Impedance seen by Distance Relays on Lines Fed from Fixed Speed Wind Turbines

Transcription

1 Ipedance seen by Distance Relays on Lines Fed fro Fixed Speed Wind Turbines Sachin Srivastava, Abhinna Chandra Biswal, Sethuraan Ganesan Power Technologies, ABB Corporate Research Bangalore Abstract This paper deals with line protection challenges experienced in syste having substantial wind generation penetration. Two types of generations: Theral Synchronous Generators and Fixed Speed Wind Turbines based on Squirrel Cage Induction Generators (SCIG) are siulated as thevenin equivalent odel, connected to grid with single circuit transission line. The paper gives coparative discussion and suarizes analytical investigations carried out on the ipedance seen by distance relays by varying fault resistances and grid short circuit MVA, for the protection of such transission lines during faults. Keywords-Wind Generation, Distance Relays, Fault Resistance, Short Circuit MVA, DFIG, WTGU, SCIG ABBREVIATIONS Z BC BC loop ipedance Z AG AG loop ipedance R f Fault resistance Z SA Source ipedance at bus A Z gb Grid equivalent ipedance at bus B Z F General representation for fault loop ipedance K r Coplex factor for fault resitance K 0 Zero Sequence current copensation factor Fault point on line fro Bus A a Coplex nuber ( ) Z LL Load ipedance R +jx Transient ipedance of Induction achine P, Q Pre fault active and reactive power Vr, Ir Relay easured voltage and current V L, I L Load Voltage and current I 1, I 2, I 0 Positive, negative & zero sequence fault point current V F1, V F2, V F0 Positive, negative & zero sequence fault point voltage V A,V B, V C Phase ground voltage at relay point I A, I B, I C Line currents at relay location Ia 1, Ia 2, Ia 0 Positive, negative & zero sequence relay current at bus A V a1, V a2, V a0 Positive, negative & zero sequence relay voltage at bus A C 1, C 2,C 0 Positive, negative & zero sequence current distribution factor Z L1, Z L2,Z L0 Positive, negative & zero sequence line ipedance Z S1, Z S2, Z S0 Positive, negative & zero sequence source side ipedance Z g1, Z g2, Z g0 Positive, negative & zero sequence grid side ipedance W I. INTRODUION ITH the recent developent of technologies, environent concerns and continuously decreasing fossil fuels, wind generation is an inevitable solution and the power grids throughout the world are going with ore and ore wind penetration. For new forat of power transfer where power can be transferred bidirectional i.e. fro EHV to LV and fro LV to EHV, a proper review of the protection U.J.Shenoy Departent of Electrical Engineering, Indian Institute of Science Bangalore ujs@ee.iisc.ernet.in philosophy is needed. These wind generators are priarily squirrel cage induction generators (SCIG) and known as type- 1 wind generators due to their rugged perforance and low aintenance. They are used widely when there is relatively constant wind speed. Modern types of wind generators are doubly fed induction generators (DFIG) which use power electronic controllers to ake wound rotor induction achines (WRIM) work on unity power factor. Line protection at HV/EHV/UHV level is priarily provided with distance protection or ipedance protection due to rugged requireents of better security and dependability. Reference [3] describes that current contribution fro induction achines will be changed with the slip of the achine. Such a property is different fro the conventional synchronous achines, which aintain the constant voltage with the help of independent excitation and Autoatic Voltage Regulator (AVR) syste. Thus there is a need to investigate the effect of these SCIG currents on the ipedance seen by the distance relays with wind far connected to the grid. Reference [4] presents an investigation of trip characteristic of distance relay for phase to earth (Ph-G) fault with change in wind far loading levels and for different sall syste perturbations. The theoretical analysis of dynaic behavior of Wind Turbine Generating Units (WTGU) during three phase and asyetrical voltage dips is explained in [7, 8]. A study is done using DIGISILENT progra with anually applying the wavefors to a distance relay through secondary injection kit and the results are presented in the paper concludes that behavior of distance relay is not as desired [6]. A MATLAB SIMULINK based study for behavior of distance protection loops has been carried out and reported in [12]. This paper presents a Thevenin equivalent odel based analytical study to estiate theaforesaid effect on the ipedance seen by distance relays fed by fixed speed wind turbines. II. IMPEDANCE PRESENTED TO MEASUREMENT LOOP Ipedance easureent by the distance relay depends on the type of fault and the selection of the easureent loop. Siultaneous easureent in three phases to earth loops AG, BG, CG and three phase to phase loops AB, BC, CA is well 1

2 docuented in the literature [1,2] and reported in IEC and IEEE standards. For phase to phase fault (B-C fault) and for phase to earth fault (A-G), ipedance seen by the relay is given by expressions, Z BC = V B V C I B I C andz AG = V A I A +K 0 I 0 respectively where K 0 = (Z L0 Z L1 ) 3Z L1. Effect of fault resistance and pre fault load flow on loop ipedance been discussed in following sections. A. No Load Condition The effect of fault resistance in the easured loop ipedance (Z Fnl (loop ) ) is given by following equation as explained in Appendix A (for 3 different loops) Z Fnl(loop) = Z L1 + K r R F (1) The coplex factor K r is based on the current contribution factors fro each end as explained in the literature [1] K r = 1 C 1 Three phase fault case (2) K r = 1 (C 1 + C 2 ) Ph-Ph fault case (3) K r = 3 C " Ph-earth fault case (4) where, C " = C 1 + C 2 + C 0 Z L0 Z L1 (5) C 1, C 2, C 0 are the coplex current distribution factors of the fault current for positive, negative and zero sequence networks respectively as shown in Figure1.Thus coplex factor K r ake the fault resistance as inductive or capacitive ipedance and the distance relay ay over reach or under reach depending on the current contribution fro the reote end Infeed. Z s C1I1 Zs1 ZL1 C2I2 Zs2 ZL2 C0I0 Zs0 ZL0 Bus A I1 I2 I0 ZL Zg1 E1 (1-)ZL1 Zg2 (1-)ZL2 Zg0 Rf (1-)ZL0 Ph-Gnd fault F C1I1 Zs1 ZL1 (1-)ZL I1 C1I1 Zs1 ZL1 Zg1 Bus B E1 (1-)ZL1 I1 Rf Z g C2I2 Zs2 ZL2 Ph-Ph fault E1 Zg1 Grid I2 (1-)ZL1 3 Ph fault (1-) Figure1Positive negative and zero sequence circuits for Ph-G, Ph-Ph and 3 Ph faults [1] Rf Zg2 ZL2 The coplex current distribution factors depend on the sources at both the ends of the line. In wind generation systes, the source ipedance varies with the speed of the wind and the nuber of wind turbines contribution to the grid. B. Load Condition Distance relay voltage (V r ) and current(i r ) in ters of prefault load voltage (V L ) and load current (I L ) fro [1] can be written as V r = A z V L + B z I L I r = C z V L + D z I L (6) A Z,B Z,C Z,D Z are the coplex ipedance plane paraeters[1]. These paraeters are calculated for different fault loops and for different type of faults.fro (5), the ipedance seen by the relay can be given as Z r = A Z(Z LL + B Z AZ ) C Z (Z LL + D Z CZ ) Where,Z LL = V L I L (7) The A Z, B Z, C Z, D Z paraeters as entioned in [1], is obtained by superposition theore for different type of faults. Coprehensive Table (Table 1) for these paraeters is given for particular type of fault. Table 1: A Z, B Z, C Z, D Z PARAMETERS FOR DIFFERENT FAULT TYPE Loop A Z B Z C Z D Z paraeters 3 Ph- Z F Gnd A Z = Fault A-G B Z = Z LZ s loop 1 C Z = D Z = 1 Z L Ph-Ph Fault A Z = (a2 a)z F BC loop B Z = a2 a Z L Z s1 C Z = (a2 a) D Z = a 2 a (1 Z L ) Ph-Gnd Fault A Z = C" Z F AG loop B Z = Z L( C 2 Z s2 + C 0 Z s0 ) C Z = C D Z = ac" Z F C Z L + a(c 2 Z s2 + C 0 Z s0 ) where = C " Z F + C 1 + C 2 Z F + C 0 Z s0 andc = C 1 + C 0 Z L0 ZL1 Where Z F is Z Fnl (loop ) as given in (1) 2

3 III. SYSTEM MODELLING The above analysis of distance protection as explained in (1) and (4) is carried out for the following systes. A. Fixed Speed Wind Fars Syste odeled for study is shown in Figure 2. WTGU far of 10.5 MW installed capacity is odeled with fixed speed squirrel cage induction generators (SCIG). WTGU Far V th Z th 33kV 10MVA Bus A 33/132kV 25MVA Z A < 25 k 25 k F Z B < Bus B Z g G2 132 kv Grid Figure 2 WTGU far connected to 132 kv grid Note: Z A and Z B are the distance relays at two ends The data used for the syste of Fig 2 is as follows. Squirrel cage induction achine ( for 1 achine) P g = 1.5 MW ; V n = 0.4 kv ; R s = pu ; X s = pu ; R r = pu ; X r = pu ; X = pu; Q c = 670e3; s nl = ; (slip on wind speed < 8/s); s nl = 0.006; (slip on average wind speed = 12/s) 132kV Syste Data X T =0.12 pu ;Z L = 0.29 Ω/k with X R = 5 Z g = different SCMVA, at 132 kv with X/R = 8;Z L0 = 4Z L1 The far contains a cluster of induction achines of 1.5 MW. Each achine is connected in series with sall distribution transforer of 0.44/33kV, 2 MVA and a 33 kv cable of approxiate 1 k length as seen infigure 3. Fixed shunt copensation of 627 kvar at each achine is provided to reduce the reactive power absorption fro the grid. SCIG/ achines SCIG/ achines Reactive Copensation Reactive Copensation... x No. Of achines 0.44/33kV 2 MVA 0.44/33kV 2 MVA 1 k line 1 k line 33kV Bus W 33/132 kv 25 MVA Figure 3: Squirrel Cage Induction Generator Wind Far The WTGU far interconnecting transforer is studied for Delta Star grounded (DYn) connection.line is odeled as pi section with standard paraeter of X and R, at 132 kv level with X/R ratio considered as 8.Grid is odeled as ideal voltage source with series ipedance. Grid Ipedance is varied as per the Grid SCMVA capacity. Tests have been perfored for 3 types of Grids (229 MVA, 690 MVA and 2800 MVA having axiu 3 ph short circuit current of 1.0 ka, 3.0 ka and 12.0 ka at Grid bus B respectively). Different fault resistances are odeled as for different type of faults. Line to Ground fault has fault resistances fro 0 to 5 Ω, Line to line fault has been siulated till axiu arc fault resistances for particular grid SCMVA calculated by Warrington forulae (5 Ω for 229 MVA grid, 1 Ω for 700 MVA grid and 0.2 Ω for 2800 MVA). A.1 Thevenin Equivalent Model of single cage SCIG The thevenin Equivalent odel for Induction achine is taken fro reference [3], which states positive sequence ipedance for the achine is calculated at unity slip, while negative sequence equivalent ipedance is calculated at rated slip fro the following Fig. 4. The voltage behind the positive sequence ipedance (E ) at the instant of fault is calculated fro (8). E = V 1 + P jq V 1 (R + jx )(8) Since this voltage depends on load flow which in turn on the wind speed hence at different wind speeds different voltage need to be odeled for induction achine. This equivalent odel of each SCIG achine is used in Figure1. Sequence networks offigure1are used to analyze the apparent ipedance seen by distance relays Z A and Z B for all the types of faults. It is not necessary to consider zero sequence odel for Induction achine as the neutral of achine is norally isolated to liit the currents during internal faults. VthPos X1 R1 X X2 R2/s Positive sequence odel of Induction Generator VthNeg X1 R1 X X2 R2/(2-s) Negative sequence odel of Induction Generator Figure 4: Positive and negative sequence equivalent of SCIG B. Synchronous Machine A synchronous achine is odeled as Xs of 0.12 pu with constant voltage source of 1.0 pu in place of WTGU far of fig 2. A separate excitation syste controlled with AVR and other auxiliaries are assued to be in place, which help to aintain the constant voltage. Figure 5shows the syste considered for analysis. 3

4 Positive, negative and zero sequence paraeters are assued to be sae for generator, transforer and grid. Line will have sae ipedance for positive and negative sequence network but different zero sequence ipedance.siilar to wind far case thevenin equivalent odel analysis is perfored on the syste of Figure 5 using pre fault load flow conditions and apparent ipedance equations of Table 1. created at 50% of line (Z=7.25 Ω) for different fault resistances as entioned in syste odeling. SynchMachine Xs Bus A 25 k 25 k F Bus B Z g G2 V th 11kV 10MVA 11/132kV 25MVA Z A < Z B < 132 kv Grid Figure 5: Synchronous achine connected to 132kV grid IV. CASE STUDIES In Thevenin Equivalent Model (TEM) analysis the pre fault load flow is found taking agnitude of the Bus A and Bus B voltage as constant, 1.0 pu. Power flow angle is calculated for three loading conditions i) no load ii) 50% load iii) full rated load. Current flow is calculated fro Bus A to Bus B based on syste paraeters. Based on the equivalent odel as explained in Sec. III, current distribution factorsc 1,C 2 and C 0 paraeters are deterined for both sides distance relays. Coplex Resistance factor K r and distance relay load dependent constants A Z, B Z, C Z and D Z are deterined for different type of faults. The R-X diagra have been plotted to see the effect of different wind speeds (different load flow conditions) on the apparent ipedances seen by both sides relays. Figure 6Ph-Gnd fault WTGU far case(r f varied till 5 Ω) Figure 7Ph-PhWTGU Far case (R f varied till 1 Ω) A. Fixed Speed WTGU Far For a WTGU Far the variation in K r factor can be seen in following table 2Table 2 Grid SCMVA Table 2: KR FAOR FOR WTGU FARM CASE WITH SCIG MACHINE K r for Z A Relay Grid Side K r for Z B Relay WTGU side Angle of K r is approxiately near to zero in all the cases since the syste considered is hoogeneous with respect to X/R ratio.the plots in Figure 6,Figure 7 and Figure 8 show the effect of K r factor (Table 2) for 690 MVA case. The fault is Figure 83 Ph fault WTGU Far case (R f varied till 1 Ω) Observations: Grid side distance relay can see the correct ipedance in all the three fault types with inial effect of fault resistance. Wind far side distance relay can see the correct values of ipedances for bolted fault of all types. However, with higher resistances, a fault resistance of 5 Ω in Ph-G, the fault resistance appear as 15 Ω. Siilarly, for a fault 4

5 resistance of 1Ω in Ph-Ph and fault appear as 9 Ω and 19Ω respectively at high wind speed. B. Synchronous Machine Siilar to WTGU Far case the variation in K r factor for a synchronous achine case is tabled as follows.angle of K r is approxiately near to zero in all the cases since the syste is considered hoogeneous with respect to X/R ratio. The plots in Figure.9,Figure.10 and Figure.11 show the effect of K r factor (Table 3) for 690 MVA case. The fault is created at 50% of line ( Z =7.25 Ω) for different fault resistances as entioned in syste odeling. Grid SCMVA Table 3: KR FAOR FOR SYNCHRONOUS MACHINE CASE K r for Z A Relay K r for Z B Relay Grid Side WTGU side Figure.11 fault synchronous achine case (R f varied till 1 Ω) Observations: Grid side distance relay can see the correct ipedance in all the three fault types with inial effect of fault resistance. Wind far side distance relay can see the correct values of ipedances for bolted fault of all types. However, with higher resistances, a fault resistance of 5 Ω in Ph-G, fault appear as 14 Ω. Siilarly, for a fault resistance of 1Ω in Ph-Ph and fault appear as 6Ω and 13Ω respectively at rated load. V. DISCUSSION It is observedfro Figure 6 to Figure.11, that apparent ipedance seen by distance relays Z A are siilar in behavior. But the adverse effect of R f and infeed is worse in WTGU case. Based on the above study percentage (%) change in apparent ipedances can be studied by analyzing the effect on K r factor. The % change for K r factor for different grid SCMVA is entioned in Table 4 Figure.9 1Ph-G fault synchronous achinecase (R f varied till 5 Ω) Table 4: % CHANGE IN K r FAOR FOR WTGU AND SYNCHRONOUS MACHINE Grid % change in K r for % change in K r for SCMVA Z A Relay (Grid Side) Z B Relay (WTGU side) Figure.10Ph-Ph fault synchronous achine case(rf varied till 1 Ω) VI. CONCLUSIONS Based on the above observation, following conclusions can be derived fro the studies: i. Availability of zero sequence current for the relay in DYn connection of the I gives distance relay a better operating condition during Ph-G fault. Thus it is recoended to have DYn connection. ii. Distance protection will operate with high reliability and selectivity for solid phase to earth as well as ulti phase faults. 5

6 iii. For higher resistance Ph-G faults additional earth fault protections such as counication added directional earth fault schees, are recoended in addition to distance protection. iv. For high resistive ph-ph and 3ph faults transfer tripping schees are recoended to suppleent distance protection. APPENDIX A (APPARENTLOOPIMPEDANCE WITH FAULT RESISTANCE) With reference to sequence network of Figure1, V a1 =V F1 + C 1 I 1.Z L1 (9) V a2 =V F2 + C 2 I 2.Z L2 (10) V a0 =V F0 + C 0 I 0.Z L0 (11) Also I a1 = C 1 I 1, I a2 = C 2 I 2, I a0 = C 0 I 0 where V F1, V F2 andv F0 are the fault point voltage for positive and negative sequence networksand C 1, C 2, C 0 are distribution factors defined in [1] by current division theore, C 1 = Z g1 + (1 )Z L1 Z g1 + Z L1 + Z s1 C 2 = Z g2 + (1 )Z L2 Z g2 + Z L2 + Z s2 C 0 = Z g0 + (1 )Z L0 Z g0 + Z L0 + Z s0 (12) Phase to phase fault : For a B-C fault in syste of Figure 2, BC loop easureent will be given by Where, Z BC = V B V C I B I C (13) V B V C = a 2 a V a1 V a2 (14) and, I B I C = a 2 a I a1 I a2 Fro Figure1, in ph-ph fault case, V F1 = I 1 R f + V F2 ; I 1 = I 2 (15) Using equation (9),(10),(11),(12),(14),(15) ineq(13), Z BC = Z L + R f C 1 + C 2 (16) Three phase fault : For 3Ph fault in Figure 2and its equivalent sequence coponent network of Figure1, V F1 = I 1 R f ; I 2 = I 0 = 0 (17) Ph-Ph loops will see the ipedance as Z BC = Z L + R f C 1 (18) Ph-Gnd loops will see the ipedance as Z AG = Z L + R f C 1 (19) V F1 + V F2 + V F0 = 3I 1 R f ; I 2 = I 0 = I 1 (21) Z AG = V a1 + V a2 + V a0 I a1 + I a2 + K 0 I a0 (22) Using equations (9),(10),(11),(12),(20),(21) in (22), the A-G loop ipedance will be given by, Z AG = Z L + 3R f C (23) Where C = C 1 + C 2 + C 0 Z L0 REFERENCES [1] V. Cook, Analysis of distance protection, JohnWilley& sons, 1985, pp [2] Walter A Elore, Protective relaying Theory and Application. ABB, 2002, pp [3] Nasser Tleis, Power Systes Modeling and Fault Analysis Elsevier 2008, Ch 5, pp [4] A.K.Pradhan and GezaJoos, Adaptive Distance Relay settings for lines connecting wind fars IEEE transaction on energy conversion, vol 22, March [5] VahanGevorgian and Eduard Muljadi, Wind Power Plant Short-Circuit Current Contribution for Different Fault and Wind Turbine Topologies NREL Conference paper, October [6] S.Jaali et al, Robustness of Distance Relay with Quadrilateral Characteristic against Fault Resistance IEEE PES T&D Conference, China, [7] Jes usl opez,et al Dynaic Behavior of the Doubly Fed Induction Generator During Three-Phase Voltage Dips, IEEE Transactions on energy conversion, vol. 22, no. 3, pp 709 (sep 2007). [8] Jes usl opez,et al Wind Turbines Based on Doubly Fed Induction Generator Under Asyetrical Voltage Dips, IEEE Transactions on energy conversion, vol. 22, no. 3,pp 321 (arch 2008). [9] K.K.Li and L.L.Lai, Ideal operating region of digital distance relay under highresistance earth fault Electric Power Systes Research, 1997 [10] Diogo de Oliveira Fialho Pereira and Arturo SuanBretas, Extended Ground Distance Relaying With Fault Resistance Copensation Modern Electric Power Systes 2010, Wroclaw, Poland [11] Rodrigo HartsteinSali,Denise PivattoMarzecand Arturo SuanBretas, Phase Distance Relaying With Fault Resistance Copensation for Unbalanced Systes IEEE Transactions on power delivery, vol. 26, no. 2, April [12] SachinSrivastava,U.J.Shenoy, Abhinna Chandra Biswal and Sethuraan Ganesan, Effect of Fault Resistance And Grid Short Circuit MVA on Ipedance Seen by Distance Relays on Lines Fed Fro Wind Turbine GeneratingUnits (WTGU) IET-DPSP 2012, April Z L Single phase to ground fault For a AG fault in syste of Figure 2, A-G loop easureent will be given by, Z AG = V AG I A + K 0 I 0 (20) And fro sequence coponent network of Figure1, 6