System restoration, sometimes simply called restoration, is the process of restoring service to customers after an outage or blackout. Prior to restoration, the cause of the blackout or outage must be addressed. Then, restoration is performed by following a sequential procedure of adding loads to the system.
First supply to match load must be established by reconnecting to an energized portion of the system or by starting up black start units. Sections of loads are then connected one at a time so that supply and load can be carefully balanced and so that reconnection does not jeopardize the stability of already energized portions of the system. The process involves opening and closing switches to divide loads into appropriately small sections and then connecting sections one by one. In some cases, a section may be intentionally isolated from the larger grid and temporarily served by local generation. This is called islanding. Eventually, each island must be synchronized with the larger grid and then reconnected.
In a large outage or blackout, system restoration can take hours or even days since great care must be taken to maintain the supply/load balance and avoid causing a new system disturbance that could shut the system down again. And if an uncontrolled outage has occurred, it is often necessary to inspect the affected areas of the system, reset protective equipment, and repair any damaged equipment in each section before it can be reconnected. While the process of system restoration can be time-consuming, it is necessary to ensure a safe restoration of power.