A black start is the process of restoring an electric power station, a part of an electric grid or an industrial plant, to operation without relying on the external electric power transmission network to recover from a total or partial shutdown.

Power to restart a generating station or plant may come from an on-site black start standby generator. Alternatively, where a large amount of power is required, a tie-line to another generating plant or to an emergency generator may be used to start the facility. Once the main generating units are running, the electrical transmission network can be re-connected and electrical loads restored.

Black-start power may be ensured by an agreement where a particular energy supplier is paid to make black start power available when required. Not all generating plants are suitable for providing black-start power to a network. The ability to perform a black start requires complex technology and is economically costly. In addition, the power plant providing the black start must be located close to the electricity users.

Electrical generating plants require electric power to operate systems required in the plant. For example, a coal-fuelled plant requires conveyors, crushers, air compressors, and combustion air fans to operate. Steam cycle plants require large pumps to circulate water for steam boilers and for cooling of condensate water. Hydroelectric plants require power to open intake gates, and to adjust the hydraulic turbines for speed regulation. Even a wind turbine plant may require a relatively small amount of electric power for such things as adjusting blade pitch and direction.

Normally, the electric power used within the plant is provided by the station's own generators. If all of the plant's main generators are shut down, station service power is provided by drawing power from the grid through the plant's transmission line. However, during a wide-area outage, off-site power from the grid is not available. In the absence of grid power, a so-called black start needs to be performed to bootstrap the power grid into operation.

To provide a black start, some power stations or plant have small on-site diesel generators, normally called the black start diesel generator (BSDG), which can be used to start larger generators (of several megawatts capacity), which in turn can be used to start the main power station generators. Generating plants using steam turbines require station service power of up to 10% of their capacity for boiler feedwater pumps, boiler forced-draft combustion air blowers, and for fuel preparation. It is uneconomical to provide such a large standby capacity at each station, so black-start power must be provided over designated tie lines from another station.

Often hydroelectric power plants are designated as the black-start sources to restore network interconnections. A hydroelectric station needs very little initial power for starting purposes (just enough to open the intake gates and provide excitation current to the generator field coils) and can put a large block of power on line very quickly to allow start-up of fossil fuel or nuclear stations. Certain types of combustion turbine can be configured for a black start, providing another option in places without suitable hydroelectric plants. In 2017, a utility in Southern California successfully demonstrated the use of an energy-storage system based on a lithium-ion battery to provide a black start, firing up a combined-cycle gas turbine from an idle state.

One method of black start (based on a real scenario) might be as follows: