# Fuzzy Approach To Critical Bus Ranking Under Normal And Line Outage Contingencies

Shobha Shankar*, Dr. T. Ananthapadmanabha**
*Research Scholar and Assistant Professor, Department of Electrical and Electronics Engineering, Vidyavardhaka College of Engineering, Mysore, INDIA 
Email:shobha.prathi@gmail.com 
** Professor, Department of Electrical and Electronics Engineering, The National Institute of Engineering, Mysore, INDIA 
Email:drapn2008@yahoo.co.in Abstract. Identification of critical or weak buses for a given operating condition is an important task in the load dispatch centre. It has become more vital in view of the threat of voltage instability leading to voltage collapse. This paper presents a fuzzy approach for ranking critical buses in a power system under normal and network contingencies based on Line Flow index and voltage profiles at load buses. The Line Flow index determines the maximum load that is possible to be connected to a bus in order to maintain stability before the system reaches its bifurcation point. Line Flow index (LF index) along with voltage profiles at the load buses are represented in Fuzzy Set notation. Further they are evaluated using fuzzy rules to compute Criticality Index. 

Based on this index, critical buses are ranked. The bus with highest rank is the weakest bus as it can withstand a small amount of load before causing voltage collapse. The proposed method is tested on Five Bus Test System. 

Key words: Criticality Index, Critical Bus Ranking, Fuzzy Set Notation, Line Flow Index 

## 1 Introduction

Voltage stability and system security are emerging as major problems in the operation of stressed power system. Line outage contingencies are the most common problem in power system and have a considerable effect on altering the base case (pre-contingency case) voltage stability margin of a load bus. Generally, the system continues to operate in the contingency condition for a considerable duration of time, on occurrence of a line outage. The altered voltage stability margins of all the load buses for the various contingency conditions are to be known prior to monitor and initiate emergency control action to avoid voltage collapse. The main cause for voltage collapse is the inability of the system to supply reactive power to cope up with the increasing load growth. The occurrence of voltage collapse is very much dependent upon the maximum load that can be supported at a particular load bus. Any attempt to increase the load beyond this point could force the entire system into instability, leading to voltage collapse. This would indicate that the power system physically could not support the amount of the connected load. Line Flow index (LF index) is used to estimate maximum loadability of a particular load bus in the system. The load buses are ranked according to their maximum loadability, where the load bus having the smallest maximum loadability is ranked highest. Hence this bus is identified as the weakest bus 