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.
