A Simple Method to Find the Most Vulnerable Generator and a Safe Value of Critical Clearing Time for a Fault in a Power System

Determination of critical clearing time (CCT) for a power system is an important component of transient stability analysis. The methods proposed so far suffer from the drawback that either they do not determine the CCTs reliably or are too complex to implement or both. Also, none of the methods easily determine the generator most vulnerable to de-synchronization for a particular fault. The present paper proposes a new method to determine CCTs and the most vulnerable generator. It first reduces the multi-machine power system to a two-machine system with one of the machines being one of the generators of the power system. It then determines the CCT for this generator from this system using known formulae. This procedure is repeated for each generator of the power system and CCTs determined for each. The least value of the CCTs obtained is declared the CCT of the power system and the corresponding generator is declared the most vulnerable one. The method uses minimum computational effort and is easy to implement. Also, it is shown that the values of CCTs obtained are always less than actual values, making the method extremely reliable. These facts are confirmed by testing the method on various test systems.

I. Introduction

For maintaining security of power system operation, transient stability analysis plays a major role and determination of critical clearing time (CCT) for a particular fault is an important component of the analysis. The CCTs are mainly determined through numerical simulation methods, where numerical integration is carried out step by step from an initial value to obtain dynamic response to disturbances. Such methods suffer from the drawback that they are time consuming and only provide a rough idea of what value the CCTs may have. Also, determining the machine which is most vulnerable to de-synchronization for a given fault location requires further effort using this method.