A busbar differential protection relay immune to the effects of CT saturation

This paper describes a busbar differential protection relay operating in conjunction with a current transformer (CT) saturation detection algorithm. 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 with CT saturation, the relay operates before the blocking signal, consequently a trip signal is issued. For external faults with saturation, the blocking signal arrives first and the trip signal remains inactive. The technique does not delay the operating time of the relay. Test results indicate that the relay can discriminate between internal and external faults even when a CT is severely saturated due to the remanent flux.