Efficient proposed solutions for current transformers saturation effects on overcurrent relays operations in distribution systems

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 time-delayed overcurreent digital relays, in distribution systems, where loads are relatively smalls and connected to switchgears with high short circuit levels. A research was performed, by testing and simulations using EMTP, to verify the operations instantaneous and time-delayed overcurrent digital relays to ensure coordination with other devices and safe isolations of faults. It is shown that instantaneous digital relays will perform properly with relatively small CTs. Also, it is shown that, time-delayed overcurrent relays will be significantly impacted by CT´s saturation and special care shall be taken to achieve coordination with other devices. The research will also utilize a well-proven and tested EMTP model of the CT´s to effectively estimate the required time delay of the overcurrent relay to achieve relay coordination. A criterion for selecting CTs for instantaneous and time-delayed digital overcurrent relays operations to achieve adequate relay operations is presented.