March 8, 2024

Ancillary Services

Table of Contents

Ancillary Services

Ancillary services refer to specialized functions that help maintain grid stability and reliability. These services include frequency regulation, voltage control, reserves and black start capabilities. Ancillary services are essential for ensuring the uninterrupted supply of electricity. They help manage fluctuations in power demand and supply, prevent grid disturbances and support the integration of renewable energy sources, ultimately safeguarding the reliability and quality of electrical services.

Types of ancillary services

Ancillary services can be divided into four categories based on their purpose.

Frequency regulation services

Uninterrupted and balanced power supply is key.

Goal: Maintain a grid’s normal frequency.

Frequency regulation services aim to maintain the frequency of a grid by continuously balancing demand and supply. In North America this frequency is 60 Hertz, in Europe and many other parts of the world it is 50 Hertz. The frequency is kept at 50 Hz by having electricity supply and demand and trying to plan by predicting how much supply and demand exists. And to make adjustments if necessary. This can be done by either increasing demand, supply or utilizing batteries to work in both directions. It is crucial as deviations of 0.2 Hertz above or below 50 Hertz (in Europe) may lead to blackouts. However, if there are still deviations from the balance after planning, balancing services can be purchased to restore the balance.

There are four main methods to regulate a grid’s frequency:

  • Generation control: Generation can be adjusted to maintain the grid’s balance. In case the frequency is too low, generation is increased – vice versa, generation is reduced, when the frequency is too high.
  • Demand response: Demand can be adjusted to maintain the grid’s frequency. Demand response requires demand-side flexibility i.e. consumers that can be controlled to reduce (i.e. load shedding) or increase their demand.
  • Energy storage: Energy storage can adjust both, demand and supply. If more supply is needed storage is discharged. If more demand is needed storage is charged.
  • Balancing energy: In some countries, balancing energy is obtained via a market based on an auction to keep costs low. In Europe, balancing energy is divided into three products: Frequency Containment Reserve (reaction time is less than 30 seconds), Automatic Frequency Restoration Reserve (less than 5 minutes) and Manual Frequency Restoration Reserve (less than 15 minutes).

Voltage support

Goal: Supply sources provide reactive power to the grid to support voltage.

As the name implies, voltage control services maintain the voltage level of an electrical grid within an acceptable range. Overvoltage and undervoltage can damage electrical equipment and disrupt power supply.

Reserve services

Goal: Balance unexpected supply shortages.

Reserve services refer to the capacity set aside within an electrical grid to respond to unexpected changes in electricity demand or supply. In case of outages of power generators or sudden spikes in power demand, reserve services are activated to address the resulting imbalances.

While the methodologies are similar, reserve services differ from frequency regulation services in that reserve services have a longer response time. This can include spinning reserve (sources of supply that are synchronized to the grid frequency and can ramp up to full capacity with 10 minutes). 

Black start services

Goal: Restore power supply after a power failure.

Transmission System Operators (TSOs) are usually responsible for quickly restoring power supply after an outage by starting up large power plants that work autonomously and without external energy supply. This is often referred to as black start services. The most common power plant types suitable for black start are hydroelectric power plants, compressed air storage plants, gas power plants or, increasingly, electricity storage facilities.

How DERs can be used for ancillary services

Distributed energy resources (DERs) are flexible small-scale assets that generate, store or consume energy. By aggregating the flexibility of these assets to create a larger dispatchable unit, such as a virtual power plant (VPP) they can also be used to provide  ancillary services. The benefits of using DERs for ancillary services include:

  • DERs are already being installed in homes at an increasingly rapid pace due to their inherent benefits (e.g. self-sufficiency, reduction in costs and emissions). Pooling their flexibility to provide ancillary services requires limited additional effort and no loss of comfort to homeowners.
  • This reduces dependence on large (fossil fuel) power plants and thus accelerates the energy transition.
  • When connected to an energy management system (EMS), residential DERs can react very quickly, thereby providing flexibility on a large scale quickly. This makes them particularly suited to use cases like frequency containment reserve (FCR).
  •  Since DERs are distributed widely across the grid, they can respond more easily to local bottlenecks, i.e. they provide locational services.

There are, however, also some challenges when utilizing DERs for ancillary services: 

  • Because of their dependency on certain weather conditions (e.g wind and sun) for some assets, DERs are not well-suited to reserve or black start services. 
  • Regulatory issues partially prevent the use of residential DERs for ancillary services by limiting access to certain markets for smaller DER portfolios. This could mean that a VPP would have to aggregate a large number of assets, which increases the barrier to entry.