Design of a harmonic restrained static relay for a single phase transformer

Abstract

Large Transformers are provided with differential relays to isolate them during severe internal faults. In the case of transformers, the differential relays need to prevent tripping for inrush currents. As a result all differential relays for transformers are equipped with mechanism to discriminate fault current from magnetizing inrush current to prevent false tripping of breakers during transformer switch on. In this project a static circuit is designed to discriminate transformer fault current and magnetizing inrush current based on identifying the second harmonic content of the transformer line current. A fault current of a transformer is high in magnitude but almost free from second harmonic and DC offset. Whereas, the magnetizing inrush current has substantial second harmonic and also it has DC offset. The circuit in this study consists of a current sensing unit (a CT and voltage divider), two notch filters to detect 50Hz and 100Hz components of the line current and the calculation circuit based on IC circuits (OPAMPS, multipliers and logic gates) to find the ratio of second harmonic to fundamental value of the line current continuously. The circuit provides the decision based on the ratio whether the relay should trip or not the circuit breaker. The circuit is designed for a single phase transformer and tested by simulation and found to serve the purpose for which it has been designed. Such three identical circuits may also provide the same degree of performance for a three-phase transformer differential protection with harmonic restraints.