Grid code requirements and advanced power electronics solutions [Part 4/8: Ancillary services]

After the introduction of grid codes, this fourth article will discuss ancillary services.

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, several types of grid connected devices including power electronics solutions and suppliers of distributed energy resources 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 and distribution system operators to keep the system precisely in balance between supply and demand in real time, to stabilise 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.

Need for 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.

Ancillary services can also be defined as the services required to support the delivery of electric power from the seller to the purchaser. These being the services needed for maintaining reliable operations of the interconnected electrical grid, securing an stable operation of the electric power system or recovering system security, ensuring power supply, and meeting in real time several technical requirements including voltage and frequency limits.

Technical challenges of modern electric power systems

Modern electric power systems face many challenges regarding the reliability, security and quality of the electric power supply. To overcome all these challenges and improve the operation of the electric power system ancillary services are required.

Some of the main technical challenges are related to system stability including frequency, voltage and rotor angle stability. Electric power system stability is the ability of a power system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most system variables bounded so that practically the entire system remains intact. Disturbances of the system may be of various types like sudden changes of load, the sudden short circuit between line and ground, line-to-line faults, all three line faults, switching, etc. 

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.

Ancillary services and grid codes

As the electricity generation capacity continues to increase globally, including a higher share of renewable generation, and more nonlinear and other challenging loads are connected to the electrical grids, stricter requirements regarding grid connection, operation and balancing of generation (renewable or non-renewable) and consumption (loads) are introduced by TSOs and DSOs to ensure that electric power systems remain reliable and robust. These requirements are compiled into grid codes. 

Grid codes typically include the necessary ancillary services that facilities have to comply with in order to achieve grid code compliance.

Technical requirements for ancillary services

Technical requirements for ancillary services are determined by the need of system operators to maintain the reliability, security and quality of the electric power supply. System operators manage the electric power system by obtaining contributions (ancillary services) as needed from service producers and carrying out the technical management of the system, while making sure there is a suitable level of security, adding their own share (implementation of controls, load dispatching function, etc.) and thus elaborating the final system services.

Controlling frequency and voltage has always been one of the main technical requirements for ancillary services and a critical task in operating an electric power system. Following the liberalisation in many countries of the electricity sector, system operators were able to obtain these support services from other participants in the electricity market. Since the liberalisation process has progressed independently in different regions and each electric power system has its own specific characteristics, technical definitions for ancillary services can vary considerably. Ancillary services are defined differently depending on the electric power system and on the regulatory framework in which they are implemented.

The next article of this series will discuss power quality improvement capabilities.

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About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, communications, sales and business development leader who holds since 2002 a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. Author of over a hundred technical publications, he delivers numerous presentations each year at major international trade shows and conferences. He has been a leading expert at several management positions at General Electric, Alstom Grid and Areva T&D, and he is currently working at Merus Power Plc.