A comparative analysis of digital relaying algorithms for the differential protection of three phase transformers

The authors present a survey and a comparative analysis of various algorithms introduced for the differential protection of three-phase transformers. After a brief review of the concept, the problems associated with the differential protection of transformers, and the existing solutions as implemented with conventional (electromagnetic) relays, the algorithms introduced for digital relaying are outlined. Some of these algorithms are derived from conventional protection methods while others represent novel methods. They are analyzed in view of how well they address the two subproblems of differential protection per se and exciting-current detection. Finally, a simple ranking procedure is introduced. Two optimality performance indices, based on time and frequency, are defined and used as a basis for a comparison of the algorithm presented