Black start
The process of restoring a power system after a total or partial blackout using units that can start without power from the grid.
Black start is the capability of a generating unit to start from shutdown without external electricity supply and then energise transmission lines to bring other plants back online. Most large thermal and nuclear plants need grid power for pumps, fans and controls, so they cannot restart on their own.
Traditional black-start providers are hydropower plants (which need little auxiliary power and can start in minutes), diesel-started open-cycle gas turbines and some combined-cycle units with on-site diesel generators. Restoration follows a pre-planned sequence: black-start units energise a skeleton network, pick up blocks of load to stabilise frequency, synchronise islands, and then start larger plants. Full restoration typically takes hours to a few days; the Electricity System Restoration Standard for Great Britain requires 60% of demand restored within 24 hours and 100% within five days in every region by end-2026.
As coal and gas units retire, operators are procuring black start from new sources:
- Battery systems with grid-forming inverters.
- VSC-HVDC links from neighbouring systems.
- Wind farms with grid-forming capability, demonstrated in the Distributed ReStart project led by NESO, SP Energy Networks and TNEI.
The April 2025 Iberian blackout highlighted the role of hydro and interconnectors: Spain and Portugal restored most supply within roughly 12 to 16 hours, drawing on hydro plants and links with France and Morocco.
Related terms
All terms →- Ancillary services
Services beyond bulk energy that system operators procure to keep the grid stable and secure, such as frequency reserves, voltage control and black start.
- Grid-forming inverter
An inverter that sets its own voltage and frequency reference, behaving like a voltage source that can stabilise weak grids and provide synthetic inertia.
- Grid frequency
The rate at which alternating current changes direction on a power system, 50 Hz or 60 Hz, which reflects the instantaneous balance of supply and demand.
- HVDC (High-Voltage Direct Current)
Transmission technology that carries bulk power as direct current, with lower losses over long distances and the ability to link asynchronous grids.
- Microgrid
A local group of generators, storage and loads with defined boundaries that can operate connected to the main grid or independently in island mode.
- Dispatchable generation
Generation whose output can be scheduled, raised or lowered on request by the operator, independently of weather.