Black start

A black start is the process of restoring a power station to operation without relying on external energy sources.

Normally, the electric power used within the plant is provided from the station's own generators. Often a transmission line will be installed to provide this "station service" power if all the main generators are shut down. However, during a wide-area outage, this off-site power supply will not be 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 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 uneconomical 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. Often hydroelectric power plants are designated as the black-start sources to restore network interconnections. A hydroelectric station needs very little initial power to start (just enough to open the intake gates), and can put a large block of power on line very quickly to allow start-up of fossil-fueled or nuclear stations.

A black start sequence

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.

Commercial value of black start services

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. [ [http://www.nationalgrid.com/NR/rdonlyres/99A34EB4-76F4-4042-AA12-35D6DD843FA7/3073/black_start.pdf Introduction to Black Start] (National Grid plc) Covers technical and commercial background.]

Similar arrangements are in place wherever generation, transmission and distribution functions have been unbundled.

Limitations on black start sources

Not all generating plants are suitable for black-start capability. For example, wind turbines are usually connected to induction generators which are incapable of providing power to a de-energized network; the same may be true of mini-hydro or micro-hydro plants, which also rely on a power network connection for frequency regulation and reactive power supply. Many large HVDC converter stations cannot operate into a "dead" system, either, since they require commutation power from the system at the load end; this is not true of PWM-based HVDC schemes.

Notes