Power system stability was
first recognized as an important problem in the 1920s (Steinmetz, 1920; Evans
and Bergvall, 1924; Wilkins, 1926). The early stability problems were
associated with remote power plants feeding load centers over long transmission
lines.
As electric power systems
have evolved over the last century, different forms of instability have emerged
as being important during different periods. The methods of analysis and
resolution of stability problems were influenced by the prevailing developments
in computational tools, stability theory, and power system control technology.
A review of the history of the subject is useful for a better understanding of
the electric power industry’s practices with regard to system stability.
With slow exciters and non continuously
acting voltage regulators, power transfer capability was often limited by
steady-state as well as transient rotor angle instability due to insufficient
synchronizing torque.
To analyze system
stability, graphical techniques such as the equal area criterion and power
circle diagrams were developed. These methods were successfully applied to
early systems which could be effectively represented as two machine systems.
As the complexity of power
systems increased, and interconnections were found to be economically attractive,
the complexity of the stability problems also increased and systems could no
longer be treated as two machine systems. This led to the development in the
1930s of the network analyzer, which was capable of power flow analysis of
multimachine systems. System dynamics, however, still had to be analyzed by
solving the swing equations by hand using step-by-step numerical integration.
Generators were represented by the classical ‘‘fixed voltage behind transient
reactance’’ model. Loads were represented as constant impedances.
Improvements in system
stability came about by way of faster fault clearing and fast acting excitation
systems. Steady-state aperiodic instability was virtually eliminated by the
implementation of continuously acting voltage regulators. With increased
dependence on controls, the emphasis of stability studies moved from
transmission network problems to generator problems, and simulations with more
detailed representations of synchronous machines and excitation systems were
required.
The 1950s saw the
development of the analog computer, with which simulations could be carried out
to study in detail the dynamic characteristics of a generator and its controls
rather than the overall behavior of multimachine systems.
Later in the 1950s, the
digital computer emerged as the ideal means to study the stability problems
associated with large interconnected systems. In the 1960s, most of the power
systems in the U.S. and Canada were part of one of two large interconnected systems,
one in the east and the other in the west. In 1967, low capacity HVDC ties were
also established between the east and west systems. At present, the power
systems in North America form virtually one large system. There were similar
trends in growth of interconnections in other countries.
While interconnections
result in operating economy and increased reliability through mutual
assistance, they contribute to increased complexity of stability problems and
increased consequences of instability. The Northeast Blackout of November 9,
1965, made this abundantly clear; it focused the attention of the public and of
regulatory agencies, as well as of engineers, on the problem of stability and
importance of power system reliability.
Until recently, most industry
effort and interest has been concentrated on transient (rotor angle) stability.
Powerful transient stability simulation programs have been developed that are
capable of modeling large complex systems using detailed device models.
Significant improvements in transient stability performance of power systems
have been achieved through use of high-speed fault clearing, high-response
exciters, series capacitors, and special stability controls and protection
schemes.
The increased use of high
response exciters, coupled with decreasing strengths of transmission systems,
has led to an increased focus on small signal (rotor angle) stability.
This type of angle
instability is often seen as local plant modes of oscillation, or in the case
of groups of machines interconnected by weak links, as interarea modes of
oscillation. Small signal stability problems have led to the development of
special study techniques, such as modal analysis using eigenvalue techniques
(Martins, 1986; Kundur et al., 1990). In addition, supplementary control of
generator excitation systems, static Var compensators, and HVDC converters is
increasingly being used to solve system oscillation problems.
There has also been a
general interest in the application of power electronic based controllers
referred to as FACTS (Flexible AC Transmission Systems) controllers for damping
of power system oscillations (IEEE, 1996).
In the 1970s and 1980s,
frequency stability problems experienced following major system upsets led to
an investigation of the underlying causes of such problems and to the
development of long term dynamic simulation programs to assist in their
analysis (Davidson et al., 1975; Converti et al., 1976; Stubbe et al., 1989;
Inoue et al., 1995; Ontario Hydro, 1989). The focus of many of these
investigations was on the performance of thermal power plants during system
upsets (Kundur et al., 1985; Chow et al., 1989; Kundur, 1981; Younkins and
Johnson, 1981). Guidelines were developed by an IEEE Working Group for
enhancing power plant response during major frequency disturbances (1983).
Analysis and modeling needs
of power systems during major frequency disturbances was also addressed in a
recent CIGRE Task Force report (1999). Since the late 1970s, voltage
instability has been the cause of several power system collapses worldwide
(Kundur, 1994; Taylor, 1994; IEEE, 1990). Once associated primarily with weak
radial distribution systems, voltage stability problems are now a source of
concern in highly developed and mature networks as a result of heavier loadings
and power transfers over long distances. Consequently, voltage stability is
increasingly being addressed in system planning and operating studies.
Powerful analytical tools
are available for its analysis (Van Cutsem et al., 1995; Gao et al., 1992;
Morison et al., 1993), and well-established criteria and study procedures are
evolving (Abed, 1999; Gao et al., 1996).
Present-day power systems
are being operated under increasingly stressed conditions due to the prevailing
trend to make the most of existing facilities. Increased competition, open
transmission access, and construction and environmental constraints are shaping
the operation of electric power systems in new ways that present greater
challenges for secure system operation.
This is abundantly clear from the
increasing number of major power-grid blackouts that have been experienced in
recent years; for example, Brazil blackout of March 11, 1999; Northeast
USA-Canada blackout of August 14, 2003; Southern Sweden and Eastern Denmark blackout
of September 23, 2003; and Italian blackout of September 28, 2003. Planning and
operation of today’s power systems require a careful consideration of all forms
of system instability.
Significant advances have
been made in recent years in providing the study engineers with a number of
powerful tools and techniques.
A coordinated set of
complementary programs, such as the one described by Kundur et al. (1994) makes
it convenient to carry out a comprehensive analysis of power system stability.
SOURCE: Electric Power
Generation, Transmission, and Distribution by Leonard L.
