Detecting with PMUs the onset of voltage instability caused by a large disturbance

This paper deals with long-term voltage instability triggered by the outage of transmission or generation equipments. It is shown how the onset of voltage instability could be detected in real-time from post-disturbance bus voltages provided by phasor measurement units (PMUs), assuming observability of the whole region prone to voltage instability. An extended set of equilibrium equations is fitted to the measurements and the change in sign of sensitivities is monitored. The latter are easily computed even for a large system. The important effects of overexcitation limiters and load tap changers are taken into account. The approach is illustrated on a 52-bus 20-machine system, where PMU outputs are simulated from detailed time simulation.