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CHAPTER 5
IMPROVEMENT OF TRANSIENT STABILITY IN IEEE 14
BUS SYSTEM WITH TCSC AND STATCOM
5.1 INTRODUCTION
The representation of IEEE 14 bus system equipped with TCSC
and STATCOM using MiPower simulation software is presented in this
chapter. The system is analyzed under severe disturbance to study the
transient behavior with and without FACTS controller. The enhancement of
the transient stability is carried out with help of TCSC and STATCOM. The
enhancement of transient stability is achieved by adjusting line reactance with
the help of TCSC and by compensating the reactive power with the help of
STATCOM. The performance curves of the system with TCSC, STATCOM
and their combination are taken for discussion. The damping of rotor angle
oscillation introduced by the individual controller is compared with combined
controllers and it is observed that the latter one shows possible improvement.
5.2 LITERATURE REVIEW
Many different techniques are available pertaining to investigation
of the effect of TCSC on power system stability. Several approaches based on
modern control theory have been applied to TCSC controller design.
Chen et al (1995) presented a state feedback controller for TCSC by using a
pole placement technique. However, the controller requires all system states
which reduce its applicability. Chang and Chow (1997) developed a time
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optimal control strategy for the TCSC where a performance index of time was
minimized. Lie et al (1995) proposed a fuzzy logic controller for TCSC. The
impedance of the TCSC is adjusted based on the machine rotor angle and the
magnitude of the speed deviation. In addition, different control schemes for
TCSC were proposed such as variable structure controller (Wang et al 1992,
Luor and Hsu 1998), bilinear generalized predictive controller (Rajkumar and
Mohler 1994), and H -based controller (Zhao and Jiang 1998). The neural
networks (Dai et al 1998 and Senjyu et al 2003) have been proposed for
TCSC-based stabilizer design. The damping characteristics of the designed
stabilizers have been demonstrated through simulation results on a multi-
machine power system. Wang et al (2002) presented a robust nonlinear
coordinated control approach to excitation and TCSC for transient stability
enhancement. The excitation controller and TCSC controller have been
designed separately using a direct feedback linearization technique. Lee and
Moon (2003) presented a hybrid linearization method in which the algebraic
and the numerical linearization technique were combined. The TCSC-based
compensation possesses a thyristor-controlled variable capacitor protected by
a metal oxide varistor (MOV) and an air gap. However, the implementation of
this technology changes the apparent line impedance, which is controlled by
the firing angle of the thyristor and is accentuated by other factors including
the MOV. The presence of the TCSC in the fault loop not only affects the
steady-state components but also the transient components. The controllable
reactance, the MOVs protecting the capacitors and the air gaps’ operation
make the protection decision more complex and therefore, the conventional
relaying scheme based on fixed settings has its limitations. Fault classification
and section identification is a very challenging task for a transmission line
with TCSC (Dash et al 2007).
5.3 PROBLEM STATEMENT
One method of analyzing the transient stability is by placing
different types of FACTS controllers in the test system. This may be done
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either by placing a single device or combination of devices at a particular
point. The problem is formulated as the insertion of TCSC and STATCOM in
IEEE 14 bus system that is to be analyzed using MiPower simulation software
for enhancing the transient stability.
5.4 GENERAL REPRESENTATION OF TCSC AND STATCOM
The TCSC power circuit shown in Figure 5.1 is similar to that of
the SVC. The key difference between the SVC and TCSC is that the TCSC is
connected in series with the transmission line. In this case, a twelve pulse
configuration is not necessary, since the current harmonics from the TCR are
able to complete their path through the capacitor more easily than through the
rest of the transmission system (Gyugyi 1998).
Figure 5.1 TCSC power circuit
The TCSC has low level controls that generate the firing pulses. In
this case, the firing pulses are synchronized with the line current. The
response of the synchronization circuit has a significant impact on the system
performance. So it is important to match this to the system that is to be
modeled. If the TCSC has multiple modules connected in series, it is necessary
to model the multiple modules for the system study. The global controls generally
produce a commanded Vc or Xc to be inserted in the line. Again, it is necessary to
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map these quantities back to produce the firing delay angle through a lookup
table. On the protection side, the converter over-current protection functions
need to be modeled carefully, if the TCSC is to be included in system
protection study. This includes the external overvoltage protection such as
MOVs and bypass breakers. A key concern in the study is the response of the
TCSC controls prior to the action of the external protection, and how they
impact the response of this protection and the relays protecting the line.
Figure 5.2 shows an example of the external protection circuitry.
Figure 5.2 TCSC with over voltage protection
In the TSC scheme, the degree of series compensation is controlled
by increasing or decreasing the number of capacitor banks in series. To
accomplish this, each capacitor bank is inserted or bypassed by a thyristor
valve (switch). To minimize the switching transients and utilize “natural”
commutation, the operation of the thyristor valves is coordinated with voltage
and current zero crossings. In the fixed-capacitor, thyristor-controlled reactor
scheme, the degree of series compensation in the capacitive operating region
(the admittance of the TCR is kept below that of the parallel connected
capacitor) is increased (or decreased) by increasing (or decreasing) the
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thyristor conduction period, and thereby the current in the TCR. Minimum
series compensation is reached when the TCR is off. The TCR is designed to
have the capability to limit the voltage across the capacitor during faults and
other system contingencies of similar effect. The two schemes are combined
by connecting a number of TCRs plus a fixed capacitor in series in order to
achieve greater control range and flexibility. The normalized power P versus
transmission angle is plotted as a parametric function of the degree of series
compensation, k=XC/X (where XC is the effective capacitive impedance of
TCSC and X is the line reactance), shown in Figure 5.3. The variable series
capacitive compensation, apart from steady-state control of power flow, is
effective in transient stability improvement, power oscillations damping and
balancing power flow in parallel lines.
Figure 5.3 P versus for series compensation
TCSC is a controller designed to be connected in series with the tie
line to control their impedance. These types of controllers can be used for
damping the rotor angle oscillations. 50% fixed compensation during steady
condition is considered. During the fault, TCSC is bypassed by MOV, in
order to protect the TCSC. After clearing the fault, the compensation is
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increased to 70% in order to transfer the high power and to maintain the
stability.
Representation of STATCOM is discussed in section 3.4.
5.5 IEEE 14 BUS SYSTEM
The IEEE 14 bus standard system is considered as the test system.
It consists of five synchronous machines with IEEE type1 AVRs, three of
which are synchronous compensators used only for reactive power support.
There are eleven loads in the system. The 40 MW generator parameter, type1
AVR parameter (type1 AVR diagram is shown in Figure 4.2), the TG data for
the generator (block diagram is shown in Figure 4.3), two winding
transformer data, three winding transformer data, bus data, line data and load
data are given in the Tables 5.1 to 5.8 respectively.
Table 5.1 40 MW generator parameter
VariableDescription
Data
MVA rating 50
MW rating 40
Rated voltage in kV 220
Ra Armature resistance in p.u. 0.004593
X2 Negative sequence reactance in p.u. 0.149
X0 Zero sequence reactance in p.u. 0.066
Xd Direct axis reactance in p.u. 2.036
Xd' Direct axis transient reactance in p.u. 0.237
Xd'' Direct axis sub - transient reactance in p.u. 0.185
Xq Quadrature axis reactance in p.u. 1.8
Xq' Quadrature axis transient reactance in p.u. 0.33
Xq'' Quadrature axis sub – transient reactance in p.u. 0.1678
Tdo' Direct axis open circuit transient time constant in p.u. 4.902
Tdo" Direct axis open circuit sub – transient time constant in p.u. 0.017
Tqo' Quadrature axis open circuit transient time constant in p.u. 0.533
Tqo" Quadrature axis open circuit sub-transient time constant in p.u. 0.1
H Inertia constant (Generator + Exciter) 4
Winding connection Y
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Table 5.2 Type1 AVR parameter
Variable Description Data
Trec Input rectifier time constant in s 0.01
Ka Amplifier gain 300
Ta Amplifier time constant in s 0.02
Ke Exciter gain 1
Te Exciter time constant in s 0.3
Kf Regulator stabilizing circuit gain 0.001
Tf Regulator stabilizing circuit time constant 1
Vse1 Saturation function at 0.75 times maximum field voltage 0.4
Vse2 Saturation function at maximum field voltage 0.7
Vrmax Maximum amplifier voltage 6
Vrmin Minimum amplifier voltage -6
Efdmax Maximum field voltage 4
Efdmin Minimum field voltage 0
Table 5.3 TG data for 40 MW generator
Variable Description Data
Droop 0.05
Pmax Maximum power limit 1
Pmin Minimum power limit 0
Cmax Rate of valve opening 0.1
Cmin Rate of valve closing - 0.33
K1 + K2 Power extraction at HP turbine 0.33
K 3 + K 4 Power extraction at IP turbine 0.33
K 5 + K 6 Power extraction at LP turbine 0.34
T1 Phase compensation 1 0.1
T2 Phase compensation 2 0.05
T3 Servo time constant 0.1
Thp HP section time constant in s 0.1
Trh Reheat section time constant in s 10
Tlp LP section time constant in s 999
Tip IP section time constant (including re-heater) in s 0.1
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Table 5.4 Two winding transformer data
Rate d MV A Rate d Vol ta ge( kV) % i mpe dan ce
150 220/ 132 10
Table 5.5 Three winding transformer data
Rated MVA Rated Voltage% impedance
(Positive)
% impedance
(Zero)
Primary Secondary Tertiary Primary Secondary Tertiary Primary Secondary Tertiary Primary Secondary Tertiary
150 105 50.25 220 132 3.3 0.31913 0.28616 0.038527 0.01 0.022 0.028
Table 5.6 Bus data
Bus No.Base
Voltage (kV)
Minimum
Voltage (kV)
Maximum
Voltage (kV)
1 220 209 231
2 220 209 231
3 220 209 231
4 220 209 231
5 220 209 231
6 132 125.4 138.6
8 3.3 3.135 3.465
9 132 125.4 138.6
10 132 125.4 138.6
11 132 125.4 138.6
12 132 125.4 138.6
13 132 125.4 138.6
14 132 125.4 138.6
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Table 5.7 Line data
Line
No.
From
bus
No.
To
bus
No.
Positive
sequence
resistance
(p.u.)
Positive
sequence
reactance
(p.u.)
Positive
sequence
susceptance
(p.u.)
Zero
sequence
resistance
(p.u.)
Zero
sequence
reactance
(p.u.)
Zero sequence
susceptance
(p.u.)
1 1 5 0.05403 0.22304 0.0246 0.0035 0.0035 0.0035
2 1 2 0.01938 0.05917 0.0264 0.0035 0.3298 0.000001099
3 2 3 0.04699 0.19797 0.0219 0.0035 0.3298 0.000001099
4 2 4 0.05811 0.17632 0.0187 0.0035 0.3298 0.000001099
5 2 5 0.05695 0.17388 0.017 0.0035 0.3298 0.000001099
6 3 4 0.06701 0.17103 0.0173 0.0035 0.3298 0.000001099
7 4 5 0.01335 0.04211 0.0064 0.0035 0.3298 0.000001099
8 6 11 0.09498 0.1989 0 0.0035 0.3298 0.000001099
9 6 12 0.12991 0.25581 0 0.0035 0.3298 0.000001099
10 6 13 0.06615 0.13027 0 0.0035 0.3298 0.000001099
11 9 10 0.03181 0.08450 0 0.0035 0.3298 0.000001099
12 9 14 0.12711 0.27038 0 0.0035 0.3298 0.000001099
13 10 11 0.08205 0.19207 0 0.0035 0.3298 0.000001099
14 12 13 0.22092 0.19988 0 0.0035 0.3298 0.000001099
15 13 14 0.17093 0.34802 0 0.0035 0.3298 0.000001099
Table 5.8 Load data
Load No. Bus No. MW rating MVAr rating
1 2 21.7 12.7
2 3 94.2 19
3 4 47.8 -3.9
4 5 7.6 1.6
5 6 11.2 7.5
6 9 29.5 16.6
7 10 9 5.8
8 11 3.5 1.8
9 12 6.1 1.6
10 13 13.5 5.8
11 14 14.9 5.6
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5.5.1 Single Line Diagram of IEEE 14 bus System
The representation of single line diagram of IEEE 14 bus system
using MiPower simulation software is depicted in Figure 5.4.
Figure 5.4 Single line diagram of IEEE 14 bus system
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The real power in MW, reactive power in MVAr, the voltage in p.u.
and angle in degree in the system at steady state condition are depicted in
Figure 5.5.
Figure 5.5 Load flow analysis of IEEE 14 bus system
The real power in MW, reactive power in MVAr, the voltage in p.u.
and angle in degree in the system with TCSC between the buses 1 and 5 are
depicted in Figure 5.6.
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Figure 5.6 Load flow analysis of IEEE 14 bus system with TCSC
The real power in MW, reactive power in MVAr, the voltage in p.u.
and angle in degree in the system with STATCOM at bus10 are depicted in
Figure 5.7.
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Figure 5.7 Load flow analysis of IEEE 14 bus system with STATCOM
The real power in MW, reactive power in MVAr, the voltage in p.u.
and angle in degree in the system with TCSC between the buses 1 and 5 and
STATCOM at bus10 are depicted in Figure 5.8.
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Figure 5.8 Load flow analysis of IEEE 14 bus system with TCSC and
STATCOM
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5.6 RESULTS WITH DISCUSSIONS
The IEEE 14 bus system is tested for transient stability by creating
three phase to ground fault at bus12 and the effect at bus2 with and without
FACTS controllers is analyzed. The voltage in p.u., frequency in Hertz and
angle in degree with respect to time in second are taken at bus2 for with and
without FACTS controllers individually and their combination, using
MiPower simulation software.
Figure 5.9 Bus2 voltage in p.u. at fault condition
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The voltage at bus2 during fault varies from 0.96 p.u. to 1.04 p.u.,
is depicted in Figure 5.9.
Figure 5.10 Bus2 voltage in p.u. at fault condition (with FACTS device)
The voltage at bus2 with FACTS devices (TCSC between the buses
1 and 5 and STATCOM at bus10) varies from 0.965 p.u. to 1.03 p.u. This is
depicted in Figure 5.10.
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Figure 5.11 Bus2 frequency in Hertz at fault condition
During steady state condition, the frequency remains constant at 50
Hz and after the fault at bus12, the frequency at bus2 oscillates and comes
below 50 Hz. This is depicted in Figure 5.11. It indicates inadequate power
generation or transmission capability.
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Figure 5.12 Bus2 frequency in Hertz at fault condition (with FACTS
device)
With TCSC (between the buses 1 and 5), the fall in frequency is
limited by means of enhancing the transmission capability. With STATCOM
(at bus10), the power transfer capability is increased by means of recovering
the voltage quickly. With both TCSC and STATCOM, the frequency is
recovered gradually towards the normal value because the power transfer
capability and voltage are restored immediately after clearing the fault. This is
depicted in Figure 5.12.
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Figure 5.13 Oscillation of rotor angle in degree at fault condition
Power system stability is basically a spring mass problem.
Whenever there is a disturbance in a spring, the mass will oscillate. Similarly,
the generators will oscillate whenever there is a fault in the network. The
oscillation of generators is basically measured with respect to reference bus.
The oscillation level of the generator depends on the disturbance severity.
During fault at bus12, the generator at bus2 oscillates from -18 to 110 degrees
whereas at steady state condition, it is 42 degrees with respect to reference
bus. This is depicted in Figure 5.13. The system is considered stable, for the
disturbance, since the swing is well within the transient limit of 180 degrees.
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Figure 5.14 Oscillation of rotor angle in degree at fault condition (with
TCSC)
With TCSC (between the buses 1 and 5), the generator at bus2
oscillates from -18 to 110 degrees whereas at the steady state condition, it is
42 degrees with respect to reference bus. This is depicted in Figure 5.14. The
system is considered stable, for the disturbance, since the swing is well within
the transient limit of 180 degrees.
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Figure 5.15 Oscillation of rotor angle in degree at fault condition (with
STATCOM)
With STATCOM (at bus10), the generator at bus2 oscillates from
-18 to 110 degrees whereas at steady state condition, it is 42 degrees with
respect to reference bus. This is depicted in Figure 5.15. The system is
considered stable, for the disturbance, since the swing is well within the
transient limit of 180 degrees.
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Figure 5.16 Oscillation of rotor angle in degree at fault condition (with
TCSC and STATCOM)
With FACTS devices (TCSC between the buses 1 and 5 and
STATCOM at bus 10), the generator at bus2 oscillates from 18 to 62 degrees
whereas at steady state condition, it is 42 degrees with respect to reference
bus. This is depicted in Figure 5.16. The system is considered stable, for the
disturbance, since the swing is well within the transient limit of 180 degrees.
When the device is placed separately in the system, the generator
oscillation is not reduced significantly whereas, it is much reduced (from 18
to 62 degrees) when the devices are placed together.
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5.7 CONCLUSION
Transient stability study for IEEE 14 bus system is carried out with
TCSC and STATCOM using MiPower simulation software. Oscillation of the
generator is reduced appreciably when connecting both TCSC and
STATCOM together. However, the oscillation of the generator is not reduced
when connecting the TCSC and STATCOM individually. Regarding voltage
and frequency, both are recovered quickly by connecting TCSC and
STATCOM together.
