Ancillary services are the services necessary to support the transmission of electric power from
generators to consumers given the obligations of control areas and transmission utilities within those control areas to maintain reliable operations of the interconnected
transmission system
:''See Transmission (mechanics) for a car's transmission system''
In telecommunications, a transmission system is a system that transmits a signal from one place to another. The signal can be an electrical, optical or radio signal.
Some transmissi ...
.
Ancillary services are specialty services and functions provided by actors within the
electric grid
An electrical grid is an interconnected network for electricity delivery from producers to consumers. Electrical grids vary in size and can cover whole countries or continents. It consists of:Kaplan, S. M. (2009). Smart Grid. Electrical Power ...
that facilitate and support the continuous flow of electricity, so that the demand for electrical energy is met in real time. The term ancillary services is used to refer to a variety of operations beyond
generation
A generation refers to all of the people born and living at about the same time, regarded collectively. It can also be described as, "the average period, generally considered to be about 20–30 years, during which children are born and gr ...
and
transmission that are required to maintain grid stability and security. These services generally include active power control or frequency control and reactive power control or voltage control, on various timescales. Traditionally, ancillary services have been provided by large production units such as generators. With the integration of more intermittent generation and the development of
smart grid technologies, the provision of ancillary services is extended to smaller
distributed generation
Distributed generation, also distributed energy, on-site generation (OSG), or district/decentralized energy, is electrical generation and storage performed by a variety of small, grid-connected or distribution system-connected devices referred to ...
and consumption units.
Types of ancillary services
There are two broad categories of ancillary services:
* Frequency related: Inertia, Frequency Containment Reserve (FCR), and Automatic Frequency Restoration Reserve (aFRR)
* Non-frequency related:
reactive power and voltage control and congestion management
Other types of ancillary services provision include:
* scheduling and dispatch
* loss compensation
* load following
* system protection
* energy imbalance
Frequency control
Frequency control refers to the need to ensure that the grid frequency stays within a specific range of the nominal frequency. Mismatch between electricity generation and demand causes variations in frequency, so control services are required to bring the frequency back to its nominal value and ensure it does not vary out of range.
If we have a graph for a generator where frequency is on the vertical axis and power is on the horizontal axis:
where P
m is the change in power of the system. If we have multiple generators, each might have its own R. Beta can be found by:
The change in frequency due to a change in power can be found with:
This simple equation can be rearranged to find the change in power that corresponds to a given change in frequency.
Reactive power and voltage control
Consumer loads expect voltage within a certain range, and the regulators require it be within a certain percent of the nominal voltage (for example, in the US it is 土5%).
Reactive power can be used to compensate the voltage drops, but must be provided closer to the loads than real power needs (this is because reactive power tend to travel badly through the grid). Notice that voltage can be controlled also using
transformer taps and
voltage regulator
A voltage regulator is a system designed to automatically maintain a constant voltage. A voltage regulator may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic components ...
s.
Scheduling and dispatch
Scheduling and dispatch are necessary because in most electrical systems energy storage is nearly zero, so at any instant, the power into the system (produced by a generator) must equal the power out of the system (demand from consumers). Since production must so closely match demand, careful scheduling and dispatch are necessary.
Usually performed by the
independent system operator
A regional transmission organization (RTO) in the United States is an electric power transmission system operator (TSO) that coordinates, controls, and monitors a multi-state electric grid. The transfer of electricity between states is considered ...
or transmission system operator, both are services dedicated to the commitment and coordination of the generation and transmission units in order to maintain the reliability of the power grid.
Scheduling refers to before-the-fact actions (like scheduling a generator to produce a certain amount of power the next week), while dispatch refers to the real-time control of the available resources.
Operating reserves
Since production and demand must match so perfectly (see
Scheduling and dispatch), operating reserves help make up the difference when production is too low.
An
operating reserve
In electricity networks, the operating reserve is the generating capacity available to the system operator within a short interval of time to meet demand in case a generator goes down or there is another disruption to the supply. Most power sys ...
is a generator that can quickly be dispatched to ensure that there is sufficient energy generation to meet load.
Spinning reserve
In electricity networks, the operating reserve is the generating capacity available to the system operator within a short interval of time to meet demand in case a generator goes down or there is another disruption to the supply. Most power sys ...
s are generators that are already online and can rapidly increase their power output to meet fast changes in demand. Spinning reserves are required because demand can vary on short timescales and rapid response is needed. Other operating reserves are generators that can be dispatched by the operator to meet demand, but that cannot respond as quickly as spinning reserves, and grid battery storage that can respond within tens of milliseconds, generally faster than even spinning reserve.
Renewable generation
The grid integration of renewable generation simultaneously requires additional ancillary services and has the potential to provide ancillary services to the grid. The inverters that are installed with distributed generation systems and roof top solar systems have the potential to provide many of the ancillary services that are traditionally provided by spinning generators and voltage regulators. These services include reactive power compensation, voltage regulation, flicker control, active power filtering and harmonic cancellation. Wind turbines with variable-speed generators have the potential to add
synthetic inertia to the grid and assist in frequency control.
CAISO
The California Independent System Operator (CAISO) is a non-profit Independent System Operator (ISO) serving California. It oversees the operation of California's bulk electric power system, transmission lines, and electricity market generated a ...
tested the 131 MW Tule wind farm's synchronverter in 2018, and found it could perform some of the grid services similar or better than traditional generators.
Hydro-Québec began requiring synthetic inertia in 2005 as the first grid operator, demanding a temporary 6% power boost when countering frequency drop by combining the power electronics with the
rotational inertia
The moment of inertia, otherwise known as the mass moment of inertia, angular mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is a quantity that determines the torque needed for a desired angular acceler ...
of a
wind turbine rotor.
Similar requirements came into effect in Europe in 2016.
Electric vehicles
Plug-in electric vehicle
A plug-in electric vehicle (PEV) is any road vehicle that can utilize an external source of electricity (such as a wall socket that connects to the power grid) to store electrical power within its onboard rechargeable battery packs, which then ...
s have the potential to be utilized to provide ancillary services to the grid, specifically load regulation and spinning reserves. Plug-in electric vehicles can behave like distributed energy storage and have the potential to discharge power back to the grid through bidirectional flow, referred to as
vehicle-to-grid
Vehicle-to-grid (V2G), also known as Vehicle-to-home (V2H) or Vehicle-to-load (V2L) describes a system in which plug-in electric vehicles (PEV) sell demand response services to the grid. Demand services are either delivering electricity or by red ...
(V2G). Plug-in electric vehicles have the ability to supply power at a fast rate which enables them to be used like spinning reserves and provide grid stability with the increased use of intermittent generation such as wind and solar. The technologies to utilize electric vehicles to provide ancillary services are not yet widely implemented, but there is much anticipation of their potential.
[Joos, G., et al. "The potential of distributed generation to provide ancillary services." Power Engineering Society Summer Meeting, 2000. IEEE. Vol. 3. IEEE, 2000.]
See also
*
National Grid Reserve Service
To balance the supply and demand of electricity on short timescales, the UK National Grid has contracts in place with generators and large energy users to provide temporary extra power, or reduction in demand. These reserve services are needed if ...
*
Availability-based tariff
Availability Based Tariff (ABT) is a frequency based pricing mechanism applicable in India for unscheduled electric power transactions. The ABT falls under electricity market mechanisms to charge and regulate power to achieve short term and long ...
*
Load following power plant
A load-following power plant, regarded as producing mid-merit or mid-priced electricity, is a power plant that adjusts its power output as demand for electricity fluctuates throughout the day. Load-following plants are typically in between base lo ...
*
Virtual power plant
A virtual power plant (VPP) is a Cloud computing, cloud-based distributed power plant that aggregates the capacities of heterogeneous distributed energy resources (DER) for the purposes of enhancing power generation, trading or selling power on the ...
References
{{Reflist
* U.S. Federal Energy Regulatory Commission 1995, Promoting Wholesale Competition Through Open Access Non-discriminatory Transmission Services by Public Utilities, Docket RM95-8-000, Washington, DC, March 29.
* E. Hirst and B. Kirby, “Ancillary Services,” Oak Ridge National Laboratory, Technical Report ORNL/CON 310, February 1996. https://web.archive.org/web/20150225160053/http://web.ornl.gov/~webworks/cpr/rpt/84170.pdf
*Operations, Power Exchange. "Guide to Ancillary Services in the National Electricity Market." (2010).
* ARENAWIRE, “What is Frequency Control Ancillary Services?” February 1996. https://arena.gov.au/blog/what-is-frequency-control-ancillary-services/
Electric power transmission