Study on Impact of Load Changes on Voltage Stability Assessment

Author Name(s): *K .Santhi Sri, Ch.V.V.S Bhaskara Reddy
Author Email: santhisri.kh@gmail.com

Abstract

Electric power system is one of the most complex man made systems. Voltage stability is the one of the instabilities among the various types of instabilities experienced by electri power systems. With the development of the technology, different types of loads with different load densities are increasing in the power system. As a result, it is necessary to assess the effectiveness of the various procedures used to monitor the power systems. In this paper a study is proposed to analyze the effect of different types of load variations on voltage stability monitoring indices. It is considered to carry out the studies on IEEE 14 and IEEE 30 bus test systems.

Introduction

Power system is a huge network consists of hundreds of nodes and thousands of lines connected to them. Operation of power system is becoming difficult day by day due to increased complexity, competition, social and environmental factors, and Imbalance in load-generation growth. All these factors are causing the power system stability problems. Voltage stability problem is one of the major concerns in power system operation. Voltage stability is concerned with the ability of a power system to maintain acceptable voltages at all nodes in the system under normal condition and after being subject to a disturbance [1]. The study of voltage stability has been analyzed under different approaches that can be basically classified into dynamic and static analysis [2]. Static analysis is carried on the assumption that at each instant the system is in steady state. Dynamic analysis involves differential algebraic models which observe the interaction of generators, tap changers, etc.

Static approach involves computation of only algebraic equations and it is faster than dynamic approach. Static analysis is preferred over dynamic since it takes less computational time. The results from the conventional power flow are used in the static analysis. There are several methods used for the static analysis they are, The P-V curves and Q-V curves are the most used method of predicting voltage security. The loading margin of system can be known from this method [3]. But this method is not efficient since lot of computation involved for complex networks. Modal analysis proposed by Gao et al. [4] that computes the eigenvalue and eigenvectors of the reduced Jacobian matrix. If one of the eigen values is negative, the system is considered to be voltage unstable. Minimum singular value method proposed by Thomas and Lof [5] is used to calculate the voltage stability margin by observing how close is the Jacobian matrix to become singular. In this paper effect of load types and its variations on voltage stability assessment are analyzed using different indices. From this analysis, one can choose the effective indices to predict the proximity of voltage instability.

These indices provide information about proximity of voltage instability in a power system. Usually, their values changes between 0 and 1. In this paper studies have been carried out using IEEE 14 an IEEE 30 bus test systems.

Conclusion

This paper presents a study on impact of load changes on voltage stability assessment. Different node based and line based indices are simulated with different types of load patterns on IEEE 14 and IEEE 30 bus test systems. From the simulations, it was observed that node based indices are varying slowly and the change is abrupt near the collapse. However, the line based indices are varying almost the constant rate and giving better indication. But, in reality, although the line based indices crosses 1or 0, it may not always result in system instability. Hence, from the studies it is observed that, instead of using a single indices, it is better to used more than one that to one line based and one node based indices will give better information when compared to single indices based approach.

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