Design and implementation of TCSC for 500KV power transmission system
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DESIGN AND IMPLEMENTATION OF TCSC FOR 500KV POWER
TRANSMISSION SYSTEM
Ali Raza
Department of Electrical Engineering, The University of Lahore, Lahore,
(Pakistan)
E-mail: ali.raza@ee.uol.edu.pk
Haroon Farooq
Department of Electrical Engineering, University of Engineering & Technology,
Lahore, (Pakistan)
E-mail: haroon.farooq@uet.edu.pk
Manzoor Ellahi
Faculty of Engineering and Technology, Superior University, Lahore, (Pakistan)
E-mail: manzoor.ellahi@superior.edu.pk
Waqas Ali
Department of Electrical Engineering, University of Engineering & Technology,
Lahore, (Pakistan)
E-mail: engr_waqasali@yahoo.com
Shahid Kaleem
Department of Electrical Engineering, The University of Lahore, Lahore
(Pakistan)
E-mail: shahid.kalim@hotmail.com
Muhammad Nasir Khan
Department of Electrical Engineering, The University of Lahore, Lahore
(Pakistan)
E-mail: muhammad.nasir@ee.uol.edu.pk
Design and implementation of TCSC for 500KV power transmission system
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ABSTRACT
Power transmission capability of a transmission line (TL) depends upon the
impedance of the TL, the magnitude and the phase angle difference of the end
voltages. Series capacitor largely employed in the transmission lines to increase
the transfer capability but create instability problems. Flexible alternating current
transmission systems (FACTs) enhance the power transfer through the existing
transmission lines with stability intact. Thyristor controlled series compensation
(TCSC) is considered in this paper. Impedance of the transmission line is
regulated by changing the firing angle of the thyristor. A 500kV transmission line
shunted with TCSC is dynamically implemented in Matlab/Simulink and tested
for different sending end voltage and, by changing the impedance of line. Results
show the significance of designed control under transient conditions of power
system.
KEYWORDS
Flexible alternating current transmission systems (FACTS) devices; transmission
lines; thyristor controlled series compensation (TCSC); stability enhancement.
1. INTRODUCTION
Energy is said to be a backbone of a nation’s economy. With the passage of time,
the world is becoming more automated and electronic and thus the consumption
of electricity per person is increased. The human race is going to be more
dependent on robots, thus it’s important to fulfill the need of electricity. There are
two ways to increase the transmission line (TL) capacity. In first choice, need to
build new transmission lines to meet the demands. Installations of new
transmission lines require feasibility studies, contract signing, electrical and
mechanical designs and, material for wires. It is an extravagant choice and
requires plenty of time for completion. Second option is to increase the power
transfer capability of the existing transmission system. This method doesn’t
require any feasibility report and cost-effective as well.
A number of researches have been conducted for increasing the bulk power
transfer capacity of existing transmission lines [1] – [3]. In early days, power flow
control is done by changing taps or via phase shifting transformers. Series reactors
were introduced in transmission lines to reduce the power flow and also used to
reduce the short circuit current level at some locations when needed. Similarly,
capacitors employed in transmission lines to reduce the electrical length and thus
to increase the power flow. Hence, series compensation was used on alternate basis
according to load condition. However, this kind of compensation introduced
transient and stability issues [1]. So, an alternative technique is required to solve
these shortcomings. Fixed series capacitors deployed in the transmission lines to
increase the power transfer capacity. The introduction of series capacitor causes
low frequency oscillations in the transmission system which introduced the effect
of sub-synchronous resonance (SSR) in electric power system. Therefore, with the
advent of power electronics devices, stated method is replaced by the flexible AC
transmission system (FACT). FACTs controllers can not only increase the power
transmission capability of the TLs but also offer advantages like damping of low
frequency oscillations and mitigation of SSR damping’s etc.
IEEE defines FACTS technology as a power electronic system that is used in AC
transmission system to enhance controllability and thus increase the power
Design and implementation of TCSC for 500KV power transmission system
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transfer capability [2], [3]. The FACTs controllers are categorized according to
their generations. Static VAR compensators (SVC), thyristor controlled phase
shifters (TCPS) and thyristor controlled series capacitors (TCSC) are known as
first generation FACTS controllers. TCSC used to control the impedance of the
TL, where a silicon rectifier connected in series combination with a capacitor [4].
In this research, a dynamic TCSC model for 500kV transmission line is
implemented in Matlab/Simulink and tested with two checks: by changing
impedance of TL, and by varying the sending end voltage.
The manuscript is organized as follows; effect of capacitors on power transfer
when employed in transmission lines is explained in section II. In section III,
detailed working of TCSC and employed capacitive mode is described. Section IV
and V deal with simulations and results. Finally, conclusions are drawn in section
VI.
2. EFFECT OF CAPACITORS ON POWER TRANSFER IN TRANSMISSION
LINES
The steady state power transfer capability of transmission line is explained
through two machine power system model as shown in Figure 1 [5]. Power is
transmitted from one terminal (sending end) bus to second terminal (receiving
end). Mathematical formulation for power transfer capability is given by:
Sin
sr
c
L
VV
P
X
(1)
Where Vs and Vr are sending and receiving ends voltages, respectively, and XL
is the indicative impedance of transmission line. δ represents the phase angle
between sending and receiving end voltages. Thus, the power transfer capability
of TL largely
Sending end
Vs
Z = R + jX
Receiving end
Vr
Figure 1. Two machine transmission system.
Figure 2. Two machine transmission system with series capacitor.
depends upon the magnitude of the sending and receiving end voltages, phase
angle between them and inductance of line.
Conversely, a series capacitor is inserted in transmission system to study the power
transfer capability as shown in Figure 2 [5]. Ability of power transmission is
increased with such an insertion of capacitors as series capacitors cancel the
inductance of transmission line and boost the power flow as:
Sin
sr
c
LC
VV
P
XX
(2)
Sending end
Vs
Receiving end
Vr
C
Z = R + jX
Design and implementation of TCSC for 500KV power transmission system
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Where Xc is the capacitive impedance of series capacitors. Due to mechanical
switching of series capacitors, transients produced and only fixed value of series
compensation achieved [5].
3. THYRISTOR CONTROLLED SERIES CAPACITOR
Thyristor controlled series capacitor is a FACTs controller. Mainly, TCSC devices
are employed in the series of TLs to increase the power flow. Impedance of TL is
regulated by controlling the firing angle of the thyristors. Practically, more than
one TCSCs are installed in TL [6]. A TCSC consists of a fixed capacitor which is
shunted by a thyristor controlled reactor (TCR) as schematically drawn in Figure
3 [7].
A TCR consists of an anti-parallel thyristors and inductor in series. With TCSC,
flexible compensation is achieved because the gate terminal of thyristor is
triggered at various firing angles to insert different values of capacitor in series of
transmission line. 75% compensation is achieved, in this paper, by using TCSC.
Normally, a TCSC has three modes of operation:
1. Thyristor blocked
2. Thyristor bypassed
3. Vernier operation
In thyristor block mode, thyristors are not conducting and the value of α is 180o.
The effective impedance of TCSC is only because of capacitive reactance of
capacitor. In thyristor bypass mode, valves are gated for full conduction and
capacitor is bypassed. Practically, some current flows though capacitor but it is
negligible. Vernier operation is further categorized into capacitive and inductive
mode as shown in Figure 4 [8].
A TCSC operates in inductive region when there is no load condition in power
system [9]. Under this mode, TCSC behaves as a source of inductive reactance
which decreases the power transfer capability of transmission line. However
practically, very rare chances of no load condition in electric power system that is
why TCSC is not employed in this scenario. In capacitive mode, TCSC behaves as
a source of capacitive reactance to cancel the inductive reactance of TL and thus,
increases power transmission capacity of existing transmission line [10].
Practically, power systems are being operated at overload condition due to which,
generally TCSC is employed in capacitive mode. TCSC in not operated in
resonance region. TCSC can operate within range from 180o to alpha minimum
for capacitive region. If TCSC is allowed to operate at an angle of 180o, no current
flows through TCR and the effective impedance of the TCSC is due to the
capacitor [11]. If the value of alpha is somewhat between 180o to alpha minimum,
then TCSC reactance is greater than the Xc. The resonance point is reached when
Xc is equal to TCR XL. Required compensation achieved, in capacitive mode, with
firing angle range of 690-900 and 163Hz oscillatory frequency, which is 2.7 times
the 50Hz [12]. Impedance of TL is at the lowest level at 900, that is power transfer
capability reduces as the firing angle increases. Impedance is about 120-136Ohm
at time of capacitive mode of TCSC.
Design and implementation of TCSC for 500KV power transmission system
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Figure 3. Thyristor controlled series capacitor module.
Capacitive
Resonance Region
180
0
Inductive
0
0
90
0
1 2
0-1-2
X TCSC (pu)
α
Figure 4. TCSC characteristics.
4. SIMULINK MODEL OF TCSC FOR 500KV TRANSMISSION LINE
Mathematical model of TCSC is developed for 500kV three phase primary
transmission line. One TCSC compensates single phase of transmission line, thus
for three phases three block of TCSCs are developed. Net reactance of TCSCs is
regulated by changing the firing angle of the thyristor. Angle alpha is
synchronized with the line current by using the phase lock loop (PLL).
Proportional integral (PI) controller is used for the feedback purpose. The
developed model includes sending and receiving ends AC voltage sources, TCSC
block and the controller. The controller of TCSC includes control unit [12] and
firing unit [13] as shown in Figure 5. System under study consists of a
programmable voltage source at sending and receiving ends of TL. The purpose
of using the programmable voltage source is to vary the voltage at different time
instants.
Control System
Firing Unit
TCSC
Programmable
Voltage Source -1
Transmission Line
Programmable
Voltage Source -2
Figure 5. Simulink model of TCSC emplyed within transmission system for 75% compensation.
Design and implementation of TCSC for 500KV power transmission system
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Figure 6. Simulink model of firing unit for single phase.
The series line compensation block consists of TCSC, controller, three phase
voltage measurement and scopes for measurement. Triggering pulses for TCSCs
are controlled through controller block which consists of control and firing units.
Control unit calculate the firing angle based upon the impedance. Initially, the
impedance is calculated using RMS values of voltage and current by applying
ohm’s law, marked as measured impedance. To remove the second order
harmonics, it is passed to second order filter [14]. Error is calculated by taking
difference of the measured and reference impedances [15]. After that the values
are passed through the proportional integral controllers.
In firing unit, line current is synchronized using PLL and then compared and,
zero crossing is checked of the line current to generate square wave and to
synchronize the pulses as shown in Figure 6. Synchronization pulses are triggered
at the start of positive and negative cycles and square wave indicates the duration
of a cycle. And then by using discrete time integrator, square wave converted into
the saw tooth wave. Rounded method is used to convert alpha into a single value
for the comparison of alpha with saw tooth wave. Finally, compared saw tooth
wave with alpha value generate triggering pulse for thyristor gate.
1000
200
800
0
600
400
1.0
2.0
3.0
4.0 5.0
Time
a) Transmitted power via 500kV transmission line
100
20
80
0
60
40
1.0
2.0
3.0
4.0 5.0
Time
140
120
b) Impedance of 500kV transmission line
Delay
Generate Firing
Pulse for TCR +
Generate Firing
Pulse for TCR -
Compute No. of
Samples per Cycle
TCR +
TCR -
Iabc
Design and implementation of TCSC for 500KV power transmission system
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90
70
80
0
85
75
1.0
2.0
3.0
4.0 5.0
Time
c) Firing angle of TCSC in series with 500kV transmission line
Figure 7. Transmitted power, impedance of transmission line and firing angle of TCSC inserted
in a two machine power system.
5. RESULTS AND OBSERVATIONS
Transmission system is designed for 500kV but due to losses in the lines,
receiving end voltages reduced to 477kV. Power, impedance and firing angle
profiles of the proposed system are shown in Fig. 7. 115MW is the power
transfer before triggering of TCSC. TCSC operates in capacitive mode and
triggered at 0.8sec, power transfer increased to 650MW and measured
impedance follow the reference impedance as shown in Figure 7 (a) and (b),
respectively.
5% change in reference impedance is applied at 2.25sec and the response
shows that TCSC successfully trace the reference impedance within around
400ms as shown in Figures 7. At 3.0sec, 5% reduction in sending end voltage
is introduced and corresponding change in impedance is shown in Figure 7
(b). TCSC tries to match with the reference impedance by compensating the
disturbances and lowering the power transfer to 500MW.
Vs returned to 1p.u. at 3.5sec. Corresponding autonomous adjustments of
power, firing angle and impedance of system are shown in their respective
Figure 7 (a), (b) and (c), respectively.
It is observed that when the voltage drop occurs from programmable voltage
source, the TCSC immediately respond to the power oscillation and damp it
out but the power transfer does not remain constant before and after the
disturbance. Power change from 530MW to 460MW during oscillations due
to voltage reduction and returned to original value (530MW) whence the Vs
= 1p.u. as indicated in Figure 7 (b).
6. CONCLUSIONS
Interconnected power system is employed using 500kV transmission line.
TCSC is used to increase the power transfer capability of transmission system
than constructing new lines. A dynamic simulation developed in
Matlab/Simulink and tested for with and without TCSC, different sending end
voltages, and by changing the impedance of transmission line. Reactive power
demand of power system is compensated through TCSC. Results show that
voltage regulation is improved, SSR mitigated and more stabled electric
power system is achieved. Simulations reveal that installation of TCSC at high
tension lines shows better performance.
Design and implementation of TCSC for 500KV power transmission system
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7. ACKNOWLEDGMENT
The authors are grateful to Prof. Xu Dianguo, IEEE Fellow and Prof. W.
Barry Williams for thorough discussion during the research work and thanks
to Punjab Higher Education Commission (PHEC) Pakistan for providing
financial support to present this research work on international forum.
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