Transmission system vulnerability assessment based on practical identification of critical relays and contingencies

Relay margin can be applied as a direct tool to evaluate relay status during system disturbances. Based on the practical dynamic security assessment criteria for relay margin, we propose the relay margin ratio and the relay staying time ratio which provide more valuable information than relay margin for relay status evaluation. Motivated by the need of considering relay tripping for contingency evaluations, the most possible tripping severity which incorporates the possibility of relay tripping to evaluate contingency severity is proposed. The relay margin ratio and relay staying time ratio are applied to identify the most critical relays for each contingency based on which the most possible tripping severity is obtained to identify the most critical contingencies in transmission systems. These two inherently related procedures together compose a framework to assess the transmission system vulnerability. Numerical test results are presented using a 23-bus test system.