Current transformer application with digital ground differential protection relays

Ground differential protection relays (device 87G) have been used to protect the windings of resistance grounded power transformers. There have been problems of relay misoperation and nonoperation with electromechanical 87G relays due to current transformer (CT) burden and saturation. Due to their low burden, it has been assumed by some that solid-state digital relays are not subject to these problems. The purpose of this paper is to evaluate the truth of this assumption. Under what circumstances, and with what types of CTs, might problems occur? An analysis is performed of digital ground differential protection over a range of power transformer and CT sizes. Guidelines are developed for CT accuracy ratings suitable for this application. The effect of different relay algorithms on scheme performance relative to CTs is also discussed