03-07-2013, 02:44 PM
Improving Voltage Stability by Reactive Power Reserve Management
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Abstract
The amount of reactive reserves at generating stations
is a measure of the degree of voltage stability. With this
perspective, an optimized reactive reserve management scheme
based on the optimal power flow is proposed. Detailed models of
generator limiters, such as those for armature and field current
limiting must be considered in order to utilize the maximum
reactive power capability of generators, so as to meet reactive
power demands during voltage emergencies. Participation factors
for each generator in the management scheme are predetermined
based on the voltage-var (V-Q) curve methodology. The Bender’s
decomposition methodology is applied to the reactive reserve management
problem. The resulting effective reserves and the impact
on voltage stability are studied on a reduced Western Electric
Coordinating Council system. Results prove that the proposed
method can improve both static and dynamic voltage stability.
INTRODUCTION
VOLTAGE collapse typically occurs on power systems
which are heavily stressed. It may or may not be initiated
by a disruption, but is usually characterized by shortage of
fast-acting reactive reserves. Voltage collapse often involves
specific areas of the power system, although the entire system
may also be involved. Many system variables may participate
in this phenomenon. However, some physical insight into the
nature of voltage collapse may be gained by examining the
production, transmission and consumption of reactive power.
Voltage collapse is associated with reactive power demands
not being met because of limitations on the production and
transmission of reactive power [1]. Reactive power demand
generally increases with a load increase, motors stalling, or a
change in load composition such as an increased proportion
of compressor load. The fast reactive sources are generators,
synchronous condensers and power electronics-based flexible
ac transmission systems (FACTS).
OPTIMIZATION FRAMEWORK
Reactive Reserves
The reactive power sources consist of synchronous generators
and shunt capacitors and reactors on the transmission
network. During a disturbance, the real power component
of line loadings does not change significantly, whereas the
reactive power flow can change dramatically. The reason is
that the voltage drops resulting from the contingency decreases
the reactive power generation from line charging and shunt
capacitors, thereby increasing reactive power losses. Sufficient
reactive reserves should be available to meet the var changes
following a disturbance. Simply speaking, the reactive power
reserve is the ability of the generators to support bus voltages
under increased load condition or system disturbances. How
much more reactive power the system can deliver depends on
the operating condition and the location of the reserves, as well
as the nature of the impending change.
STRESSED CASE SUBPROBLEM
By considering the reactive margin constraints of (11), the
constrained optimization problem (6)–(11) becomes one of reactive
reserve management with dynamic security constraints.
At the optimal point, the system has the maximum reactive
reserves under normal operation condition, and each case in
set C is feasible. It can be hierarchically decomposed into two
parts, a base case subproblem and a stressed case subproblem
that can be solved by the Bender’s cuts decomposition method
[11]–[13]. For some stressed cases, the power flow may become
infeasible. However, in order to apply a decomposition
methodology, there is a need to have some measure of this infeasibility.
Since this kind of infeasibility is caused by reactive
power load increase, fictitious reactive injections are used as
slack variables and added to the constraints corresponding to
the reactive power balance equation at a bus. Therefore, the
objective function for the stressed cases is to minimize the sum
of the fictitious injections of reactive power.
CONCLUSION
This paper discusses the management of dynamic reactive
power reserves in order to improve voltage stability. The method
is based on optimal power flow and the Bender’s decomposition
technique. The proposed RRMP is decomposed into two parts
giving it a hierarchical structure that gives the optimal problem
added flexibility. Both armature and field current limiting are introduced
into the optimization problem so as to make the model
more accurate for mid-term voltage stability analysis. The robust
interior point method is utilized to solve the problem. Detailed
generator models were considered in the optimization.