Application of percentage cross differential relay in EHV double lines

The cross differential protection used to protect parallel double transmission lines is able to offer fast operation without the need for communication link. However, the conventional cross differential relay is not sensitive enough to protect EHV transmission lines because of the requirement for high setting values. In this paper, a percentage cross differential element is used to improve the performance of the conventional cross differential relay. A Voltage start and a phase selection element are also used to improve the stability of the percentage cross differential relay. The percentage cross differential relay is implemented in to latest numeric platform. Extensive tests using EMTP and RTDS show that the relay is able to give significantly improve the performance of the conventional cross differential relay by extending the coverage of the instant tripping zone and increasing the reliability of the protection scheme.