120 CHAPTER 6 POWER QUALITY IMPROVEMENT OF SCIG IN WIND FARM USING STATCOM WITH SUPERCAPACITOR 6.1 INTRODUCTION For a long time, SCIG has been the most used generator type for wind turbines because of the robust technology and low cost. Main drawback of such generator is that it needs reactive power for their operation, which is normally provided using FC compensation. But, reactive power consumption depends on the real power produced by SCIG, which in turn relies on the fluctuating wind speed. FC cannot provide dynamic compensation thereby leading to voltage fluctuations in the grid. As the wind penetration level is increasing day by day, these problems are also increasing. So, STATCOM is preferred for providing dynamic compensation. But during grid fault conditions, STATCOM is not able to provide sufficient amount of reactive power, which makes the WEG to get tripped off from the grid. So, in order to increase the transient stability margin of WEG, which is the measure of FRT capability, use of new energy storage technology supercapacitor with STATCOM is proposed in this chapter. This chapter also focuses on the use of charging and discharging tests on supercapacitor 100PP14, to develop the equivalent circuit model to characterize symmetric supercapacitors which is used for simulation with STATCOM in MATLAB Simulink. It also deals with the application of STATCOM with supercapacitor for mitigating other power quality issues related to SCIG based windfarms such as voltage
121 fluctuations, harmonics, power transients, STATCOM DC link voltage overshoots and dips apart from improving the FRT capability of WEG. 6.2 SUPERCAPACITOR Supercapacitor technology has been available commercially for over the past decade.they can store more energy than conventional capacitors and are available in various sizes. They can be charged and discharged faster than batteries. Supercapacitors integrated with a power conversion system can be used to assist the electric utility by providing voltage support, power factor correction, active filtering, and reactive and active power support. They also have higher cycle life than batteries, which results in longer life span. There is a strong need to gain a better understanding of supercapacitors when used in electric utility applications.this requires suitable models that can be incorporated into different software programs such as MATLAB Simulink, PSPICE, PSCAD etc. used to create dynamic simulations for different applications (Stanley 2000). 6.3 DETERMINATION OF EQUIVALENT CIRCUIT PARAMETERS OF SUPERCAPACITOR A supercapacitor can be modeled in a similar manner to conventional capacitors. There are many models developed to characterize the electrical behavior of supercapacitor (Faranda 2007).The multi branch model defines the capacitance of the supercapacitor as a constant capacitor with a parallel capacitor dependent on voltage. This voltage dependence of capacitance implies that more energy can be stored in it than expected. The transmission line model of supercapacitor is a complex network of non-linear capacitors connected between them by resistors. Figure 6.1 shows the
122 classical model of supercapacitor, where ESR is equivalent series resistance, C is the capacitance and EPR is the equivalent parallel resistance of supercapacitor. Figure 6.1 Classical model of supercapacitor In short, the most important parameters of a supercapacitor include capacitance, ESR and EPR. Capacitance decides the energy capability that can be stored in a supercapacitor.esr consists of electrode resistance, electrolyte resistance and contact resistance. Power is wasted for internal heating when charging or discharging. For the supercapacitor, ESR is in the range of milliohms and it influences the energy efficiency and power density. EPR is an inner equivalent parallel resistance, usually in hundreds of ohms and decides the leakage current when the supercapacitor is in stand-by-mode. 6.3.1 ESR Measurement Figure 6.2 shows the experimental setup for determining the ESR of supercapacitor (Yao 2006). Initially the supercapacitor is charged to the rated voltage and then it is discharged. Instantaneous voltage drop and current at the beginning of the discharging are recorded by two probes of an oscillograph. The voltage drop and discharging current can be measured
123 through resistor sampling. The ESR is the quotient of voltage drop to discharge current. Figure 6.2 Experimental circuit to find ESR by voltage drop method 6.3.2 EPR Measurement The supercapacitor is charged to a specified voltage. Then the power supply is disconnected and left in the self discharging state. The voltage of supercapacitor declines approximately according to equation (6.1)(Yao 2006).EPR in is given by EPR=(t 2 -t 1 )/(ln (U 2 /U 1 )*C) (6.1) where U 1 and U 2 are the voltages in V at t 1 and t 2 (in s )respectively, C is the supercapacitor s rated capacitance in Farads. EPR varies with the environment temperature. Self discharging becomes more serious when temperature rises. 6.3.3 Capacitance Measurement Supercapacitor is charged to full rated voltage. Then it is allowed to discharge through a known value of resistance and the time taken for the rated voltage to reduce to half the rated value is noted using stop watch.
124 Then the capacitance in Farads is calculated using the Equation (6.2) (Yao 2006). C = t/(r ln 2) (6.2) where t = discharge time in s and R = Known load discharge resistance in. 6.3.4 Testing Results of Supercapacitor 100PP14 Supercapacitor used in this work is 100PP14, which is rated for 100V and has an energy density of 14.2 kj. It is an Electrochemical Double Layer Capacitor (EDLC) having bipolar symmetric carbon/carbon electrodes and an aqueous KOH electrolyte. It has internal balancing circuits. Its characteristics are high power cycling capacity of 300,000 cycles, wide operating temperature of -45 degrees to +55 degrees, quick recharge and free form fire and explosion hazards because of rugged construction. Its equivalent circuit parameters can be found by conducting charging and discharging tests on the supercapacitor. 100PP14 supercapacitor is charged to the rated voltage of 100V from an AC source through an autotransformer. A filter capacitance of 470 microfarad and 250V is used to remove ripples in DC voltage output. Once it reaches the rated voltage, supercapacitor is discharged through a load resistance of 28.6 ohms, 250W. Figure 6.3 shows the charging and discharging set up of the supercapacitor. Figure 6.4 shows the charging and discharging characteristics of 100PP14. Table 6.1 and 6.2 show the results.
125 Figure 6.3 Charging and Discharging set up of 100PP14 (a) (b) Figure 6.4 (a) Charging and (b) discharging characteristics of 100PP14 Table 6.1 Self discharge results of 100PP14 Time(s) Voltage(V) 0 100 240 96.9 600 95 Table 6.2 Charge and discharge results of 100PP14 Transient Voltage drop(v) Current(A) Time to discharge to half the rated voltage(s) Load resistance ) 0.44 3.496 65 28.6
126 From the results, 100PP14 supercapacitor s equivalent circuit parameters are found to be: C CALC = 3.278F DC ESR = 0.125 DC EPR = 5398.166 6.4 STATCOM WITH SUPERCAPACITOR STATCOM is operated as shunt connected static VAR compensator whose inductive or capacitive output current can be controlled independent of AC system voltage. It can rapidly supply dynamic VAR required during system disturbances and faults for voltage support. However, because of less energy density of DC link capacitor used in STATCOM, there is a large voltage dip in DC link voltage which limits the reactive power capability of STATCOM (Zhengping Xi 2008). Recent developments in the field of supercapacitors have led to the achievement of high specific energy and high specific power devices which are suitable for energy storage in high power electronic applications (Barker 2002). As supercapacitors have time constants from fractional seconds to seconds, compared to the time duration of power line transients in the range of microseconds, these devices can be able to withstand short duration surges specified in standards (Nihal Kularatna 2010). FRT Capability of SCIG can be improved by STATCOM to preserve the power system security. But during the fault,the reactive power capability of STATCOM is limited which can be enhanced by connecting a supercapacitor with STATCOM. Also after the fault is cleared, the electromagnetic torque should be developed quickly by SCIG to counterbalance the mechanical torque produced by wind turbine. Because of the fast dynamic characteristic of supercapacitor, this is achieved by SCIG so
127 that it remains connected to the grid without being tripped by over speed protection devices. When FC compensation is used for WEG, it is seen that there are no harmonics. But when STATCOM is used, it introduces voltage harmonics at PCC, which causes current harmonics also. When supercapacitor is used with STATCOM,it is found that harmonics are reduced in both voltage and current. Also, when there are random wind speed variations, voltage fluctuations are very much reduced when supercapacitor is used with STATCOM. 6.5 SIMULATION RESULTS - TRANSIENT PERFORMANCE OF WEG It was found in section 6.2.4 that 100PP14 supercapacitor is having an equivalent series resistance of 0.125, equivalent parallel resistance of 5398.16 and capacitance of 3.278 F. In transmission and distribution applications, supercapacitors have to be connected in series in order to withstand high voltage stress (Srithorn 2006). The supercapacitor used here is required to be connected in parallel with the STATCOM DC link capacitor rated for 600V. So, six numbers of 100PP14 supercapacitor have to be connected in series and accordingly a modified equivalent circuit with capacitance 0.55F, equivalent series resistance of 750 m and equivalent parallel resistance of 900 is considered for simulation. The schematic diagram of the two machine system shown in Figure 3.6 with VAR compensation as STATCOM with supercapacitor is considered for the study. The load connected to the system is assumed to be RL load of 0.9 power factor lagging. A STATCOM of 250kVAR with the modified model of supercapacitor is installed at PCC. The transient stability of SCIG under different fault conditions of various fault duration using STATCOM with supercapacitor compensation is studied. Performance with different penetration levels are also analyzed for each type of fault.
128 6.5.1 250 kw SCIG Connected to 2000kVA Alternator (Medium Penetration) The penetration level of WEG is 12.5% for Case-1 as a steam turbine- alternator of 2000 kva capacity is connected to the 250 kw SCIG coupled to a wind turbine. 6.5.1.1 Single line to ground fault A single line to ground fault is simulated at PCC for the considered system operating at full load. Wind speed is assumed to be 10m/s as this is the speed normally occurring in practice. Simulation is repeated for different fault durations and corresponding values of the performance indices are given in Table 6.3. STATCOM DC link voltage Vdc is maintained at 600V before and after fault. Alternator speed and Vpcc settle at 1 pu. Vpcc settles at 0.989 and 0.982pu after the fault clearance for 100ms and 625ms faults respectively. Figure 6.5 shows the plots of the parameters for a fault duration of 100ms. It is found that all parameter variations are reduced when super capacitor is used with STATCOM. Table 6.3 Range of transients in different parameters at SCIG terminals for single line to ground fault at PCC for a wind speed of 10m/s at full load and 0.9 power factor lagging (case 1) Fault duration (ms) (rad/s) P (kw) Q (kvar) Te(Nm) Vpcc(pu) Vdc(V) 100 157.4-160.8 110-210 76-111 330-1750 0.977-1 595-604 625 156.5-160.8 110-209 73-111 330-1750 0.97-1 594-605
129 Figure 6.5 System performance indices for single line to ground fault of 100ms duration at PCC for a wind speed of 10m/s at full load of 0.9 power factor lagging (case 1)
130 6.5.1.2 Double line to Ground fault A double line to ground fault is implemented at PCC. Figure 6.6 System performance indices for double line to ground fault of 100ms duration at PCC for a wind speed of 10m/s at full load of 0.9 power factor lagging (case 1)
131 Table 6.4 shows the results for double line to ground fault for different durations. Figure 6.6 shows the plots for 100ms fault duration. For 100ms fault duration, Vpcc and Vdc settle at 1pu and 600V respectively. For 400ms fault duration, Vpcc and Vdc settle at respective values of 0.93 pu and 580V. When the fault duration is increased to 550ms, SCIG speed increases indefinitely and the system becomes unstable. Table 6.4 Range of transients in different parameters at SCIG terminals for double line to ground fault at PCC for a wind speed of 10 m/s at full load and 0.9 power factor lagging (case 1) Fault duration (ms) 100 (rad/s) P (kw) Q (kvar) Te(Nm) Vpcc(pu) Vdc (V) 152.8-170.9 200 153-173 400 152.9-185.6-95 to +365-95 to +255-95 to +245-380 to +650-540 to +650-680 to +650 +4000 to -6280 +4000 to -6280 +4000 to -6280 0.41-1.05 535-625 0.39-1.045 532-620 0.39-0.945 395-655 Figure 6.7(i) shows the plots of and T e for a wind speed of 10m/s at full load corresponding to 550ms fault. When the wind speed is reduced to 8m/s from 10m/s, for the same type of fault and duration, the system comes back to original condition and the system becomes stable. Table 6.5 shows the transients for 8m/s during fault condition. Figure 6.7(ii) shows the plots of and T e corresponding to this condition.
132 Table 6.5 Range of transients in different parameters at SCIG terminals Fault duration (ms) 550 for double line to ground fault at PCC for a wind speed of 8 m/s at full load and 0.9 power factor lagging (case 1) (rad/s) P (kw) Q (kvar) Te(Nm) Vpcc(pu) Vdc(V) 147.4-167.5-150 to +142-440 to +660-7300 to +4900 0.42 to 1.035 440-675 Figure 6.7(i) and T e for double line to ground fault of 550ms duration at PCC for a wind speed of 10m/s at full load of 0.9 power factor lagging (case 1) Figure 6.7(ii) and T e for double line to ground fault of 550ms duration at PCC for a wind speed of 8m/s at full load of 0.9 power factor lagging (case 1)
133 For the double line fault of 550ms duration at 10m/s, the system becomes unstable. But when the load demand is reduced to half load, the system retains its stability by returning to original condition. Alternator speed settles at 1.017 pu. and Te respectively settle at 171 rad/s and 975 Nm. Table 6.6 shows the parameters variations for half load. Figure 6.7(iii) show the plots of and T e. Table 6.6 Range of transients in different parameters at SCIG terminals for double line to ground fault at PCC for a wind speed of 8 m/s at half load and 0.9 power factor lagging (case 1) Fault duration (ms) (rad/s) 550 160-182 P (kw) Q (kvar) Te(Nm) Vpcc(pu) Vdc (V) -150 to +255-540 to +640 +5200 to - 8350 0.41-1 370-660 Figure 6.7 (iii) and T e for double line to ground fault of 550ms duration at PCC for a wind speed of 10m/s at half load of 0.9 power factor lagging (case 1)
134 6.5.1.3 Three phase to Ground fault PCC. A three phase to ground fault of different durations is simulated at Figure 6.8 System performance indices for three phase to ground fault of 100ms duration at PCC for a wind speed of 10m/s at full load of 0.9 power factor lagging (case 1)
135 Table 6.7 shows the transients of different parameter and system becomes stable after the clearance of the fault. Alternator speed varies over 0.96 to 1.06pu during fault. For 50ms, 100ms and 200ms fault durations, Vpcc settle at 1pu, 0.99 pu and 0.94 pu respectively. Figure 6.8 shows the plots for 100ms fault. Vdc settles at 600V for 50 and 100ms fault and 580V for 200ms fault. Table 6.7 Range of transients in different parameters at SCIG terminals for three phase to ground fault at PCC for a wind speed of 10 m/s at full load and 0.9 power factor lagging (case 1) Fault duration (ms) 50 100 200 (rad/s) P (kw) Q (kvar) Te(Nm) Vpcc(pu) Vdc (V) 143.2-179.5 143.2-180.5 143.2-202.4-275 to +420-130 to +350-90 to +265-375 to +200-450 to +200-575 to +200 +2670 to -8100 +2675 to -8100 +2700 to -8030 0-1.04 548-620 0-1.04 430-625 0-0.95 185-682 When the fault duration is increased to 300ms, the system becomes unstable. Figure 6.9(i) shows the plots of Te and corresponding to this condition. For 300ms duration, the system becomes unstable for RL load of 0.9 power factor lagging. If unity power factor load is used for same type and duration of fault, the system retains its original condition thereby stability is attained. Vpcc settles at 0.9 pu after the fault. Table 6.8 gives the parameter variations and Figure 6.9(ii) shows the plots of Te and for this condition.
136 Table 6.8 Range of transients in different parameters at SCIG terminals for three phase to ground fault at PCC for a wind speed of 10 m/s at full load and unity power factor (case 1) Fault duration (ms) 300 (rad/s) 217.4-139 P (kw) Q (kvar) Te(Nm) Vpcc(pu) -90 to +220-650 to +210 +2740 to -8540 0-0.86 Vdc (V) 215-745 Figure 6.9(i) and T e for three phase to ground fault of 300ms duration at PCC for a wind speed of 10m/s at full load of 0.9 power factor lagging (case 1) Figure 6.9(ii) and T e for three phase to ground fault of 300ms duration at PCC for a wind speed of 10m/s at full load of unity power factor (case 1)
137 6.5.2 SCIG Connected to 910kVA Alternator (High Penetration) In the second case, the penetration level of WEG is 27.5% as a steam turbine- alternator of 910 kva capacity is connected to the 250 kw SCIG coupled with a wind turbine. 6.5.2.1 Single line to ground fault A single line to ground fault is simulated at PCC. Table 6.9 shows the transients of various parameters for different fault durations at high penetration. Figure 6.10 shows the plots for 100ms single line to ground fault. Vpcc settles at 0.985pu after the fault clearance. It shows that the settling value of Vpcc is decreasing for higher penetration. Alternator speed settles at 1 pu. Table 6.9 Range of transients in different parameters at SCIG terminals for single line to ground fault at PCC for a wind speed of 10 m/s at full load and 0.9 power factor lagging (case 2) Fault duration (ms) (rad/s) P (kw) Q (kvar) Te(Nm) Vpcc(pu) Vdc (V) 100 156.2-161.5 104-232 30-110 65-2135 0.95-1.04 578-619 625 154.4-161.5 95-232 43-111 65-2135 0.955-1.02 572-619
138 Figure 6.10 System performance indices for single line to ground fault of 100ms duration at PCC for a wind speed of 10m/s at full load of 0.9 power factor lagging (case 2)
139 6.5.2.2 Double line to ground fault at PCC. A double line to ground fault of different durations is implemented Figure 6.11 System performance indices for double line to ground fault of 100ms duration at PCC for a wind speed of 10m/s at full load of 0.9 power factor lagging (case 2)
140 Table 6.10 Range of transients in different parameters at SCIG terminals for double line to ground fault at PCC for a wind speed of 10 m/s at full load and 0.9 power factor lagging (case 2) Fault duration (ms) 100 (rad/s) P (kw) Q (kvar) Te(Nm) Vpcc(pu) Vdc (V) 145.4-176.7 200 145.4-195 250 145.5-201 -70 to 315-70 to 250-70 to 208-310 to +560-450 to +560-440 to +560 +5050 to -8900 +5015 to -8920 +5015 to -8920 0.37-1.065 390-663 0.27-0.97 260-860 0.27-0.8 260-640 Table 6.10 shows the transients of different parameters corresponding to various durations. Results show that, as penetration level is increasing, the transients are also increasing. Alternator speed varies over 0.96 to 1.06pu during fault. For 100ms, 200ms and 250ms fault durations, Vpcc settles at 0.96 pu, 0.92pu and 0.85pu respectively. Vdc settles at 600V for 100 and 200ms faults and 560V for 250ms fault. Figure 6.11 shows the plots for 100ms fault. When the fault duration is increased to 300ms, the SCIG speed increases indefinitely and becomes unstable. But, for same fault duration, when the wind speed is reduced to 8m/s from 10m/s, the system remains stable. Table 6.11 shows the results.
141 Figure 6.12 and T e for double line to ground fault of 300ms duration at PCC for a wind speed of 8m/s at full load of 0.9 power factor lagging (case 2) Table 6.11 Range of transients in different parameters at SCIG terminals for double line to ground fault at PCC for a wind speed of 8 m/s at full load and 0.9 power factor lagging (case 2) Fault duration (ms) (rad/s) P (kw) Q (kvar) Te(Nm) Vpcc(pu) Vdc (V) 300 146.5-168 -95 to +145-390 to +550 +4040 to -6530 0.36-1.06 345-675 Figure 6.12 shows the plots of Te and. Vpcc, Vdc, and Te settle at 0.95pu,600V,157 rad/s and 450Nm respectively. 6.5.2.3 Three phase to ground fault A three phase to ground fault is simulated at PCC. Variations in different parameters during fault condition are given in the Table 6.12. Alternator speed varies over 0.96 to 1.07pu. Vpcc settles at 0.96pu, 0.94pu,
142 0.91 pu and 0.85pu for 50,100,150 and 190ms faults. Vdc settles at 600V for 50 and 100ms faults. Vdc settles at 580V and 560V for 150 and 190ms fault durations. Figure 6.13 shows the plots for 50ms fault. Table 6.12 Range of transients in different parameters at SCIG terminals for three phase to ground fault at PCC for a wind speed of 10 m/s at full load and 0.9 power factor lagging (case 2) Fault duration (ms) (rad/s) P (kw) Q (kvar) Te(Nm) Vpcc(pu) Vdc (V) 50 145.5-178 100 145.5-181.5-185 to +340-70 to +260-300 to +160-350 to +160 +2400 to -7290 +2400 to -7290 0-1.07 510-645 0-1.045 110-640 150 145.5-191.2-45 to +245-420 to +160 +2400 to -7290 0-0.98 75-655 190 145.5-202 -80 to +210-440 to +160 +2400 to -7290 0-0.79 70-700 When the fault duration is increased to 200ms, the system becomes unstable at full load. But at half load, for same fault, the system regains to original condition., P, Q, Vdc, Vpcc and Te respectively settle at 172 rad/s,160kw,85kvar,600v,0.94pu and 950Nm. Alternator speed settles at 1.01 pu in 9s. Table 6.13 shows the corresponding results. Figure 6.14 shows the plots of Te and for this condition.
143 Figure 6.13 System performance indices for three phase to ground fault of 50ms duration at PCC for a wind speed of 10m/s at full load of 0.9 power factor lagging (case 2) Table 6.13 Range of transients in different parameters at SCIG terminals for three phase to ground fault at PCC for a wind speed of 10 m/s at half load and 0.9 power factor lagging (case 2) Fault duration (ms) 200 (rad/s) 165.6-190 P (kw) Q (kvar) Te(Nm) Vpcc(pu) - 60 to +245-485 to +200 3300 to - 6000 0-0.975 Vdc (V) 150-675
144 Figure 6.14 and T e for three phase to ground fault of 200ms duration at PCC for a wind speed of 10m/s at half load of 0.9 power factor lagging (case 2) Table 6.14 and Table 6.15 give the summary of the transient stability margin of SCIG (in ms) for a wind speed of 10m/s at different loading conditions for medium and high penetration levels respectively. Table 6.14 Transient stability margin of SCIG (in ms) for a wind speed of 10m/s at different loading conditions for medium penetration Type of fault Full load Fraction of Load Half load Nature of load RL load R load RL load R load Single line to ground fault 625 625 625 625 Double line to ground fault 500 510 625 625 Three phase to ground fault 280 310 330 350
145 Table 6.15 Transient stability margin of SCIG ( in ms) for a wind speed of 10m/s at different loading conditions for high penetration Type of fault Fraction of Load Full load Half load Nature of load RL load R load RL load R load Single line to ground fault 625 625 625 625 Double line to ground fault 250 300 370 400 Three phase to ground fault 190 250 250 280 From Table 6.14 and Table 6.15, it is inferred that the transient stability margin of SCIG is improved at half load for all types of faults. For highly resistive load, the transient stability margin is increasing, as the resistance component of load offers damping effect to rotor acceleration. Table 6.16 shows the maximum reactive power consumption of SCIG (Q max ) and maximum SCIG speed ( max ) during fault, settling time (t s ) after the fault and transients in STATCOM DC link capacitor voltage (V dc ) during fault for double line to ground fault (L-L-G) and three phase to ground fault (L-L-L-G) with different compensation techniques. When higher rating of STATCOM is used, it provides a compromising solution with respect to settling time (t s ) after the fault and maximum SCIG speed ( max ) during the fault for symmetrical and unsymmetrical faults. Transients in DC link capacitor voltage are adequately suppressed when supercapacitor is used with STATCOM. Recovery voltage is also improved when supercapacitor is added to STATCOM. Table 6.17 shows the comparison of transient stability margin of SCIG for different faults under various compensations at 10m/s wind speed and 27% wind penetration level. The results show that, if lesser rating of STATCOM is used with supercapacitor, it provides better transient stability
146 margin than STATCOM for symmetrical faults. But for unsymmetrical faults, the transient stability margin remains unaltered. When higher rating of STATCOM is used, improvement in transient stability margin of SCIG for both symmetrical and unsymmetrical faults are 12.8% and 11.5% respectively, when compared to STATCOM. Table 6.16 Comparison of system performance indices for different compensations at 10m/s wind speed and 27% wind penetration level Type and duration of fault L-L-G fault of 230ms duration L-L-L-G fault of 170ms duration Performance indices 250kVAR STATCOM with supercapacitor 1000kVAR STATCOM 1000kVAR STATCOM with supercapacitor max ( rad/s) 198.8 197.5 198 Q max (kvar) 450 455 455 Transients in V dc (V) 260 to 650 290 to 850 455 to 630 t s (ms) 330 300 300 Recovery voltage at PCC (pu) 0.86 0.87 0.89 max ( rad/s) 197.5 198.5 198.5 Q max (kvar) 415 420 420 Transients in V dc (V) 100 to 690 60 to 1460 220 to 690 t s (ms) 380 450 420 Recovery voltage at PCC(pu) 0.85 0.84 0.85
147 Table 6.17 Comparison of transient stability margin of SCIG in milliseconds for different faults under various compensations at 10m/s wind speed and 27% wind penetration level Type of compensation 250kVAR STATCOM with supercapacitor Type of Full load Half load fault RL load R load RL load R load L-L-G 250 300 370 400 L-L-L-G 190 250 250 280 1000kVAR L-L-G 250 300 370 400 STATCOM L-L-L-G 170 230 220 270 1000kVAR STATCOM with supercapacitor L-L-G 260 325 385 420 L-L-L-G 195 260 255 285 6.6 SIMULATION RESULTS - HARMONICS For the first case of 12.5% penetration, simulation of the considered system is carried out at a wind speed of 10m/s under STATCOM compensation and STATCOM with supercapacitor compensation. Total Harmonic Distortion (THD) for both voltage and current at PCC are noted and tabulated. Table 6.18 shows the results and Figure 6.15 shows the voltage and current waveforms at PCC without and with supercapacitor for 12.5% penetration. Table 6.18 Voltage THD and Current THD at PCC without and with supercapacitor at 12.5% penetration Type of compensation Voltage THD (%) Current THD (%) 250 kvar STATCOM 4 1.1 250 kvar STATCOM with supercapacitor 3.49 0.94
148 (a) (b) Figure 6.15 Voltage and current waveforms (a)with Supercapacitor (b)without supercapacitor at 12.5% penetration The above simulation is repeated for 27% penetration and the voltage and current THD are tabulated. Table 6.19 shows the results. In both cases, it is seen that both voltage and current harmonics are reduced when supercapacitor is added to STATCOM.
149 Table 6.19 Voltage and Current THD at PCC without and with supercapacitor at 27% penetration Type of compensation Voltage THD (%) Current THD (%) 250 kvar STATCOM 3.88 1.16 250 kvar STATCOM with supercapacitor 3.76 0.97 6.7 SIMULATION RESULTS-VOLTAGE FLUCTUATIONS DUE TO WIND SPEED AND LOCAL LOAD For the considered system, random wind speed variations from 6 m/s to 10 m/s as shown in Figure 6.16 are applied and the voltage fluctuations are noted with and without supercapacitor. Figure 6.17 and Table 6.20 show the results. From the results, it can be seen that the range of voltage fluctuations are minimized on including supercapacitor with STATCOM. Similarly voltage fluctuations and recovery voltage for the considered system are noted for local load fluctuations with and without supercapacitor. Table 6.21 shows the results.here also, the voltage fluctuations are reduced and recovery voltage is improved when supercapacitor is used. Figure 6.16 Wind speed profile considered for the simulation study
150 (a) (b) Figure 6.17 Voltage fluctuations for random wind speed variations ranging from 6m/s to 10m/s at 27% penetration (a)without supercapacitor (b)with supercapacitor Table 6.20 Voltage fluctuations for random wind speed variations ranging from 6m/s to 10m/s at 27% penetration Type of compensation Voltage fluctuations(pu) 1000 kvar STATCOM 1.055 to 0.965 1000 kvar STATCOM with supercapacitor 1.05 to 0.98 Table 6.21 Voltage fluctuations for local load variations at constant wind speed of 10m/s at 27% penetration Type of compensation 1000 kvar STATCOM 1000 kvar STATCOM with supercapacitor Voltage Recovery voltage(pu) fluctuations(pu) 1.06 to 0.93 0.92 1.03 to 0.94 0.93
151 6.8 SUMMARY Simulation and analysis of SCIG based WEG performance with STATCOM and supercapacitor was done. Results show that the transient stability margin of WEG in increased thereby improving its FRT capability according to the grid code requirements. Reactive power consumption of SCIG during fault and settling time after the fault are considerably reduced when STATCOM with supercapacitor is used. It can also be inferred that harmonics in voltage and current are minimized when supercapacitor is added to STATCOM. Also voltage fluctuations due to wind speed variations and load variations are reduced when supercapacitor is used with STATCOM. STATCOM DC link voltage dips and overshoots, real and reactive power transients are also reduced to a larger extent because of the addition of energy storage device with STATCOM.