Selection of an Appropriate Filter Algorithm for Transformer Differential Relays

In view of the numerous benefits of digital relaying in terms of economics, performance, flexibility and system interaction, significant efforts have been made in the development of digital relaying algorithms. For transformer differential protection, the second and fifth harmonic magnitude of the differential current compared with that of the fundamental allows a tripping decision. This paper compares various filter algorithms introduced for the differential protection of three-phase transformers. The algorithms are investigated as to their speed of response, computational effort and the capability to discriminate between a fault within the protection zone and inrush or overexcitation, respectively.