Relay margin trajectory based identification of transmission vulnerability for power system security assessment

In this paper relay margin is derived in terms of bus voltage. With the variation of bus voltage during system disturbance the relay margin will also consequently vary to form a trajectory which is proposed as the relay margin trajectory. The sensitivity of the relay margin trajectory is investigated to quantify the rate of the relay margin variation. Based on the relay margin we propose a vulnerable relay indicator which samples the smallest relay margin over the interested time span. Based on the vulnerable relay indicator, another two indicators--severe contingency indicator and system security indicator are proposed. These three inherently related indicators together compose a framework for system security analysis. Vulnerable relay indicator is applied to identify the most vulnerable relays based on which the severe contingency indicator is obtained. System security indicator is sequentially gained based on those two indicators to identify the transmission vulnerability for the power system security analysis. Numerical test result on a test system is presented.