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
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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
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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
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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.
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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
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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.
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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)