65 CHAPTER 4 TRANSIENT STABILITY MARGIN OF SCIG IN WIND FARM USING STATCOM 4.1 INTRODUCTION Angular stability assessment of WEG is one of the main issues in power system security and operation. Rotor speed stability refers to the ability of an induction machine to remain connected to the electric power system and running at a mechanical speed close to the speed corresponding to the actual system frequency after being subjected to a disturbance (Kanabar 2008). In practice, overspeed protection circuit disconnects the WEG from the grid when its speed exceeds 1.2pu. From the power quality study undertaken in one 110kV/11kV substation at Anthiyur windfarm it was observed that nearly 60% of power quality issues in windfarms are contributed by voltage sags,29% by voltage swells,8% by transients and 3% by interruptions (Thirumoorthy 2009). Normally, LVRT requirements are stringent in regions with high penetration of wind power. In order to promote the integration of wind farms into the electrical network, FACTS are widely used. STATCOM is one of them (Hingorani 2000). STATCOM stimulates voltage stability by reactive power regulation. STATCOM provides or absorbs reactive power to or from the grid to compensate small voltage variations at PCC. Many studies show that STATCOM helps the wind farm to stabilize voltage especially after a voltage dip occurs. With regard to maintaining the short term voltage
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65
CHAPTER 4
TRANSIENT STABILITY MARGIN OF SCIG IN WIND
FARM USING STATCOM
4.1 INTRODUCTION
Angular stability assessment of WEG is one of the main issues in
power system security and operation. Rotor speed stability refers to the ability
of an induction machine to remain connected to the electric power system and
running at a mechanical speed close to the speed corresponding to the actual
system frequency after being subjected to a disturbance (Kanabar 2008). In
practice, overspeed protection circuit disconnects the WEG from the grid
when its speed exceeds 1.2pu. From the power quality study undertaken in
one 110kV/11kV substation at Anthiyur windfarm it was observed that nearly
60% of power quality issues in windfarms are contributed by voltage
sags,29% by voltage swells,8% by transients and 3% by interruptions
(Thirumoorthy 2009). Normally, LVRT requirements are stringent in regions
with high penetration of wind power. In order to promote the integration of
wind farms into the electrical network, FACTS are widely used. STATCOM
is one of them (Hingorani 2000). STATCOM stimulates voltage stability by
reactive power regulation. STATCOM provides or absorbs reactive power to
or from the grid to compensate small voltage variations at PCC. Many studies
show that STATCOM helps the wind farm to stabilize voltage especially after
a voltage dip occurs. With regard to maintaining the short term voltage
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stability, all grid codes demand that the voltage in the transmission power grid
is re-established without subsequent disconnection of large wind farms.
In this chapter, the effect of STATCOM on the transient stability
margin of SCIG is studied under different penetration levels in the event of
unbalanced or balanced fault in the grid. The performance of WEG with
STATCOM is studied using MATLAB/Simulink taking into account the
nature of the load and the results are presented.
4.2 WIND FARM STABILITY AND REACTIVE POWER
COMPENSATION
A system experiences a state of voltage instability when there is a
progressive or uncontrollable drop in voltage magnitude after a disturbance,
increase in load demand or change in operating condition. The main factor,
which causes these unacceptable voltage profiles, is the inability of the
distribution system to meet the demand for reactive power (Alejandro Jurado
2009). The reactive power absorbed by the induction generator coupled to
wind turbine depends on the generator parameters and its operational points
(generated electric power, terminal voltage magnitude and slip). During the
fault, the generator speed is increased by the difference between
electromagnetic torque of SCIG and mechanical torque of WT. Once the fault
is cleared, the SCIG draws a large amount of reactive power from the grid
because of its high rotational speed. If the rotor accelerates faster than the
terminal voltage is restored, the reactive power consumption continues to
increase. This leads to a decrease in the terminal voltage and thus to a further
deterioration of the balance between mechanical and electrical power and to a
further acceleration of the rotor. Owing to this reactive power consumption, it
can happen that the terminal voltage recovers only relatively slowly after the
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fault is cleared. This decline in the electromagnetic torque causes a decrease
in the value of the CCT and hence the transient stability margin of SCIG.
4.3 EFFECT OF ADDITIONAL REACTIVE POWER SUPPORT
ON TRANSIENT STABILITY MARGIN OF SCIG
Figure 4.1 shows the torque-slip characteristics of a SCIG with two
different values of reactive power compensation (Kanabar 2008). For a given
set of machine parameters, the electromagnetic torque developed by the WEG
depends on the value of reactive power compensation .The additional value of
reactive power compensation will shift the torque-slip characteristic of SCIG
upwards. Consequently, the value of critical clearing slip will increase from
Scr1 to Scr2. which will enhance the rotor stability margin of SCIG. This, in
turn improves the CCT, which is in compliance with the LVRT requirements
in new grid code.
Figure 4.1 Torque-slip characteristic of SCIG with nominal and
additional reactive power
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4.4 ADVANTAGES OF STATCOM
Due to the low cost, shunt capacitors are the most commonly used
scheme to compensate reactive power in WEGs. Shunt capacitors are used in
banks and switched in and out of the circuit using contactors. Due to the surge
current taken by the capacitors while switching in, the lifetime of the
contactors is limited. The switching of capacitors excites transients and the
switching has to be done by keeping the transients minimum. Also the voltage
support provided will be discontinuous. STATCOM has better characteristics
than FC compensation and SVC. Reactive power output of STATCOM is
independent of the actual voltage at PCC. In contrast, the reactive output of
FC and SVC is proportional to the square of the voltage magnitude at PCC.
This makes the reactive power output from SVC to decrease rapidly when the
voltage at PCC decreases, thus reducing the system stability.
Nevertheless, FACTS systems provide faster and smoother
response to changes in wind farm voltage. On the other hand, shunt capacitors
give a poor response. Power quality issues in Anthiyur windfarm near
Udumalpet in Tamilnadu, show frequent failure of lightning arrestors and
studies show that switching out of capacitor may be one of the reasons which
would have caused the transients that leads to the failure of insulation.
4.5 STATCOM
4.5.1 Principle of Operation
During the last few decades, development of power electronics
technology has helped to propose and implement FACTS devices for
overcoming power quality problems in power system. A STATCOM is a
regulating device used on alternating current electricity transmission
networks. It can act as either as a source or sink of reactive power to an
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electricity network. When system voltage is low, the STATCOM generates
reactive power (STATCOM capacitive). When system voltage is high, it
absorbs reactive power (STATCOM inductive).The variation of reactive
power is performed by means of a VSC connected on the secondary side of
a coupling transformer. The VSC uses forced-commutated power electronic
devices GTOs, IGBTs or IGCTs which can be operated at high switching
frequency to synthesize a voltage from a DC voltage source. The
STATCOM can be operated in two different modes:
In voltage regulation mode (the voltage is regulated within
limits)
In VAR control mode (the STATCOM reactive power output is
kept constant)
When the STATCOM is operated in voltage regulation mode, it
implements the V-I characteristic shown in Figure 4.2.
Figure 4.2 STATCOM V-I characteristic
As long as the reactive current stays within the minimum and
minimum current values (-Imax, Imax) imposed by the converter rating, the
voltage is regulated at the reference voltage Vref. However, a voltage droop is
normally used (usually between 1% and 4% at maximum reactive power
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output), and the V-I characteristic has the slope indicated as shown in the
Figure 4.2.
4.5.2 Mathematical Model of STATCOM Control System
The STATCOM used is a standard 3-phase inverter with PWM
switching. The passive elements, namely, the series choke and the dc-bus
capacitor are designed to limit the ripple in the ac side current and dc bus
voltage of the STATCOM, respectively.
Figure 4.3 Schematic diagram of STATCOM connected to the grid
Figure 4.3 shows the schematic diagram of STATCOM connected
to grid. Assuming that the control of STATCOM is successful, the current
that will flow through R and L is equal to the reference current Ii .The voltage
that the inverter should generate is given below by applying Kirchhoff’s
voltage law. R is the equivalent loss resistance which includes winding
resistance, switch power loss etc. L is the filter inductance, Vg is the grid
voltage and Vi is the inverter output voltage before filtering (Arun
Karuppusamy 2007). Since the current references in the Synchronous
Reference Frame strategy are in the d-q plane, the equations are first written
in the R-Y-B plane and then they are transformed to the plane and
subsequently to the d-q plane. Applying KVL to the R-L circuit shown in
Figure 4.3, the Equations (4.1) to (4.3) are obtained:
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iaia ga ia
di (t)(t) (t) R.i (t) L.
dt(4.1)
ibib gb ib
di (t)(t) (t) R.i (t) L.
dt(4.2)
icic gc ic
di (t)(t) (t) R.i (t) L.
dt(4.3)
Converting the above equations to plane, Equation (4.4) to (4.5) are
obtained.
ii g i
di (t)(t) (t) R.i (t) L.
dt (4.4)
i
i g i
di (t)(t) (t) R.i (t) L.
dt(4.5)
In general, Equation (4.6) can be written for STATCOM.
ii g i
dIV V R.I L
dt(4.6)
As it is known that plane is related to d-q plane by the relation
given by Equation (4.7), Equation (4.8) is obtained.
( + j ) = (d cos - q sin ) + j ( d sin + q cos ) = (d + jq).e j
(4.7)
j
id iqj j j
id iq id iq gd gq
d i ji .ej .e R i ji .e L j .e
dt(4.8)
The above equation , when multiplied by e-j is transformed to d-q
plane. Since d-axis is aligned with grid voltage, Equation (4.9) to (4.10) are
obtained.
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idid id iq
diR.i L L.i | V |
dt(4.9)
iq
iq iq id
diR.i L L.i
dt (4.10)
From the above two Equations, the d axis and q axis currents Iid and
Iiq can be represented as shown in Figure 4.4.
Figure 4.4 Representation for d axis and q-axis currents of STATCOM
Where
id id iq' Li | V | (4.9)
iq iq id' Li (4.10)
Above mathematical equations can be represented as block diagram
shown in Figure 4.5, in which the d-axis and q-axis reference voltage vid and
viq of STATCOM are obtained. Reactive power control is achieved by control
of Iiq and active power control by control of Iid .
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Figure 4.5 Representation of d-axis and q-axis voltage at PCC
Similarly for DC bus voltage controller, Equation (4.11) can be
obtained.
Iid = C dVdc/dt + Vdc/R (4.11)
Figure 4.6 shows the representation for DC bus voltage controller
of STATCOM.
Figure 4.6 Representation of STATCOM DC bus voltage controller
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4.5.3 Design and Control of STATCOM
The design of a STATCOM has three broad sections (Arun
Karuppaswamy 2007):
1. Current reference generation (It involves computing the
reactive current absorbed by SCIG).
2. Design of the DC bus capacitor and inductor
3. Design of closed loop controller, that makes the STATCOM
current to follow the reference.
The first part of the design is to generate the current reference.
There are several methods to generate the current reference. The present study
is based on the application of co-ordinate transformations to separate the
active and reactive components of the current. The strategy used is the Vector
control method (Arun Karuppaswamy 2007). Once the current reference has
been generated, the next work is to find the values of DC capacitor and
inductor of STATCOM, according to the requirement of the reactive power
compensation.
The reactive current injected is controlled so as to obtain full rated
grid voltage before, during and after the fault. It is based on the measurement
of voltage at PCC. The voltage error signal is obtained by comparing the
actual and reference voltage, which is fed to a PI controller. There needs to be
another voltage controller to maintain a constant DC bus voltage. The
STATCOM current is continuously compared with reference current received
from two voltage controllers and error signal is fed into the Hysteresis
comparator.
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Hysteresis current control is a method of controlling a voltage
source inverter so that an output current is generated which follows a
reference current waveform. This method controls the switches in an inverter
asynchronously to ramp the current through an inductor up and down so that
it tracks a reference current signal. This scheme is employed for generation of
pulses to the STATCOM. This is a continuous current variable switching
current control scheme. The STATCOM current is continuously compared
with the reference current waveform and the error signal after amplification is
fed into the hysteresis comparator. The comparator changes state when the
error exceeds a preset value in positive and negative directions. The
comparator state switches is used to decide which of the switches should be
on and which of the switches should be off. When the STATCOM current
actually goes above the reference current by the comparator hysteresis band,
the comparator changes state. This state change is used to switch off the boost
switch and current ramps down. When the STATCOM current goes below the
reference current by comparator hysteresis band, it changes state again and
state change is used to turn the boost switch on. Thus the STATCOM current
is always maintained within half of the hysteresis band. A hysteresis current
controller is implemented with a closed loop control system and is shown in
diagrammatic form in Figure 4.7(a) (David 2009). An error signal, e(t), is
used to control the switches in an inverter. This error is the difference
between the desired current, iref(t), and the current being injected by the
inverter, iactual(t). When the error reaches an upper limit, the IGBTs are
switched to force the current down. When the error reaches a lower limit the
current is forced to increase. The minimum and maximum values of the error
signal are emin and emax respectively. The range of the error signal, emax –
emin, directly controls the amount of ripple in the output current from the
inverter and this is called the Hysteresis Band. The hysteresis limits, emin and
emax, relate directly to an offset from the reference signal and are referred to
as the Lower Hysteresis Limit and the Upper Hysteresis Limit. The current is
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forced to stay within these limits even while the reference current is changing.
The ramping of the current between the two limits is illustrated in Fig 4.7(b).
Figure 4.7 Block diagram and operational waveform of Hysteresis
current controller
Figure 4.8 shows the total control block diagram of the vector
control scheme for STATCOM.
Figure 4.8 Block diagram of the Vector Control Scheme for STATCOM
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4.6 SIMULATION RESULTS
One of the methods to meet the LVRT requirements is by providing
additional reactive power support which can improve the terminal voltage
during a disturbance. For the purpose of analysis, the system shown in Figure
2.7 is considered with VAR compensation as STATCOM. The SCIG acts as a
load requiring variable reactive power. It was found that SCIG is drawing a
reactive power of nearly 900kVAR during severe three phase to ground fault
with no VAR compensation. The STATCOM’s power rating is to be decided
based on the reactive power requirement. It is discussed in section 4.3, that
additional reactive power improves the transient stability margin of SCIG. A
STATCOM of 1000kVAR is assumed to be installed at PCC as SCIG is
drawing approximately 900kVAR during severe three phase to ground fault
without any compensation. Simulation studies have been carried out assuming
that the system is operating at full load and 12m/s wind speed. Different types
of faults are simulated at PCC. Simulations are repeated for the system with
1000kVAR FC compensation.
Table 4.1 shows the maximum reactive power (Q) consumption of
WEG, maximum SCIG speed and settling time after the fault for different
fault conditions with FC compensation and STATCOM compensation. The
slip of SCIG after the fault clearance is larger than that prior to the fault. The
larger the slip, the larger will be the reactive power demand of SCIG. Results
show that most of the parameters are reduced when 1000 kVAR STATCOM
is used for compensation instead of 1000 kVAR FC, which means that
STATCOM is responding faster than FC.
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Table 4.1 Comparison of FC compensation with STATCOM
compensation for different fault conditions at the wind speed of
12m/ s
FC compensation STATCOM compensationNature of
fault and
fault
duration
Maximum
Q (kVAR)
Maximum
SCIG
speed
(rad/s)
Settling
time
after the
fault (s)
Maximum
Q(kVAR)
Maximum
SCIG
speed
(rad/s)
Settling
time
after the
fault (s)
Single line to
ground fault
(600ms)
500 168 1.5 400 167 1.25
Double line
to ground
fault(100ms)
1080 175 1.25 920 178 0.8
Three phase
to ground
fault(50ms)
925 186 1 620 179 0.9
For considering the effect of wind penetration level on transient
stability of SCIG, the two machine system shown in Figure 3.6 is taken for
study with VAR compensation as STATCOM. Assuming that the system
under consideration is operating at full load, the transient stability of SCIG
under different fault conditions of various fault durations with STATCOM
compensation is studied.
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4.6.1 250 kW SCIG Connected to 2000kVA Alternator (Medium
Penetration)-Case 1
4.6.1.1 Single line to ground fault
A single line to ground fault is simulated at the instant of 2 seconds
from the start at PCC. The fault is cleared after 100ms. The wind speed is
assumed to be 10m/s, which is the normal case prevailing in practice.
Simulation is repeated for different fault durations and corresponding values
of the performance indices are given in Table 4.2. It is observed that for
longer duration faults, dip in DC link capacitor voltage is more. STATCOM
DC link voltage Vdc is maintained at 600V before and after fault. Alternator
speed and Vpcc settle at 1 pu.
Figure 4.9 shows the plots of the parameters for a fault duration of
625ms. From Figure 4.9 , it is inferred that grid code is satisfied for single line
to ground fault as the system returns to stable condition without getting
tripped for 625ms fault duration.
Table 4.2 Range of transients in different parameters at SCIG terminals