Application of Distance and Line Current Differential Relays in Breaker-and-a-Half Configurations

Line protection in breaker-and-a-half configurations requires main protection functions to respond to the sum of the two currents. This concerns many protection functions particularly when sensitive settings are applied and/or significant Ct saturation occurs. If the remote system is relatively weak, the current supplied through the line towards a close-in reverse fault on the breaker-and-a-half terminal may be overwritten with a spurious current produced by saturated Cts. This may affect directionality and impact not only fast and sensitive ground protection functions used by pilot-aided schemes, but also distance, and current differential protection elements. This paper addresses the problem by providing practical solutions for ground directional overcurrent, distance and differential functions. A new design for the digital line current differential relay is presented. The relay is capable of protecting three-terminal applications with each terminal fed from up to four currents