Relay performance considerations with low-ratio CTs and high-fault currents

Low-ratio toroidal current transformers (CTs) are in existence in many locations on power systems. The majority of these CTs are in the auxiliary switchgear at generating stations, but are not limited to this application. 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. In many cases the fault duty of this switchgear is very high and, because the low-ratio CTs also are of low accuracy classes, the CTs will saturate during a fault. The 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 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 resolving low-ratio/high-fault current situations