The research of the overcurrent relays based on phase-to-phase differential current -adaptive setting and coordination

This paper presents an adaptive overcurrent relay based on the principle of phase-to-phase differential currents, denominated as I˙_AB, I˙_BC and I˙_CA, to detect phase fault of transmission and distribution line. The characteristics of phase- to-phase differential currents under phase fault condition are analyzed. The trip criterion and coordination principle are described. This relay is adaptive to the type of phase fault, the operating conditions of power system and the disconnection of PT. It can also provide an adaptive backup protection for the secondary-side overcurrent relay of a delta-wye transformer. Comparing with conventional overcurrent relays, this relay has a larger zone of protection and higher sensitivity.