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Usually, the voltage instability will occur after one or more rotor angle instabilities have occurred. Though not always the case. The way to identify instability conditions in bus voltages is an underdamped highly oscillatory response for a long duration (>10 seconds) coupled with failure in recovery. Also, track the terminal and field voltages at all 10 machines to track long and spiky underdamped responses. Mostly if that stage is reached for the system, rotor angle instability for a few machines has already occurred.