Application of the line differential protection scheme for radial transmission lines

Modern communication networks have dramatically increased the implementation of the line differential scheme (87L) as one or both primary protection for transmission lines. The transmission network is usually meshed which also provides fault current contribution from both ends of the line. When supplying electricity to specific loads, for instance mining in rural zones, the transmission network could be expanded radially with single or double circuit applications. In a radial system, in the event of fault, fault current contribution is drawn mainly from the source side. Very weak or absent current contribution from the remote end of the line presents a menace to the traditional current supervisory elements of the 87L, such in the cases of low load and a three phase fault or where the power transformers at the remote end do not provide a zero sequence path. This paper reviews this specific application with particular attention to solutions which balance the need for security, speed and dependability of the line differential protection scheme. First, some practices of 87L and supervisory elements used in today's relays are investigated. The complications and challenges for the application of 87L on radial lines are then examined. Two genuine phase-to-ground fault events on the same 132 kV line in Australia have been reviewed to analyze the performance of the 87L supervision. Most important, several practical solutions are proposed and compared to enhance the functionality of the 87L supervision and meet the need of increasing the dependability of the 87L without affecting security, simplicity and speed of operation.