Voltage stability assessment of a large power system

Modal analysis and reactive power reserve method are applied in the Entergy electrical system to find the buses and zones prone to voltage instability. In the modal analysis method, weak zones are identified by monitoring the participation factor of the buses in the critical modes. In the reactive power reserve method, reactive power reserves of the zones are monitored for single line outages, generator outages, and double line outages and the zones that have exhausted their reactive power reserves are identified as critical. The critical contingencies resulting in the smallest stability margins are also computed, and are ranked in the order of severity. The voltage stability assessment tool (VSAT) developed by Powertech Labs, is used to determine the weak zones and to determine the critical contingencies.