Common differential relaying scheme for the protection of various transformer configurations

A Transformer is one of the most important and expensive devices in a power system. This paper presents new, simple and common method for transformer protection based on percentage biased differential principle. Fast Fourier Transform (FFT) technique is used to provide the operating quantity to the dual slope differential relay. The operating criterion of the relay depends on the relationship between differential current (IDIFF) and restraining current (IREST). Various power system components such as generator, transformer, loads, relay, etc. have been modeled in PSCAD/EMTDC software package. The proposed differential protection scheme of transformer operates only for the internal fault which includes ground and phase faults. The scheme remains stable and does not operate for any fault outside the transformer as well as during normal load condition. Moreover, it operates for high resistance in-zone fault and is unaffected by Current Transformer (CT) saturation in case of an external fault. The proposed method is implemented for different transformer configurations such as Star-Star, Star-Delta, Delta-Delta and Delta-Star. Simulation results and relay response for different types of fault are included in the paper.