Definition

Ancillary services ensure a proper operation of the power grid. The grid operators (transmission grid operators and distribution grid operators) are responsible for ancillary services. To ensure a reliable power supply, it is necessary that frequency, voltage, and power load remain within certain limits. This does not happen automatically, but through continuous corrections: namely, ancillary services. The most renowned service is probably the frequency control/balancing energy. Even in the event of a blackout, ancillary services help restart the power grid as quickly as possible. Ancillary services can be divided into four sections: Frequency measure, voltage measure, supply reconstruction, and operational management.

Frequency Measure

System services for frequency maintenance

The frequency within a power grid is either 50 or 60 Hertz. It is important that the frequency be within a defined range so that the power grid does not collapse. In Europe, this is a positive or negative deviation of max. 0.2 Hertz from 50 Hertz. Therefore, power feed-in and power consumption have to be in balance at all times. This also applies in cases of unforeseen events, such as power plant outages, a strong wind period with the resulting high feed-in of wind power, or a sudden increase in electricity consumption. The responsibility for a stable frequency lies with the balancing group managers, who must keep their balancing groups in balance, and with the grid operators, who are responsible for stabilizing the frequency in case balancing groups – and subsequently the frequency itself – are out of balance.

Spinning reserve

In the narrower sense, the spinning reserve is not part of the ancillary services, since it is already created by the inertia of rotating masses: If a heavy mass is set in rotation like a flywheel, it continues for a while, even after the energy supply has been stopped. For the sake of completeness, however, the spinning reserve is often mentioned in connection with ancillary services and is a kind of preliminary stage to balancing energy.

Due to the kinetic energy of their rotors, power plant generators are able to compensate for a short-term power failure. Renewables do not yet provide a spinning reserve. However, it is conceivable that the kinetic energy of large wind turbines could be used for this purpose.

Frequency control / balancing energy

If the effect of the spinning reserve cannot balance the frequency sufficiently, balancing energy is used to restore equilibrium. In some countries, balancing energy is obtained via a market. This is often based on an auction procedure to keep costs low. In countries with a vertically integrated energy industry, such as China or parts of the USA, this task is performed directly by the grid operator. Balancing energy is often divided into different products depending on how fast they have to react. In Germany, for example, balancing energy is divided into three products: Frequency Containment Reserve (FCR: reaction time < 30 seconds), Automatic Frequency Restoration Reserve (aFRR: reaction time < five minutes), and Manual Frequency Restoration Reserve (mFRR: reaction time < 15 minutes).

Sheddable loads

Switchable loads, sometimes also called “sheddable loads”, are loads that are deactivated for a short time in order to guarantee network stability. Who provides these loads and in which procedure the needed power is tendered, varies from country to country. Oftentimes, heavy industries with a high degree of power consumption and automation in their production lines, such as aluminum or steel manufacturers, provide sheddable loads.

System services for voltage compensation

In addition to the frequency, voltage must also be kept within a certain range in order to ensure the safety of persons, equipment, and final consumption devices. In the event of a voltage drop, it is the task of the network operators to contain the voltage instability.

Power factor correction

For alternating current (AC) to maintain its real power, active power and reactive power need to be out of phase by 90°. Transformers, the grid itself or various consumption processes can destabilize the phase relation between active and reactive power, which leads to a loss in voltage. Grid operators need to take measures to ensure stable voltage range, such as providing additional load.

Loss energy

Energy losses occur during the transport of energy. Grid operators have to forecast this power loss and purchase additional amounts of energy on the electricity market accordingly. The amount of power dissipated is influenced by the capacity utilization of the networks and represents one of the largest costs of system services. Loss energy is a by-product of power production or a system efficiency problem, which the TSOs and DSOs try to counteract.

Ancillary services for the reconstruction of supply after a power failure

After a power failure, the power supply must be restored as quickly as possible. Usually, this is the responsibility of the TSO. Power plants that can start up without an external power supply play a central role in rebuilding the supply.

Black start

Many large power plants are thermal power plants. They are able to generate large amounts of electricity but need an external power source to start their electricity production. Without this, neither nuclear nor coal-fired power plants can be put into operation, as there is no energy source for the operation of the cooling and monitoring systems of the reactor and the coal extraction plants.
In contrast to these types of power plants, there are power plants which can carry out a black start. They can start up autonomously and without external energy supply. Power plant types that are suitable for a black start are, for example, hydroelectric power plants, compressed air storage power plants, or gas power plants. Electricity storage facilities are also increasingly being used on a large scale to ensure black start capability.

Ancillary services for operational management and grid bottleneck management

Operational management means controlling and monitoring of the grid by the network operators. This includes the coordination of ancillary services. The expansion of renewables increases the demands placed on grid operators. Different system services are used for these measures.

Redispatch

The redispatch is a means of grid congestion management - especially in grids with a high share of renewable energies. The TSOs consider the expected feed-in and feed-out at grid level on the previous day. They then use this information to analyze the utilization of the electricity grid. The TSOs use this analysis, also known as load flow calculation, to instruct the power plant operators the day before to postpone the planned electricity production. This avoids foreseeable potential grid bottlenecks. This instruction to shift electricity production is called a redispatch.
The redispatch does not change the amount of electricity fed into the grid, but its local distribution. Meanwhile, there are also pilot projects for the use of renewable energy plants in redispatch.

Feed-in management

Feed-in management refers to the regulation of electricity feed-in from renewable energy systems. If sections of the distribution or transmission grid are overloaded and the stability of the grid is threatened, feed-in management comes into play. The grid operator takes renewable energy systems off the grid in the event of a drastic power surplus.

Available capacities and capacity mechanisms

In addition, the provision of electricity without the factual delivery is a popular capacity mechanism. Services of this kind can either be grid reserves or the safety readiness of power plants. This gives these ancillary services a capacity market character. The design of such reserves differs greatly from country to country.

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