A Unified Approach to Critical Clearing Time Estimation Considering Rotor Angle, Voltage, and Frequency Stability

Abstract

Critical Clearing Time (CCT) is an important parameter in determination of power system stability because it helps in deciding the protection relay settings. A wrong CCT value results in unstable power system operation which may cause catastrophic accidents, cascading outages and blackouts. Previous literature primarily determines CCT based on rotor angle stability, which does not ensure complete system stability since voltage collapse and frequency deviations also contribute to system instability. It is therefore quite significant to determine the CCT by considering the rotor angle, voltage, and frequency ( CCTδ,V,f ). This paper proposes a novel approach for determining comprehensive CCTδ,V,f by simultaneously considering rotor angle, voltage and frequency stability. This proposed method is implemented using in PSS®E software on the IEEE 9-bus test system and IEEE 14-bus system respectively. In IEEE 9-bus system, simulation results show that while rotor angle based critical clearing time ( CCT) value is 180 ms, the proposed CCTδ,V,f value is 80 ms, reflecting rigorous and more realistic protection requirement. These results confirm that relying solely on rotor angle based CCT may lead to maloperation of circuit breakers and compromise system instability, whereas configuring the settings based on CCTδ,V,f ensures overall system stability and protection reliability. The proposed method is also validated on IEEE 14-bus system. It shows that the proposed method provides a robust framework for dynamic security assessment and more reliable protection coordination.