RELAY PERFORMANCE CONSIDERATIONS WITH LOW RATIO CTS AND HIGH FAULT CURRENTS

Low ratio toroidal cts are in existence in many locations on power sys tems. The majority of these cts are in the auxiliary switchgear at gen erating stations, but are not limited to this application. The ratio o f the cts used in this application is usually selected to accommodate metering and overload protection. The problem addressed here is the ab sence of consideration in this ratio selection for fault protection. I n many cases the fault duty of this switchgear is very high and, becau se the low ratio cts also are of low accuracy classes, the cts will sa turate during fault. Analysis of some fault cases shows that the prote ctive relaying will not operate, or will not operate properly, due to the saturated cts. The significance and consequences of the use of the se cts and the accompanying saturation during high fault currents are discussed. Considerations when testing low ratio ct applications for s aturation are presented as well as a computer program to demonstrate t he volt-time concept of ct saturation. Alternate ct connections and co rrective alternatives are given along with conclusions to aid in resol ving low ratio/high fault current situations.