Relay performance considerations with low ratio CTs and high fault currents

Low-ratio toroidal current transformers (CTs) are used in power systems. The majority are in the auxiliary switchgear at generating stations. The ratio of the CTs used in this application is usually selected to accommodate metering and overload protection. The problem addressed is the absence of consideration in this ratio selection for fault protection. Analysis of some fault cases shows that the protective relaying will not operate, or will not operate properly, due to the saturated CTs. The significance and consequences of the use of these CTs and the accompanying saturation during high fault current are discussed. Considerations when testing low-ratio CT applications for saturation are presented, and a computer program that demonstrates the volt-time concept of CT saturation is described. Alternate CT connections and corrective alternatives are given along with conclusions to aid in resolving low ratio/high fault current situations.<>