Enhancing differential protection stability during CT saturation with Transient Bias

The paper describes a Transient Bias technique designed to overcome the effects of Current Transformer (CT) saturation, which might severely affect the behaviour of a protection system. This Transient Biasing, as the name implies, is only active during transient conditions in the power system, and it decays quickly once the transients disappear. A complementary algorithm for fast detection of external faults is proposed, which prevents the Transient Bias feature from increasing the differential protection operating time in case of internal faults. The operation of current differential protection with Transient Bias is checked using Simulink® models of the relay and current transformer. In addition, the paper reviews the results of RTDS® tests carried out with actual current differential relays and provides detailed information on CT requirements testing method adopted by Alstom Grid.