Current transformers for HV protection

Current transformers or CTs take up a lot of space in HV cubicles. Thorough knowledge of how they work makes it possible to: - reduce their dimensions and thus their cost, - use standard CTs in a larger number of configurations. The purpose of this study is to learn more about CT operation in association with protection relays and to lay down a few rules for sizing them properly. After a brief theoretical review of CT operation and current protection devices, the behaviour of the CT-protection relay combination is studied in two particularly important cases in HV: - overcurrent relay supplied by a heavily saturated CT, - protection relay connected to two CTs in parallel in duplex cubicles. This study is completed by experimental results.