Sizing current transformers for line protection applications

This paper discusses the factors to consider for sizing current transformers (CTs) for line protection applications. We first cover CT basics, with emphasis on errors and ac and dc saturation. We also discuss the criteria to avoid CT saturation. Then we analyze the effect of CT saturation on overcurrent, distance, directional, and differential elements. Further, we present the advances in protection element design to improve security and speed under CT saturation conditions. Finally, we discuss the tools available to the protection engineer for CT sizing and provide some guidelines.