Abstract: Due to the incorrect tripping of nonfault busbar with the simulated disconnected bustie circuit current transformer(CT) during the pretesting process in the centralized network test for a 220kV busbar protection device, the limitation existing in big difference and small difference protection logic for singlesectionalized doublebusbar connection mode was proposed. After introducing the medium differential protection criterion, the protection ranges of three busbars were divided again in order to locate the specific fault busbar accurately. The differential protection range, calculation of the differential current, ratiorestraint characteristic equation and their specific applications in the protection logic in singlesectionalized doublebus connection mode were described. Through a simulation test, it was verified that adding medium differential protection criterion could effectively avoid maloperation, improve the quickness and sensitivity of the protection.

Key words: singlesectionalized doublebus connection, bustie CT disconnection, busbar protection, medium differential protection criterion, ratiorestraint characteristic