [] M. Rahimi, M. Parniani,:.Grid-ault ride-through analysis and ontrol o wind turbines with doubly ed indution generars. Elsevier Si. Eletr. Power Syst. Res., 8 ():84 95 [] D. Xiang, L. Ran, P.J. Tavner, S. Yang,:.Control o a Doubly Fed ndution Generar in a Wind Turbine During Grid Fault Ride- Through. EEE Trans. Energy Convers. 6, (3):65-66 [] X. Yan, G. enkataramanan, P.S. Flannery, Y. Wang, Q. Dong, B. Zhang,:.oltage-Sag Tolerane o DFG Wind Turbine With a Series Grid Side Passive-mpedane Network. EEE Trans. Energy Conver., 5 (4):48-56 [3] M. M. Kyaw*,.K. Ramahandaramurthy, Fault ride through and voltage regulation or grid onneted wind turbine, Renew. Energy., 36 ():65 [4] X. Yan, G. enkataramanan, P.S. Flannery, Y. Wang,:.Evaluation the Eet o oltage Sags Due Grid Balane and Unbalane Faults on DFG Wind Turbines. nt. Con. on Sustainable Power Generation and Supply, 9:- [5] J. L opez, E. Gub ıa, P. Sanhis, X. Roboam, L. Marroyo,:.Wind Turbines Based on Doubly Fed ndution Generar Under Asymmetrial oltage Dips. EEE Trans. Energy Convers. 8, 3 ():3-33 [6].T. Phan, H.H. Lee,:. Enhaned Proportional- Resonant Current Controller or Unbalaned Stand-alone DFG-based Wind Turbines. Elet. Eng. And Teh., 5 (3): [7] R. Pena, J.C. Clare, G. M. Asher,:.Doubly ed indution generar uising bak--bak PWM onverters and its appliation variable speed wind-energy generation. Pro. nst. Elet. Eng. Eletri Power App., 996, 43 (3):34 [8] F.M. Mahdianpoor, R. Hooshmand, M. Ataei,:.A New Approah Multiuntional Dynami oltage Resrer mplementation or Emergeny Control in Distribution Systems. EEE Trans. Power Deliv.,, 6():88-89 [9]. Axente, M. Basu, M.F. Conlon, K. Gaughan,:.Protetion o uniied power quality onditioner against the load side short iruits. ET Power Eletron. 9, 3 (4):54-55 [] J. Tian, Q. Chen, B. Xie,:.Series hybrid ative power ilter based on ontrollable harmoni impedane. ET Power Eletron., 5 ():4-48 [] F. Katiraei, M.R. ravani, d.p.w. Lehn,:.Miro-Grid Aunomous Operation During and Subsequent slanding Proess. EEE Trans. Power Deliv. ():4857 7