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. 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 supply from the grid 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.
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A typical sequence (based on a real scenario) might be as follows:
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]
In the North American Independent System Operators the procurement of black start varies somewhat. Traditionally black start was provided by integrated utilities and the costs were rolled into a broad tariff for cost recovery from ratepayers. In those areas which are not part of organized electricity markets this is still the usual procurement mechanism. In the deregulated environment this legacy of cost-based provision has persisted, and even recent overhauls of black start procurement practices, such as that by the ISO New England, have not necessarily shifted to competitive procurement, despite the fact that deregulated jurisdictions have a bias for market solutions rather than Cost-of-Service (COS) solutions.
In the United States, there are currently three methods of procuring black start. The most common is Cost of Service, as it is the simplest and is the traditional method. It is currently used by the California Independent System Operator (CAISO), the PJM Interconnection[2] and the New York Independent System Operator[3] (NYISO). Although the exact mechanisms differ somewhat the same approach is used, namely that units are identified for black start and their documented costs are then funded and rolled into a tariff for cost recovery. The second method is a new method used by the Independent System Operator of New England[4] (ISO-NE). The new methodology is a flat rate payment which increases black start remuneration to encourage provision. The monthly compensation paid to a generator is determined quite simply by multiplying a flat rate (in $/KWyr and referred to as the $Y value) by the unit's Monthly Claimed Capability for that month. The purpose of this change was to simplify procurement and incent provision of black start. The final method of procurement is competitive procurement as used by the Electric Reliability Council of Texas[5] (ERCOT). Under this approach ERCOT runs a market for black start services. Interested participants submit an hourly standby cost in $/hr (e.g. $70 per hour), often termed an availability bid, that is unrelated to the capacity of the unit. Using various criteria ERCOT evaluates these bids and the selected units are paid as bid, presuming an 85% availability. Each black start unit must be able to demonstrate that it can startup another unit in close proximity to begin the islanding and synchronization of the grid.
In other jurisdictions there are differing methods of procurement. The New Zealand System Operator[6] procures the Blackstart capability via competitive tender. It solicits closed tenders from potential providers and enters into contracts with those parties that it identifies. In addition to New Zealand there are a number of other jurisdictions that also appear to have some sort of competitive procurement, although not as structured as ERCOT. These include the Alberta Electric System Operator[7], as well as Independent Electric System Operator of Ontario[8] , both of which use a long-term Request For Proposals approach similar to New Zealand and ERCOT.
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 high-voltage direct current 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.
2007: Isemonger, A.G. “The Viability of the Competitive Procurement of Blackstart: Lessons from the RTOs” The Electricity Journal Volume 20, Issue 8, October 2007, Pages 60-67 doi:10.1016/j.physletb.2003.10.071
Brendan Kirby and Eric Hirst, Maintaining System Black Start In Competitive Bulk-Power Markets, American Power Conference, Chicago, Illinois, April 1999, available online at: http://www.ornl.gov/sci/btc/apps/Restructuring/pub.htm
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