06-05-2014, 02:30 PM
A Load Shedding Algorithm for Improvement of Load Margin to Voltage Collapse
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Abstract
Voltage stability is concerned with the ability of a
power system to maintain acceptable voltages at all buses. A
measure of the power system voltage stability is the distance to
the saddle node bifurcation of the power flow equations, which is
called the load margin. When the power system load is very high,
and/or there exists a large generation-demand imbalance in the
power system areas the load margin to the saddle node
bifurcation may be too low, and the power system may become
close to voltage collapse. In case that active and reactive power
generation resources in the importing areas are exhausted,
corrective load shedding may become the last option. This paper
presents an LP-based optimization load shedding algorithm to
improve the load margin. The objective function consists of
minimizing the total system demand decrease. First order
sensitivities of the load margin with respect to the load to be shed
are considered. The performance of the method is illustrated with
highly loaded scenario of the Spanish power system.
INTRODUCTION
In recent years, voltage stability has become one of the
most important problems in the power system operation.
Voltage stability is concerned with the ability of a power
system to maintain acceptable voltages at all buses in the
system under normal conditions and after being subjected to a
disturbance [1].
Voltage stability is a dynamic phenomena. However, the
voltage instability of a power system can be measured
obtaining the distance of the static power flow equations from
the initial point of operation to its saddle node bifurcation
point, known as the voltage collapse point. To calculate this
distance, the bus injections are increased in each bus by a
constant rate of initial injections and a scale factor called load
margin [2].
OVERVIEW OF THE ALGORITHM
Fig. 1 depicts a flowchart of the proposed algorithm. The
algorithm starts from the initial saddle node bifurcation
solution of the base case. The algorithm has been designed as
an iterative algorithm, where limits on the demand reduction
have been imposed in each iteration to avoid the loss of
accuracy caused by the linear approximation of the load
margin. While the desired security load margin is not reached,
a new iteration is performed updating the sensitivities and
solving a new LP optimization problem, obtaining the optimal
location of both generation and demand buses (and their
corresponding power reduction) and calculating the new load
margin. Section III details the mathematical expressions of the
sensitivities, and Section IV contains the LP-optimization
problem of each iteration to obtain the minimum load
shedding.
CONCLUSIONS
This paper has proposed an optimization algorithm to
determine the amount and location of the minimum load
shedding to improve the load margin to voltage collapse.
The algorithm is based on LP optimization. First order
sensitivities of the load margin with respect to the load to be
shed have been considered. The objective function consists of
minimizing the total system demand decrease. The problem
constraints consist of a target improvement of the load margin,
the power balance equation and the generation and demand
limits
To overcome the loss of accuracy of the linear
approximation of the load margin, the algorithm has been
designed as an iterative process that imposes an additional
constraint of the total demand reduction in each iteration
The performance of the algorithm is illustrated considering
an hourly scenario of the operation of the Spanish power
system