OVERVIEW OF THYRISTOR CONTROLLED SERIES CAPACITOR (TCSC)
IN POWER TRANSMISSION SYSTEM
E03421 MAZILAH BINTI A RAHMAN
1. Introduction
In early 90s, a general concept in term of operation and
planning of power systems was proposed and widely discussed
since then. The Flexible Alternating-Current Transmission
Systems (FACTS), incorporating a wide range of possibilities for
better utilization. Improvement of voltage and current limits on
the power electronics devices leads to a fast development of
FACTS in the last decade. FACTS are defined by the IEEE as
“AC transmission systems incorporating power electronics-based
and other static controllers to enhance controllability and
increase power transfer capability. And among the proposed
FACTS devices, possibly the Thyristor Controlled Series
Capacitor (TCSC) has given the best results in terms of
performance and flexibility. It can have various roles in the
operation and control of power systems, such as scheduling
power flow; decreasing unsymmetrical components; reducing net
loss; providing voltage support; limiting short-circuit currents;
mitigating sub-synchronous resonance (SSR); damping the
power oscillation; and enhancing transient stability.
Research on control strategies for TCSC can be traced back
to 1966 when Kimbark [3] analysed the improvement in transient
stability of power systems by using switched series capacitors.
This was a very simple control; the maximum amount of
compensation was inserted at the same time that the faulted line
was switched out. Ramarao et al.[4] proposed an optimal control
of the capacitance. Pontryagin’s maximum principle was used to
obtain bang-bang control. It was one of the earliest closed-loop
control systems for series compensation.
2. TCSC in Power Transmission System
reactor
thyristor
Fig. 1 Simple Diagram of TCSC
I. Circuit and Function
Figure 1 shows the simple diagram of TCSC. This structure
is modeled as
where
X = X 0 + ∆X
∆X = f ( ∆ω g ) . ∆ X is the capacitive reactance.
TCSC comprised of a series capacitor bank, shunted by a
Thyristor Controlled Reactor (TCR), to provide a smoothly
ADVISOR
GORO FUJITA
variable series capacitive reactance. It is a one-port circuit in
series with transmission line; it uses natural commutation; its
switching frequency is low; it contains insignificant energy
storage and has no DC-port. Insertion of a capacitive reactance in
series with the line’s inherent inductive reactance lowers the total,
effective impedance of the line and thus virtually reduces its
length. As a result, both angular and voltage stability gets
improved. Furthermore, in contrast to capacitors switched by
circuit breakers, TCSC will be more effective because thyristors
can offer flexible adjustment, and more advanced control
theories can be easily applied.
II. Characteristics and Problems
In this chapter, we will be discussing about some
characteristics of TCSC that have been discussed all over the
region for about 20 years ago. These chapter have been
summarized and we could see clearly some part of the character
and the problem occurs within it. Two major characteristic of
TCSC that will be discussed below is sub-synchronous resonance
that would appear in the generator oscillation and transient
voltage stability controlling.
(i) Sub-synchronous Resonance (SSR)
SSR has gained its name from the fact that the frequency of
interest happen to lie in a region below the synchronous
frequency of the network. The main concern in SSR studies is a
significant oscillatory electromagnetic torque developed on the
generator rotor and the possibility of shaft damage from torsional
stresses [5]. As in general, any device that controls or responds
rapidly to power or speed variations in the sub-synchronous
frequency range is a potential source for excitation of oscillations
[5].
Simulations of the IEEE First Benchmark Model [6] show
that even without a fault, sub-synchronous oscillations appear in
that system. They just take more time to develop. In other words,
the occurrence of a fault may speed up the process (Torque
Amplification), while the possibility of SSR is mainly due to the
nature of the combined mechanical-electrical system. The
eigenvalue results for the first benchmark model show that the
thyristor switching can have a significant effect on system
stability and that the effect varies with the firing angle [7].
The papers on the inclusion of the TCSC in SSR studies are
centered on two field installations, Western Area Power
Administration’s Kayenta site [8,9], and Bonneville Power
Administration’s Slatt substation [10,11]. The Kayenta system
was analyzed with time-domain simulation of a detailed model
of the AC system, a machine and TCSC with controls [8]. While
for the Slatt system, the effect of the TCSC was evaluate by
measuring the electrical damping torque as a function of machine
rotor speed. Analog and digital simulators modeled the AC
system, machine and TCSC controls.
(ii) Transient voltage stabilizing control
The inserted series capacitor will also affects the reactive
power distribution in the interconnected power system. Thus,
paper [12] suggests that TCSC be used to enhance voltage
stability. In a transient voltage stability analysis, TCSC is
operated in a big range for a long period, which is very different
from small signal voltage stability. And this means that the
operation limits of a TCSC must be well considered. For other
practical reasons, such as availability of the measurement and the
burden of calculation, relatively simple controllers are used in
the installed TCSC [13,14].
There are also other characteristics regarding TCSC and it’s
such as power damping oscillation, fault current, non-linear
control model, dynamic characteristic etc.
3. Discussions
For two major characteristic that have been discussed above,
MATLAB, the friendly user of technical computing have been
used as study-system case. In this case, we used 3-Machine and
9-Bus System as transient stability study model. The analysis is
done first with the model and the result are simulated by
MATLAB.
Fig. 2
3-Machine and 9-Bus System Transient Stability
Study Model (MATLAB).
Fig. 2 shows a MATLAB diagram of the study- system case.
The research procedures are divided into two cases; that are
study system with non-TCSC control and with TCSC control.
The result has been simulated by using MATLAB programming.
Then, we will see frequency deviation comparison for both
studies. The result of the system model for non-TCSC control is
that we can see after the fault, amplitude of frequency deviation
is becoming large. Then, after the model were inserted with
TCSC, the system becoming stepped-out. Further studies shows
that TCSC will prevent the system becoming stepped out only if
relatively simple weighting matrix been operated. The
comparison of eigenvalue in the system also been studied. For a
result, the stability level of the system is improved by TCSC. The
difference of the improvement mode is according to TCSC
installation location in the study-system. The result is the best
installation for TCSC is near generator which is colce to the fault
generator.
The results of the small-signal investigations in this section
have thus shown that in both modes of operation, the closed-loop
power flow controller has an increasing influence on the
damping of the system’s electromechanical oscillations as its
time is made shorter.
4. Conclusion
This paper has addresses a quick overview of thyristor
controlled series capacitor as one the best proposed devices in
FACTS family and its applications in power transmission system.
There are also two major characteristic of TCSC and that are the
existing of sub-synchronous resonance in generator oscillation
controller and TCSC as transient voltage stabilizing controller.
And for both characteristic, we proposed a 3-machine and 9-Bus
System Transient Stability Study Model simulated by MATLAB
friendly technical programming as study-case system.
It has considered the application of TCSC for closed-loop
control of power flow in both constant power and constant angle
modes of operation. The results indicate that the power flow
controller (TCSC) operation has an important influence on both
the small signal and transient stability characteristic of the
system．
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