Design of a symmetrical component based current differential protection scheme

Increasing availability of digital communication systems has ensured current differential schemes can now widely be used for the protection of transmission feeders. With ideal current transformers (CT) stability should be achieved under all steady state operating conditions and for all faults outside the protection zone, referred to as external faults. For real CTs, saturation of the CT core due to a high current external fault will cause a nonzero differential current to circulate. To ensure stability, the operating threshold is raised as the through fault current is increased. This has a detrimental effect on sensitivity and may prevent the detection of internal resistive faults. This paper discusses how symmetrical components allow the biased operating threshold to be reduced when CT saturation occurs.