Black start

A black start is the process of restoring an electric power station or a part of an electric grid 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 may come from an on-site standby generator. Alternatively, where a large amount of power is required, a tie-line to another generating plant 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. 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 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.