A Microprocessor Based Three-Phase Transformer Differential Relay

This paper presents an algorithm for digital protection of a three-phase, three-winding power transformer. Simple recursive expressions are given for the harmonics using a sampling rate of 12 times a cycle. These expressions are used as the basis for a one cycle algorithm which is compatible with proposed digital line protection schemes. The algorithm was tested on data obtained from a model transformer. Test results are given for 19 cases of energizations and faults.