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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=(t2-t1)/(ln (U2/U1)*C) (6.1)
where U1 and U2 are the voltages in V at t1 and t2 (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:
CCALC = 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)
(rad/s) P (kW) Q (kVAR) Te(Nm) Vpcc(pu) Vdc (V)
100152.8-
170.9
-95 to
+365
-380 to
+650
+4000 to
-62800.41-1.05 535-625
200 153 -173-95 to
+255
-540 to
+650
+4000 to
-62800.39-1.045 532-620
400152.9-
185.6
-95 to
+245
-680 to
+650
+4000 to
-62800.39-0.945 395-655
Figure 6.7(i) shows the plots of and Te 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 Te corresponding to this condition.
132
Table 6.5 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 1)
Fault
duration
(ms)
(rad/s) P (kW) Q (kVAR) Te(Nm) Vpcc(pu) Vdc(V)
550147.4-
167.5
-150 to
+142
-440 to
+660
- 7300 to
+4900
0.42 to
1.035
440-
675
Figure 6.7(i) and Te 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 Te 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 Te.
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)
P (kW) Q (kVAR) Te(Nm) Vpcc(pu) Vdc (V)
550 160-182-150 to
+255
-540 to
+640
+5200 to -
83500.41-1 370-660
Figure 6.7 (iii) and Te 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
A three phase to ground fault of different durations is simulated at
PCC.
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)
(rad/s) P (kW) Q (kVAR) Te(Nm) Vpcc(pu) Vdc (V)
50143.2-
179.5
-275 to
+420
-375 to
+200
+2670 to
-81000-1.04 548-620
100143.2-
180.5
-130 to
+350
-450 to
+200
+2675 to
-81000-1.04 430-625
200143.2-202.4
-90 to+265
-575 to+200
+2700 to
-80300-0.95 185-682
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)(rad/s)
P (kW) Q (kVAR) Te(Nm) Vpcc(pu)Vdc
(V)
300217.4-
139-90 to +220
-650 to
+210
+2740 to
-85400-0.86
215-
745
Figure 6.9(i) and Te 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 Te 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
A double line to ground fault of different durations is implemented
at PCC.
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)
(rad/s) P (kW) Q (kVAR) Te(Nm) Vpcc(pu) Vdc (V)
100145.4-
176.7
-70 to
315
-310 to
+560
+5050 to
-8900
0.37-
1.065390-663
200 145.4-195-70 to
250
-450 to
+560
+5015 to
-89200.27-0.97 260-860
250 145.5-201-70 to
208
-440 to
+560
+5015 to
-89200.27-0.8 260-640
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 Te 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
-65300.36-1.06 345-675
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)
50 145.5-178-185 to
+340
-300 to
+160
+2400 to
-72900-1.07 510-645
100145.5-
181.5
-70 to
+260
-350 to
+160
+2400 to
-72900-1.045 110-640
150145.5-
191.2
-45 to
+245
-420 to
+160
+2400 to
-72900-0.98 75-655
190 145.5-202-80 to
+210
-440 to
+160
+2400 to
-72900-0.79 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)(rad/s)
P (kW) Q (kVAR) Te(Nm) Vpcc(pu)Vdc
(V)
200165.6-
190- 60 to +245
-485 to
+200
3300 to -
60000-0.975
150-
675
144
Figure 6.14 and Te 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
Fraction of LoadType of fault
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 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
Fraction of LoadType of fault
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 (Qmax) and maximum SCIG speed ( max) during fault , settling time (ts)
after the fault and transients in STATCOM DC link capacitor voltage (Vdc)
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 (ts) 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
Performance
indices
250kVAR
STATCOM
with
supercapacitor
1000kVAR
STATCOM
1000kVAR
STATCOM
with
supercapacitor
max ( rad/s) 198.8 197.5 198
Qmax (kVAR) 450 455 455
Transients in Vdc (V) 260 to 650 290 to 850 455 to 630
ts (ms) 330 300 300
L-L-G fault
of 230ms
duration
Recovery voltage at
PCC (pu)
0.86 0.87 0.89
max ( rad/s) 197.5 198.5 198.5
Qmax (kVAR) 415 420 420
Transients in Vdc (V) 100 to 690 60 to 1460 220 to 690
ts (ms) 380 450 420
L-L-L-G
fault of
170ms
durationRecovery 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
Full load Half loadType of
compensation
Type of
fault RL load R load RL load R load
L-L-G 250 300 370 400250kVAR
STATCOM
with
supercapacitorL-L-L-G 190 250 250 280
L-L-G 250 300 370 4001000kVAR
STATCOM L-L-L-G 170 230 220 270
L-L-G 260 325 385 4201000kVAR
STATCOM
with
supercapacitorL-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 Voltage
fluctuations(pu)
Recovery voltage(pu)
1000 kVAR
STATCOM
1.06 to 0.93 0.92
1000 kVAR
STATCOM with
supercapacitor
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.
Recommended