CHAPTER 6 POWER QUALITY IMPROVEMENT OF …shodhganga.inflibnet.ac.in/bitstream/10603/24514/11/11...with the application of STATCOM with supercapacitor for mitigating other power quality

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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

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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

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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.

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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

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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.

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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


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.


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


for double line to ground fault at PCC for a wind speed of 8 m/s


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



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.


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


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



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.


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)


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


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



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.


for single line to ground fault at PCC for a wind speed of 10 m/s


Fault

duration

(ms)


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


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



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)


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




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)


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,

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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.


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)


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




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


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.

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CHAPTER 6 POWER QUALITY IMPROVEMENT OF …shodhganga.inflibnet.ac.in/bitstream/10603/24514/11/11...with the application of STATCOM with supercapacitor for mitigating other power quality