Black start

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For the species of bird, see Blackstart.

A black start is the process of restoring power after a wide-area power outage has occurred. This can be difficult, as power stations need to be brought back on-line. Normally, this is done with the help of power from the rest of the grid and drastic measures (such as disconnecting large numbers of customers) will be taken to avoid complete shutdown of the grid. 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 are typically equipped with small diesel generators 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 uneconomic to provide such a large standby capacity at each station, so black-start power must be provided over the electrical transmission network from other stations.

A typical sequence (based on a real scenario) might be as follows:

A battery starts a small diesel generator installed in a hydroelectric generating station.
The power from the diesel generator is used to bring the hydroelectric generating station into operation.
Key transmission lines between the hydro station and other areas are energized.
The power from the hydro dam is used to start one of the coal-fired base load plants.
The power from the base load plant is used to restart all of the other power plants in the system including the nuclear power plants.
Power is finally re-applied to the general electricity distribution network and sent to the consumers. Often this will happen gradually; starting the entire grid at once may be unfeasible. (In particular, after a lengthy outage during summer, all buildings will be warm, and if the power were restored at once, the demand from air conditioning units alone would be more than the grid could supply; in colder climates a similar issue can occur in winter with the use of heating devices).

In a larger grid, black start will often involve starting multiple "islands" of generation (each supplying local load areas) and then synchronising and reconnecting these islands to form a complete grid. The power stations involved have to be able to accept large step changes in load as the grid is reconnected.

In the United Kingdom the grid operator has commercial agreements in place with some generators to provide black start capacity, recognising that black start facilities are often not economic in normal grid operation. ^[1]