DESIGN AND DEVELOPMENT OF .A COMPOSITE PROTECTION SCHEME FOR POWER TRANSFORMER

Abstract

The present work aims at development of a combined percentage biased harmonic restrained differential protection together with•restricted earth fault protection for a three phase power transformer. The protective scheme is based on the fact that transformer currents contain large amount of harmonics during heavy external fault, during magnetizing inrush and during the condition of over excitation due to dynamic overvoltages. On the other hand in currents during internal fault harmonic content is very low. 'In the protective scheme followed here the presence of large second harmonic in the operating current, which is the difference between primary and secondary currents, has been chosen to indicate the presence of magnetizing inrush condition. During over excitation condition it has been observed that third and fifth harmonics predominate. Here fifth harmonic has been chosen to Indicate the condition of over excitation. Whenever second harmonic or fifth harmonic content of the operating current is greater than a fixed percentage of fundamental of the operating current the relay is restrained from issuing trip signal to the circuit breaker. The fundamental of zero sequence current has been chosen to test the condition of earth fault. To filter out different harmonics a digital filtering technique based on discrete fourier transform has been used. This has some obvious advantages over analog filters.