Provision of ancillary services by electric vehicles and energy storage systems [Part 1/2: Introduction]

The electric power system is a complex system. Operators of the system need a variety of system services to be able to guarantee a secure and reliable operation of the electric power system. Generators, flexible loads, different types of grid connected devices including power electronics solutions and suppliers of distributed energy resources (DER for short) can provide these system services, also known as ancillary services. Some services are set as requirements in grid codes and some services are procured as needed by transmission system operators (TSO for short) and distribution system operators (DSO for short) to keep the system precisely in balance between supply and demand in real time, to stabilize the system, to maintain all the parameters of the electric power system including frequency and voltage within operational limits or to recover the system in case of disturbances or failures.

Ancillary services

Modern electric power systems are made up of different types of generating plants and energy storage systems that handle electricity and feed it through an electrical grid (transmission grid, distribution grid, minigrid or microgrid) to be able to meet the demand for power from the end users. The operators of the grid must transmit electricity to these end users in the instant it is consumed, without interruption, in an efficient, cost-effective and reliable manner.

Ancillary services can be defined in many ways but are often considered as the services and functions necessary to support the secure and reliable operation of the electric power system. These services have varying technical requirements and regulatory frameworks across the world, mainly influenced by the individual deregulation processes that have taken place in different countries. Ancillary services are an important piece of grid operations and have traditionally been provided by integrated electric utilities as part of their normal operations. Back then, these services did not need to be explicitly defined and monetized. The deregulation of electric power systems brought changes in many aspects of ancillary services, for example, how these services are defined, procured, activated and remunerated.

There is an increased interest in providing ancillary services by new actors in the grid business, for example, owners of distributed generation, energy storage systems and intelligent loads, either individually or as virtual power plants (VPP for short). The focus is mainly on participating in different markets based on active power with a specific focus on energy markets. Creating VPPs, by the use of, for example, aggregators, is a very practical way to utilise the capabilities from small generators and consumers.

Types of ancillary services

There are three main groups of ancillary services these being services related to frequency support, to voltage support and to system restoration support.

• Frequency support services are required for the short-term balance of energy and frequency of the electric power system, and they include frequency regulation and operational reserves. These services are usually provided by generators, flexible loads and energy storage systems.
• Voltage support services are required for maintaining the electric power system voltage within the prescribed limits during normal operation and during disturbances by keeping the balance of generation and consumption of reactive power. The main targets of these services are voltage profile management and maintaining voltage stability.
• System restoration support services are required to return the electric power system to normal operation after a blackout. They include black start and islanding operation.

There are also some other ancillary services closely related to system operability and stability like power oscillation damping, subsynchronous resonance damping, congestion management and grid loss compensation that help to maintain the reliability of the system and its operation within pre-defined limits safely.

The next article of this series will discuss the use of electric vehicles and energy storage systems for providing ancillary services.

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About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, sales, business development and project leader who holds since 2002 a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. He has been a leading expert at several management positions at Merus Power, General Electric, Alstom Grid and Areva T&D, and he is currently working at Circutor.