Performance of transformer differential relay at different neutral grounding resistance based on wavelet transform

Power transformer protective relay should block the tripping during magnetizing inrush and rapidly should operate the tripping during internal faults. There are a variety of protective relays using different measuring techniques to provide reliable and secure transformer protection. These types of the protective relays include electro-mechanical, solid state, digital and numerical relays. Within each group of them, various algorithms are existed. A stand-alone digital relay has been designed and implemented, where its function can be modified easily to accommodate the protection of different types of transformers by only changing its attendant software. This paper suggests a possibility of improving the digital power transformer algorithm for the protection of three-phase power transformers based on the discrete wavelets transform. This algorithm is able to discriminate between the inrush and internal fault currents even in the case of an inrush with a low second harmonic component and an internal fault current with a high second harmonic component. A sample of a power transformer is simulated using the Electro Magnetic Transient Program, and the generated data are used by the MATLAB to test the performance of the technique, such as its speed of response and reliability. The obtained results showed that the proposed algorithm is reliable, accurate and fast. Also in this paper, the performance of the proposed algorithm is tested for different neutral grounding resistances of the transformer, as it greatly affects the relay sensitivity in the case of turn-to-ground fault.