RELAY PERFORMANCE CONSIDERATIONS WITH LOW-RATIO CTS AND HIGH-FAULT CURRENTS

Low-ratio toroidal CT's are in existence in many locations on power systems, The majority of these CT's are in the auxiliary switchgear at generating stations, but are not limited to this application. The ratio of the CT's used in this application is usually selected to accommodate metering and overload protection, The problem addressed here is the absence of consideration in this ratio selection for fault protection, In many cases the fault duty of this switchgear is very high and, because the low-ratio CT's also are of low accuracy classes, the CT's will saturate during fault, Analysis of some fault cases shows that the protective relaying will not operate, or will not operate properly, due to the saturated CT's, The significance and consequences of the use of these CT's and the accompanying saturation during high-fault currents are discussed. Considerations when testing low-ratio CT applications for saturation are presented as well as a computer program to demonstrate the volt-time concept of CT saturation, Alternate CT connections and corrective alternatives are given along with conclusions to aid in restoring low-ratio/high-fault current situations.