Saturation of Current Transformers and its Impact on Digital Overcurrent Relays

Improper selections of current transformers will result in CT's saturation during fault conditions which can affect protection relays operations. With the absence of clear and practical guidelines for CT's selections, designers are facing great difficulties to ensure proper operations of digital relays. The impact of CT saturation is different for different protection devices and schemes. This paper investigates the influence of CT saturation on over-current digital relays, in distribution systems where loads are relatively small and connected to switch-gears with high short circuit levels. A research was performed to verify the operations of instantaneous and time-delayed over-current digital relays to ensure coordination with other devices and safe isolations of faults. It is shown that instantaneous digital relays will perform properly with a relatively small CT's. A criterion for selecting CT's for instantaneous digital over-current relays operations is presented. However, time-delayed over-current relays will be significantly impacted by CT's saturation and special care shall be taken to achieve coordination with other devices