Impact of decaying DC component on the characteristics of over-current protective relays

The response of an over-current relay towards a decaying DC component, in the current signal measured by the relay, is dependent on how much this decaying DC component is filtered out. Classic types of relays (i.e. electromechanical and static) might have insufficient or even no filtering and in this case the relay tripping characteristics are deteriorated to the limit that it might trip for steady state currents less than its setting value. This mal-operation might be avoidable when the relay has a relatively long tripping time delay. However, for high-set instantaneous over-current relays, the possibility of mal-tripping of the relay due to the decaying DC component is high and might lead to unsatisfactory system operation. This paper assesses the performance of over-current protective relays when a decaying DC component exists in the current signal. The paper depicts how the characteristics of the relay could deviate more than expected. From those characteristics, three new performance indices are proposed; from which reassessment of the relay performance for a more satisfactory protection operation could be achieved. The method used for the assessment could be used in analysis of protective system performance during system faults and also for selecting the proper relay for a specific application that requires the relay operation to be more independent of decaying DC components.