Protection of Distribution Transformer in Radial Networks by a Microprocessor-Based Relay in the Source

This paper describes a source protection for the distribution transformers in radial feeders. In this protection scheme the transformers are intended to be protected by their source circuit-breaker, rather than local overcurrent devices, such as RV switchfuse used in the present conventional protection scheme.The source protection algorithm is based on the measurement of increments in the sequence components of electrical quantities, such as currents. By this means a detectable quantity can be provided which will be used on an incremental increase in current rather than its absolute value, since faults normally cause a more rapid increase in current compared to variation in load.The Electro-Magnetic Transient Program (EMTP) has been used to simulate the internal transformer faults on an Iranian 20KV network to test the validity of the proposed incremental protection The simulation results show a very good sensitivity of the relay, even for the worst inter-turn faults which could not be detected by the conventional protection unless becoming a serious transformer fault.