The Influence of Communication Channel on the Performance of Line Differential Protection

Differential protection of transmission-lines requires the exchange of current data between the relays installed at both sides of the line. For this purpose, point-to-point connection over dedicated optical fiber is preferred in terms of reliability. However, the optical fiber bandwidth is much higher than that required by differential relays and therefore, a dedicated fiber may not be an economically optimal design. Thus, the use of digital communication networks is considered as an alternative solution. This not only ties the reliability of the protection system to communication network equipment, but also can affect the performance of the differential protection scheme. Performing simulation studies on a typical line in PSCAD/EMTDC program, this paper shows that the channel asymmetry of a digital communication network will cause a false differential current, resulting in spurious tripping of the relay if not properly considered in the relay settings.