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POWER SYSTEM STABILITY ENHANCEMENT USING FACTS CONTROLLERS
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INTRODUCTION
Since the development of interconnection of large electric power systems, there have been spontaneous system
oscillations at very low frequencies in order of 0.2–3.0 Hz. Once started, they would continue for a long period of
time. In some cases, they continue to grow causing system separation due to the lack of damping of the mechanical
modes [1; 2].
In the past three decades, power system stabilizers (PSSs) have been extensively used to increase the system
damping for low frequency oscillations. The power utilities worldwide are currently implementing PSSs as effective
excitation controllers to enhance the system stability [1–12]. However, there have been problems experienced with
PSSs over the years of operation. Some of these were due to the limited capability of PSS, in damping only local and
not interarea modes of oscillations. In addition, PSSs can cause great variations in the voltage profile under severe
disturbances and they may even result in leading power factor operation and losing system stability [13]. This
situation has necessitated a review of the traditional power system concepts and practices to achieve a larger stability
margin, greater operating flexibility, and better utilization of existing power systems.
Flexible AC transmission systems (FACTS) have gained a great interest during the last few years, due to recent
advances in power electronics. FACTS devices have been mainly used for solving various power system steady state
control problems such as voltage regulation, power flow control, and transfer capability enhancement. As
supplementary functions, damping the interarea modes and enhancing power system stability using FACTS
controllers have been extensively studied and investigated. Generally, it is not cost-effective to install FACTS
devices for the sole purpose of power system stability enhancement.
In this work, the current status of power system stability enhancement using FACTS controllers was discussed
and reviewed. This paper is organized as follows. The development and research interest of FACTS is presented in
Section 2. Section 3 discusses the potential of the first generation of FACTS devices to enhance the low frequency
stability while the potential of the second generation is discussed in Section 4. Section 5 highlights some important
issues in FACTS installations such as location, feedback signals, coordination among different control schemes, and
performance comparison. Major real-world installations and recent developments in power electronic devices used in
FACTS controllers have been summarized in Section 6. Applications of FACTS to optimal power flow and
deregulated electricity market as steady state problems have been discussed in Section 7. Some concluding remarks
are highlighted in Section 8. About two hundred research publications are reviewed, discussed, classified, and
appended for a quick reference.
FACTS DEVICES
Overview
In the late 1980s, the Electric Power Research Institute (EPRI) formulated the vision of the Flexible AC
Transmission Systems (FACTS) in which various power-electronics based controllers regulate power flow and
transmission voltage and mitigate dynamic disturbances. Generally, the main objectives of FACTS are to increase the
useable transmission capacity of lines and control power flow over designated transmission routes. Hingorani and
Gyugyi [14] and Hingorani [15; 17] proposed the concept of FACTS. Edris et al. [18] proposed terms and definitions
for different FACTS controllers.
There are two generations for realization of power electronics-based FACTS controllers: the first generation
employs conventional thyristor-switched capacitors and reactors, and quadrature tap-changing transformers, the
second generation employs gate turn-off (GTO) thyristor-switched converters as voltage source converters (VSCs).
The first generation has resulted in the Static Var Compensator (SVC), the Thyristor- Controlled Series Capacitor
(TCSC), and the Thyristor-Controlled Phase Shifter (TCPS) [19; 20]. The second generation has produced the Static
Synchronous Compensator (STATCOM), the Static Synchronous Series Compensator (SSSC), the Unified Power
Flow Controller (UPFC), and the Interline Power Flow Controller (IPFC) [21–24]. The two groups of FACTS
controllers have distinctly different operating and performance characteristics.
The thyristor-controlled group employs capacitor and reactor banks with fast solid-state switches in traditional
shunt or series circuit arrangements. The thyristor switches control the on and off periods of the fixed capacitor and
reactor banks and thereby realize a variable reactive impedance. Except for losses, they cannot exchange real power
with the system.
M. A. Abido
April 2009 The Arabian Journal for Science and Engineering, Volume 34, Number 1B 155
The voltage source converter (VSC) type FACTS controller group employs self-commutated DC to AC
converters, using GTO thyristors, which can internally generate capacitive and inductive reactive power for
transmission line compensation, without the use of capacitor or reactor banks. The converter with energy storage
device can also exchange real power with the system, in addition to the independently controllable reactive power.
The VSC can be used uniformly to control transmission line voltage, impedance, and angle by providing reactive
shunt compensation, series compensation, and phase shifting, or to control directly the real and reactive power flow
in the line [24].
Interest Measure for FACTS
For the purpose of this review, a literature survey has been carried out including two of the most important and
common databases, namely, the IEEE/IEE electronic library and ScienceDirect electronic databases. The survey
spans over the last 15 years from 1990 to 2004. For convenience, this period has been divided to three sub-periods;
1990–1994, 1995–1999, and 2000–2004. The number of publications discussing FACTS applications to different
power system studies has been recorded. The results of the survey are shown in Figure 1. It is clear that the
applications of FACTS to different power system studies have been drastically increased in last five years. This
observation is more pronounced with the second generation devices as the interest is almost tripled. This shows more
interest for the VSC-based FACTS applications. The results also show a decreasing interest in TCPS while the
interest in SVC and TCSC slightly increase.
Generally, both generations of FACTS have been applied to different areas in power system studies including
optimal power flow [25–29], economic power dispatch [30], voltage stability [31; 32], power system security [33],
and power quality [34–35].
Applications of FACTS to power system stability in particular have been carried out using same databases. The
results of this survey are shown in Figure 2. It was found that the ratio of FACTS applications to the stability study
with respect to other power system studies is more than 60% in general. This reflects clearly the increasing interest to
the different FACTS controllers as potential solutions for power system stability enhancement problem. It is also
clear that the interest in the 2nd generation of FACTS has been drastically increased while the interest in the 1st
generation was decreased.
The potential of FACTS controllers to enhance power system stability has been discussed by Noorozian and
Anderson [36], where a comprehensive analysis of damping of power system electromechanical oscillations using
FACTS was presented. Wang and Swift [37] have discussed the damping torque contributed by FACTS devices,
where several important points have been analyzed and confirmed through simulations.
Number of Publications (General)
SVC TCSC TCPS STATCOM SSSC UPFC
1990-94 1995-99 2000-04
Figure 1. Statistics for FACTS applications to different power system studies
M. A. Abido
The Arabian Journal for Science and E 156 ngineering, Volume 34, Number 1B April 2009
Number of Publications (Stability)
SVC TCSC TCPS STATCOM SSSC UPFC
1990-94 1995-99 2000-04
Figure 2. Statistics for FACTS applications to power system stability
. FIRST GENERATION OF FACTS
. Static VAR Compensator (SVC)
It is known that the SVCs with an auxiliary injection of a suitable signal can considerably improve the dynamic
stability performance of a power system [38–61]. In the literature, SVCs have been applied successfully to improve
the transient stability of a synchronous machine [38]. Hammad [39] presented a fundamental analysis of the
application of SVC for enhancing the power systems stability. Then, the low frequency oscillation damping
enhancement via SVC has been analyzed [40–46]. It is shown that the SVC enhances the system damping of local as
well as interarea oscillation modes. Self-tuning and model reference adaptive stabilizers for SVC control have been
also proposed and designed [47–49]. Robust SVC controllers based on H∞ , structured singular value μ, and
quantitative feedback theory QFT have been presented to enhance system damping [50; 51]. However, the
importance and difficulties in the selection of weighting functions of H∞ optimization problem have been reported. In
addition, the additive and/or multiplicative uncertainty representation can not treat situations where a nominal stable
system becomes unstable after being perturbed [52]. Moreover, the pole-zero cancellation phenomenon associated
with this approach produces closed loop poles whose damping is directly dependent on the open loop system
(nominal system) [53]. Genetic algorithms and fuzzy logic based approaches have been proposed for SVC control
[54–60]. The superiority of these approaches over the conventional methods is confirmed through time domain
simulations. Messina and Barocio [61] studied the nonlinear modal interaction in stressed power systems with
multiple SVC voltage support. It was observed that SVC controls can significantly influence nonlinear system
behavior especially under high-stress operating conditions and increased SVC gains.