Algorithm to Prevent Breaker-Failure Protection Mal-operation Due to Subsidence Current

Abstract

Duplication of circuit breakers (CBs) in substations is not realistic due to economic and operational constraints. In the event of failure of a breaker, adjacent breakers need to be called in to isolate the fault. Typically, a breaker-failure protection (BFP) function is integrated within commercial relays to monitor such situations. However, these functions are prone to mal-operation because of subsidence in current transformers (CTs) resulting from faults with significant decaying DC component. To this end, a reliable reset algorithm for the BFP function is presented in this paper. After measuring the decaying DC component within a one-cycle moving window, the proposed algorithm accurately distinguishes between the alternating pattern of fault current and exponential decaying pattern in subsidence current. Its performance remains unaffected during change in fault current level, decaying dc component, CT saturation and level of subsidence. Results from simulation as well as laboratory setup indicate that the proposed algorithm is able to prevent the mal-operation of an existing BFP scheme in the conventional relay with safety margin in the range of 20-40%. A comparative evaluation with available techniques testifies its superiority.