Design, evaluation and implementation of a busbar differential protection relay immune to the effects of current transformer saturation

The design, evaluation and implementation of a busbar current differential protection relay that operates in conjunction with a current transformer (CT) saturation detection algorithm is described. The detection algorithm detects the start and end of each saturation period based on the third-difference function of the current. A blocking signal is activated during saturation and for one cycle after it ends. For internal faults that cause CT saturation, the relay operates before activation of the blocking signal and consequently, a trip signal is issued. For external faults that cause saturation, the blocking signal arrives first and the trip signal remains inactive. The technique does not delay the operating time of the delay. Test results indicate that the relay can discriminate between internal and external faults even when a CT is severely saturated due to the effect of remanent flux. The relay is implemented in a protection scheme based on a digital signal processor. The new relay achieves a high level of stability on external faults and a major improvement in sensitivity for internal faults.