Design and evaluation of a current differential protection scheme with enhanced sensitivity for high resistance in-zone faults on a heavily loaded line

Current transformer saturation, resulting from an external fault, may cause the malfunction of a differential protection relay if measures are not taken to automatically change the operating sensitivity. Adequate stability is achieved by raising the operating threshold as the through or bias current increases. This has a detrimental effect on the sensitivity and may prevent the detection of an in-zone resistive fault. This paper explains how symmetrical component based current quantities enhance the relay sensitivity and ensure it can detect in-zone resistive faults. The results show that, by using an adaptive biasing technique, a significant improvement in relay sensitivity is achieved. This allows resistive faults to be detected where the differential current magnitude is below the biased operating threshold.