28-06-2013, 01:02 PM
Probabilistic vs Deterministic Power System Stability and Reliability Assessment
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Introduction
The power industry has undergone the significant restructuring throughout
the world since the 1990s. In particular, its traditional, vertically monopolistic
structure has been reformed into competitive markets in pursuit of increased
efficiency in the electricity production and utilization. Along with the introduction
of competitive and deregulated electricity markets, some power
system problems have become difficult to analyse with traditional methods,
especially when power system stability, reliability, and planning problems are
involved. Traditionally, the power system analysis was based on deterministic
frameworks; but they only consider the specific configurations, which ignore
the stochastic or probabilistic nature of real power systems. Moreover, many
exterior constraints as well as growing system uncertainties now need to be
taken into consideration. All these have made existing challenges even more
complex. One consequence is that more effective and efficient power system
analysis methods are required in the deregulated, market-oriented environment.
The mature theory background has facilitated effective employment of
probabilistic based analysis methods. The study of probabilistic approaches
based power system analysis has become highly important.
Power System Stability Analysis
A power system is said to be stable if it has the capacity to retain a state of
equilibrium under normal operating conditions and to regain an acceptable
state of equilibrium after being subjected to a disturbance (Kundur et al.,
2004; Kundur, 1994). The classification proposes the categories of power system
stability, shown in Fig.5.1 (Kundur et al., 2004). The following sections
focus on the discussions of transient stability and small signal stability.
Transient Stability
Transient stability is the ability of power systems to maintain synchronism
in case of a severe transient disturbance, such as faults on transmission
lines, generating units, or load outages (Kundur, 1994). It has been widely
applied in power system dynamic security analysis for years. Traditionally,
the power systems transient stability was studied using deterministic stability
criteria. In such criteria, several extreme operation conditions and critical
contingencies are manually selected by expert experience, such as the load
levels, fault types, and fault locations. The designed system should withstand
all the extreme conditions after most severe disturbances. Although the deterministic
method has served the power industry well, acquiring satisfactory
performance, it ignores the stochastic or probabilistic nature of a real power
system, which is unrealistic in the complex system analysis.
Small Signal Stability
Small signal stability analysis explores the power system security conditions
in the space of power system parameters of interest, including load
flow feasibility, saddle node and Hopf bifurcations, maximum and minimum
damping conditions, in order to determine suitable control actions to enhance
power system stability (Dong et al., 1997; Makarov and Dong, 1998;
Makarov et al., 2000). Therefore, studying the small signal stability is of great
importance to ensure the secure and healthy operation of power systems with
growing uncertainties. In order to investigate the small signal stability of a
power system, the dynamic components (e.g., generators) and relevant control
systems (such as excitation control system, and speed governor systems)
should be modelled in detail (Dong et al., 2005). The accuracy of small signal
stability analysis depends on the accuracy of the models used, which means
more accurate models could result in increased overall power system transfer
capability and associated economic benefits. Traditionally, the system security
is evaluated under the deterministic framework, which was based on
given network configurations, system loading conditions, disturbances, etc.
Power System Reliability Analysis
The reliability of a bulk system is a measure of the ability to deliver electricity
to all points of utilization within accepted standards and in the amount
desired (Ringlee et al., 1994). The reliability is a key aspect of power system
design and planning, which can be assessed using deterministic methods. The
most common deterministic method for assessing power system reliability is
the N-1 criterion. It defines that the power system a considered reliable if it
is able to withstand any prescribed outage situations or contingencies within
acceptable constraints (Zhang et al., 2004).
Power System Planning
Power system planning is an important topic and a general problem in the
modern power system analysis, that of energy and economic development
planning. The general fundamental objective of system planning is to determine
a minimum cost strategy for expansion of generation, transmission,
and distribution systems adequate to supply the load forecast within a set
of technical, economic, and political constraints (Xu et al., 2006a; Xu et al.,
2006b; Zhao et al., 2009). The power system behavior is stochastic in nature,
and therefore the theoretically system planning should be carried on
with probabilistic techniques. However, most of the present planning, design,
and operating criteria are based on deterministic techniques which have been
widely used for decades. Along with market deregulation, the operation of
large-scale power systems needs more careful study, usually guided by safety
and environmental requirements, legal and social obligations, present and future
power demands, and maximizing the values of generating resources (Operation,
website).