Bulk electric systems are operated to maintain voltage within specific predefined ranges. But with the occurrence of certain system disturbances (such as equipment failure, weather-related natural disasters, unexpected system behavior, or human performance errors), voltage across the system may decline and voltages may move outside of the acceptable range. This can lead to the triggering of protection equipment, which further exacerbates voltage problems. Voltage collapse is a particular type of blackout that occurs when voltage decline coupled with triggering of protection equipment leads to a cascading outage.
In the initial stage of a voltage collapse, voltage magnitudes at substations may fall gradually with temporary voltage recovery due to control actions from generators and equipment such as capacitors. But if control actions are insufficient given the magnitude of voltage problems, the voltages will continue to fall and protective devices such as underfrequency relays will observe the unacceptable conditions and open circuit breakers to deenergize all or part of the system. At that point, blackouts will occur.