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.
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 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 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 black start, providing another option in places without suitable hydroelectric plants. In 2017, a utility in Southern California successfully demonstrated the use of a battery-based energy-storage system to provide a black start, firing up a combined-cycle gas turbine from an idle state.
A black start sequence.
One method of black start (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.
There are multiple methods of commencing a black start: hydro-electric dams, diesel generators, open cycle gas turbines, grid scale battery stores, compressed air storage, and so on. Different generating networks take different approaches, dependent on factors such as cost, complexity, the availability of local resources (i.e. suitable valleys for dams), the interconnectivity with other generating networks, and the response time necessary for the black start process.
Procurement 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. It is typical of power stations from the CEGB era to have a number of open-cycle gas turbines (i.e. no heat recovery modules attached) that can run the entirety of the plant necessary to operate a full generating unit; these would normally be started by diesel generators, fed in turn by battery backups. Once up to speed, these gas turbines are capable of running the entire plant associated with the rest of the power station, negating the need to bring power in from other sources.
In the North American independent system operators, the procurement of black starting varies somewhat. Traditionally, black-start capability 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, even though 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 and the New York Independent System Operator (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 (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 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 encourage provision of the black start service. 
The final method of procurement is competitive procurement as used by the Electric Reliability Council of Texas (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 start another unit in close proximity, in order to begin the islanding and synchronization of the grid.
In other jurisdictions there are differing methods of procurement. The New Zealand System Operator procures the blackstart capability via competitive tender. Other jurisdictions also appear to have some sort of competitive procurement, albeit perhaps not as structured as that of ERCOT. These include the Alberta Electric System Operator, as well as Independent Electric System Operator of Ontario, both of which use a long-term "request for proposals" approach similar to New Zealand and ERCOT.
The first black start on Germany's grid was tested in 2017 at WEMAG battery power station in Schwerin on a disconnected, isolated grid. The WEMAG battery plant proved that it can restore the power grid after major disruption or blackout.
Limitations on black start sources.
Not all generating plants are suitable for black-start capability. Wind turbines are not suitable for black start because wind may not be available when needed. Wind turbines, mini-hydro, or micro-hydro plants, are often connected to induction generators which are incapable of providing power to re-energize the network. In 2020, the 69 MW Dersalloch wind farm black-started part of the Scotland grid, using virtual synchronous machines. The black-start unit must also be stable when operated with the large reactive load of a long transmission line. Many high-voltage direct current (HVDC) converter stations cannot operate into a "dead" system, either, since they require commutation power from the system at the load end. A pulse-width modulation (PWM)-based voltage-source converter HVDC scheme has no such restriction.