14-08-2012, 02:26 PM
Comparison of FACTS Devices for Power System Stability
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INTRODUCTION
Modern power system is a complex network comprising of
numerous generators, transmission lines, variety of loads and
transformers. As a consequence of increasing power demand,
some transmission lines are more loaded than was planned when
they were built. With the increased loading of long transmission
lines, the problem of transient stability after a major fault can
become a transmission limiting factor [1]. Now power engineers
are much more concerned about transient stability problem due
to blackout in northeast United States, Scandinavia, England and
Italy. Transient stability refers to the capability of a system to
maintain synchronous operation in the event of large
disturbances such as multi-phase short-circuit faults or switching
of lines [2]. The resulting system response involves large
excursions of generator rotor angles and is influenced by the
nonlinear power angle relationship.
FACTS CONTROLLERS
FACTS controllers may be based on thyristor devices with no
gate turn-off or power devices with gate turn-off capability.
FACTS controllers are used for the dynamic control of voltage,
impedance and phase angle of high voltage AC transmission
lines. The basic principles of the following FACTS controllers,
which are used in the two-area power system under study, are
discussed briefly.
Static Var Compensator (SVC)
Static var systems are applied by utilities in transmission
applications for several purposes. The primary purpose is usually
for rapid control of voltage at weak points in a network.
Installations may be at the midpoint of transmission
interconnections or at the line ends. Static Var Compensators are
shunt connected static generators / absorbers whose outputs are
varied so as to control voltage of the electric power systems. In
its simple form, SVC is connected as Fixed Capacitor-Thyristor
Controlled Reactor (FC-TCR) configuration as shown in Fig. 1.
The SVC is connected to a coupling transformer that is connected
directly to the ac bus whose voltage is to be regulated. The
effective reactance of the FC-TCR is varied by firing angle
control of the antiparallel thyristors. The firing angle can be
controlled through a PI (Proportional + Integral) controller in
such a way that the voltage of the bus, where the SVC is
connected, is maintained at the reference value.
Static Synchronous Series Compensator(SSSC)
The SSSC is one of the most recent FACTS devices for power
transmission series compensation. It can be considered as a
synchronous voltage source as it can inject an almost sinusoidal
voltage of variable and controllable amplitude and phase angle,
in series with a transmission line. The injected voltage is almost
in quadrature with the line current. A small part of the injected
voltage that is in phase with the line current provides the losses
in the inverter. Most of the injected voltage, which is in
quadrature with the line current, provides the effect of inserting
an inductive or capacitive reactance in series with the
transmission line. The variable reactance influences the electric
power flow in the transmission line.
TWO AREA POWER SYSTEM MODEL
Consider a two-area power system (Area-1 & Area-2) with series
and shunt FACTS devices, connected by a single circuit long
transmission line as shown in Fig. 5 and Fig. 6. Here, the series
FACTS devices such as UPFC (combination of STATCOM and
SSSC), SSSC, and TCSC are equipped between bus-2 and bus-3
and the shunt FACTS device such as SVC is equipped at bus-2.
The direction of real power flow is from Area-1 to Area-2. In the
two-area power system model, the Area-1 consists of Generator 1
(G1) and Generator 2 (G2) and the Area-2 consists of Generator
3 (G3) and Generator 4 (G4). The system data are given in [21].