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

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.

I. Introduction

Keeping the stability and operational consequences of a fault in extra high voltage (EHV) substations in mind, the narrower margin for fault clearance is always preferred. Some of the utilities mandate further reduction of this margin. Hence, it has become more difficult to accommodate the BFP trip time. Selection of excellent mathematical tools to provide reliable BFP has the utmost priority in this regard. Traditionally, BFP is achieved either by physically monitoring the auxiliary contacts or by checking current flow in the circuit breaker. The former method is typically troublesome and has been historically proved to be incorrect. The latter one checks two inputs: (i) the trip signal from the protective relay (BFI), (ii) an over current element (50BF) with threshold set at a fraction of CT nominal current. A delay timer (62) is set at the duration of normal clearing time of the breaker plus a safety margin. If both inputs remain high for this predefined duration, the BFP issues trip signal to the adjacent breakers [1]. Any false tripping inside the relay may lead to large scale power outage or possible blackouts [2]–[4].