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Glossary · Energy industry & energy transition

Black Start

Black start black start capability electricity grid system restoration Blackout security of supply grid operation ancillary service resilience

Black start refers to the ability of a generating plant, a storage unit or a unit designated for this purpose to start up without electricity from the public grid. The plant must then provide voltage, frequency and power to rebuild a failed power grid. In German, the term black-start capability is commonly used. This does not simply mean that a plant can be switched on again after a fault. It means starting from a condition in which no external electrical supply is available at the connection node. In its procurement concept, the German Federal Network Agency (Bundesnetzagentur) describes the capability in essence as follows: a plant can re-energise a grid from the de-energised state within a defined period and keep voltage and frequency stable within certain limits.

This distinction matters because most large power plants themselves need electricity before they can supply it. Pumps, lubricating oil systems, fuel supply, control systems, excitation systems, cooling, valves, converters, protection systems and communications require auxiliary power. In normal operation it comes from the grid or from the station service busbars of the running plant. After a large-scale grid outage, this source is missing. A black-start-capable plant can take this first step on its own, for example with its own diesel generating set, a battery or a small auxiliary turbine. It thus forms the starting point for restoration.

Black start is therefore a special form of ancillary service. It does not serve the ongoing balancing of generation and consumption in the way balancing power does. Nor does it replace a reserve for bottlenecks in normal operation. Black start belongs to the restoration of grid operation after a severe disturbance. It therefore lies at the interface of technology, crisis organisation, market rules and the state's responsibility for security of supply.

What must be provided technically during a black start

A de-energised grid is not an empty wire into which power can be fed at will. For a part of the grid to return to operation, an electrical reference is needed. Voltage must be built up and frequency must be controlled. Lines and transformers must be connected without impermissible inrush currents. Loads must be picked up in such a way that generation and consumption match at every step.

Black-start-capable plants must therefore be able to do more than start up mechanically. In early restoration they must act as grid-forming units. Grid-forming means that the plant sets voltage and frequency and keeps them stable, instead of following an existing grid voltage. In normal operation, many wind and solar plants and many battery storage systems are grid-following. Their converters feed into an existing grid and synchronise to its voltage and frequency. In a completely de-energised grid, this reference is missing. For black start, therefore, synchronous machines, grid-forming converters or a combination of several plants that together build a stable partial grid are needed.

Besides voltage and frequency, reactive power plays an important role. It is needed to maintain voltages in the AC grid and to build up magnetic fields in lines, transformers and machines. Long lines running at no load, for example, generate capacitive reactive power and can raise the voltage. Large transformers draw high inrush currents when switched on. A black-start concept therefore does not only clarify which plant starts first. It also defines which grid paths are energised one after another, which transformers may be connected, which loads initially remain disconnected and which plants are synchronised and in what state.

Frequency control is also demanding. In a small partial grid, every load change has a stronger effect than in the European interconnected grid. There is less rotating mass, less control reserve and no balancing across large areas. If a larger load is connected, the frequency can drop abruptly. If generation is connected without enough load being present or without properly coordinated control, the frequency can rise. Black start is therefore a process made up of many controlled steps, not a single switching-on operation.

Distinction from blackout, island operation and emergency power

A blackout is a large-scale collapse of the power supply. The European network code on emergency and restoration describes the blackout state as a state in which the operation of part or all of the transmission system is interrupted. It distinguishes this from the restoration state: in this state, all activities aim to restore system operation and safeguard operational security. Black start is therefore a capability and a part of this restoration, not the disturbance state itself. A power outage in one city district or a planned shutdown is not automatically a blackout. Not every restart after a disturbance is a black start.

Island operation is related but not the same. An island grid is an area separated from the rest of the grid in which generation and consumption are balanced locally. Black start often leads first to an island grid, because restoration begins with small partial grids. However, island operation also exists by design, for example on islands, in industrial networks or in microgrids. A plant can be capable of island operation without being able to start from the completely de-energised state. German rules therefore treat the two properties separately. There, plants that provide both black-start capability and island operation count as black-start plants.

Emergency power supply must also be distinguished. Diesel generators in hospitals, data centres or waterworks secure certain consumers. They start when the grid fails and supply internal facilities. This is important for the protection of critical infrastructure. But it does not mean that the facility can rebuild the public grid. To do so, it would have to control voltage and frequency for a grid segment, comply with protection concepts, enable switching operations, be capable of synchronising with other plants and be integrated into the grid operator's planning. The same applies to uninterruptible power supplies, home storage systems or backup power systems in buildings.

Which plants can be black-start capable

Traditionally, hydropower plants, pumped-storage power plants, gas turbines or special diesel generating sets provide black start. Hydropower plants start comparatively quickly and often need only little auxiliary power. Pumped-storage power plants are suitable if they can start up in turbine mode without an external supply or have their own starting units. Gas turbines deliver power quickly. However, they need fuel, control and starting systems. For gas-supplied plants, the gas supply itself must also withstand a power outage. German transmission system operators require separate proof of this in their tenders, where relevant.

Large thermal power plants such as coal-fired or nuclear power plants are generally not the first black-start plants. They need extensive auxiliary systems and long start-up times. They can become important later in restoration, once an island grid supplies their own consumption. In practice, therefore, a small, robust unit is often started first. A grid path is then built to a larger power plant in order to supply its auxiliary systems.

Battery storage systems can be technically suitable. For this, their converters must operate in grid-forming mode, supply sufficient short-circuit current or suitable protection functions, have enough energy stored and be able to stabilise voltage, frequency and reactive power. Their strength is fast and precise controllability. Limits are set by the energy content, the duration of restoration, the design of the inverter and the integration into protection and control systems. A storage system optimised in the market for short-term flexibility is not automatically a reliable black-start resource. German law explicitly names storage systems as possible providers of black-start capability.

Wind and photovoltaic plants can also contribute if they are equipped accordingly. However, their output depends on weather and time of day. Many plants are also set up to switch off under abnormal grid conditions or to feed in only after a stable grid voltage has returned. Installed capacity alone is therefore not enough. What is needed are defined control functions, assured availability, communication capability, coordinated grid protection and procedures for small island grids. With more generation coupled via power electronics and fewer large synchronous generators, the question of which plants will take on grid-forming tasks in future becomes more pressing.

Restoration as a planned sequence

In the EU, Regulation (EU) 2017/2196 governs this sequence. It is a network code on the emergency and restoration of the transmission system. Under it, each transmission system operator must maintain a system defence plan and a restoration plan, in English the Restoration Plan. The plans had to be drawn up by 18 December 2018 in coordination with distribution system operators, significant grid users, regulatory authorities and neighbouring grid operators. They describe the steps by which supply is restored with the help of plants with black-start or island-operation capability. This also includes the desired geographical distribution of such plants. The network code also requires suitable means of communication and regular testing of the planned equipment and capabilities.

The sequence usually begins with a black-start-capable plant that builds a small island grid. Initially, it often consists only of the plant, a substation, individual lines and a few loads. Further parts of the grid are then connected in a targeted way. Via start-up paths or restoration paths, energy reaches power plants that must cover their own consumption. With each additional generator, the island grid grows. After stabilisation, several island grids are synchronised with one another. For this, frequency, voltage and phase angle must match sufficiently. German transmission system operators describe their approach as a bottom-up strategy. Restoration is thus intended to be possible from the bottom up, even independently of neighbouring grids. If a neighbouring grid is still intact, restoration can also begin from there.

Picking up load is a critical step in its own right. After a prolonged power outage, consumers behave differently than in normal operation. Refrigeration units, heating systems, pumps, chargers and industrial plants may start up almost simultaneously. This phenomenon is called cold load pickup. The load after reconnection can be significantly higher than immediately before the outage. Grid operators therefore restore supply to consumers in stages. If connection is too fast, an unstable island grid can collapse again.

Restoration also depends on communication and personnel. The following must work: switching operations in substations, coordination between transmission system operators and distribution system operators, the reachability of power plant control rooms, emergency communications, fuel supply and access to the plants. Black start is therefore not a single property of a machine. The capability arises from the plant, grid path, personnel, contracts, tests and organisational procedures.

Legal framework and procurement in Germany

In Germany, section 12h of the Energy Industry Act (EnWG) classifies black-start capability as a non-frequency-related ancillary service. In principle, it is to be procured in a transparent, non-discriminatory and market-based manner. The Federal Network Agency can set exceptions if procurement via the market would not be economically efficient. For the transmission system operators, it has not granted such an exception. With determination BK6-21-023 of 13 January 2023, it instead prescribed a procurement concept. Under it, each control-area-responsible transmission system operator procures black-start capability independently in procurement regions. The subject matter is the number of black-start plants required. Suitable plants in distribution grids are to be included.

The changeover takes place in stages. No later than twelve months after the determination, a procedure had to begin in each control area for at least one procurement region. No later than six years thereafter, procedures must have been initiated for all regions. An award is followed by a lead time of usually three to five years. During this time, the provider can build or retrofit its plant. In the case of a short-term need that could not be foreseen beforehand, the lead time can exceptionally be shorter. The agreed remuneration covers all services relating to black-start capability.

By decision BK6-21-360 of 22 February 2023, the Federal Network Agency exempted distribution system operators from the obligation of market-based procurement. Reason: grid restoration is essentially the responsibility of the transmission system operators. Such exceptions are to be reviewed at least every three years. Distribution system operators nevertheless remain involved, because many switching operations, loads, decentralised generators and critical consumers are located in their grids.

Testing obligations are also regulated. Under the EU network code, each provider that delivers black start with a power-generating facility must test the capability at least every three years. A corresponding obligation applies to HVDC systems with black-start capability. In Germany, the terms and conditions for providers of restoration ancillary services approved by the Federal Network Agency in 2020 specify these requirements in more detail. They provide for regular checks, additional tests after repairs or longer outages, operational trials and joint simulator training with the transmission system operator. A manufacturer's statement alone is therefore not enough. Black-start capability must be demonstrated and maintained in operation.

Economic particularities

Black start is rarely needed, but in an emergency it has a very high value. The electricity price does not reflect this value. Black-start capability causes additional investment and obligations: starting units, battery systems, control technology, tests, personnel readiness, documentation, fuel logistics and special protection concepts. Without separate procurement and remuneration, there would be little incentive to maintain such capabilities permanently. The costs of the transmission system operators generally flow into grid costs.

Plants also change over the years. A power plant is decommissioned, a storage system is used differently, a control system behaves differently after a software update, a grid path is lost through conversion work, or experienced personnel leave the operation. Black-start capability is therefore not a permanent entry in a plant list. Recurring tenders, contracts and tests are intended to ensure that it actually remains available.

Why black start is becoming more difficult and more important with the energy transition

In the past, large synchronous generators were available in many regions. They supplied voltage, frequency, short-circuit power and rotating mass, without these properties being visible as separate products. If such plants are operated less often or decommissioned, their system functions must be provided in other ways. At the same time, wind energy, photovoltaics, storage, electrolysers, heat pumps and charging infrastructure change the power flows. Many of these facilities are connected to the distribution grid via power electronics. This can support restoration if the facilities are grid-forming, controllable and integrated into the plans. It can make restoration harder if they switch off under unstable conditions, communication paths are missing or grid operators know too little about the state of the facilities.

The electrification of heating, transport and industry makes more consumers dependent on the power grid. Charging parks, heat pumps or electrolysers must not start up uncontrolled during restoration. When controlled in a targeted way, however, they can help absorb generation in an island grid or stabilise frequency and voltage. This requires technical controllability and clear responsibilities.

Common misunderstandings

  • Decentralised does not automatically mean easier to restore: many small plants without coordinated control can create additional uncertainty during restoration. Protection devices trip, converters wait for a grid reference, and feed-in fluctuates with the weather. Decentralisation does not replace restoration planning.

  • Batteries are not automatically black-start units: they must deliver defined voltage, stable frequency, sufficient reactive power and overload capability, fit the grid protection and store enough energy for the whole restoration step. A storage system that was discharged in the market shortly before the disturbance may fail. If it is to serve as a black-start resource, it needs operating rules for a minimum state of charge.

  • Emergency generators do not start the public grid: a hospital diesel generator protects the hospital, not the distribution grid. It is usually deliberately wired so that it does not feed power back into de-energised lines, so that nobody is endangered. Feeding into the grid requires synchronisation, coordinated grid protection and the grid operator's approval.

  • A grid cannot be started up like a single device: depending on the damage, extent, plant availability, communication situation and grid structure, restoration can take many hours. If equipment is damaged, fuel is lacking or telecommunications fail, it takes longer.

Black start and neighbouring terms

Grid stability describes the ability of the running system to keep frequency, voltage and synchronism within permissible limits. Black start only comes into play when this stability has been lost on a large scale or an area has become de-energised. Balancing power stabilises the running system; black start rebuilds a failed system. Flexibility can be useful during restoration but does not replace the grid-forming starting capability. Grid reserve, capacity reserve and redispatch are intended to prevent bottlenecks and stability problems in advance. Black start is held in readiness for the case that this precaution was not enough. Resilience encompasses avoiding, limiting and withstanding disturbances as well as recovering afterwards. Black-start capability belongs to the last link of this chain and is one building block of resilience, not its entire content.

What the term makes visible

Security of supply cannot be measured by the amount of energy generated alone. A plant can deliver many kilowatt-hours a year and still be hardly usable for restoration. Another runs rarely but is indispensable for the first start-up path. A power plant fleet can generate enough electricity on balance and still offer too little restoration capability. Those who discuss decommissioning, power plant strategies, storage subsidies or grid-forming converters should therefore not treat black-start capability as a marginal detail. However, it is also no argument for preserving arbitrary old plants permanently. The factual question is: which restoration functions are needed in which region, what alternatives exist, how are they tested, and who bears responsibility and costs?

Black start is thus neither an ordinary restart nor a blanket crisis resilience. The term describes the proven capability to build up orderly grid operation again from a de-energised state: first locally, then via defined grid paths and finally through the synchronisation of larger parts of the grid.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 16:01

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.