Ancillary services (electric power)

Last updated

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.

Contents

"Ancillary services are all services required by the transmission or distribution system operator to enable them to maintain the integrity and stability of the transmission or distribution system as well as the power quality". [1]

Ancillary services are specialty services and functions provided by actors within the electric grid 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 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 and consumption units. [2]

Types of ancillary services

There are two broad categories of ancillary services:

Other types of ancillary services provision include:

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. [3]

If we have a graph for a generator where frequency is on the vertical axis and power is on the horizontal axis:

where Pm 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. [4]


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 regulators. [5]

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 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 is a generator that can quickly be dispatched to ensure that there is sufficient energy generation to meet load. Spinning reserves 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. [6] Wind turbines with variable-speed generators have the potential to add synthetic inertia to the grid and assist in frequency control. [7] [8] [9] CAISO 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. [10] [11] 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 of a wind turbine rotor. [12] Similar requirements came into effect in Europe in 2016. [13]

Electric vehicles

Plug-in electric vehicles 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 (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. According to the study cited in the reference, [14] which compared the profitability of offering operating reserve ancillary service with the sales of V2G energy from a fleet of vehicles, providing operating reserve regulation service is more profitable than selling V2G energy alone. However, the technologies to utilize electric vehicles to provide ancillary services are not yet widely implemented, but there is much anticipation of their potential. [15]

See also

Related Research Articles

<span class="mw-page-title-main">Distributed generation</span> Decentralised electricity 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 as distributed energy resources (DER).

<span class="mw-page-title-main">Electric power industry</span> Industry that provides the production and delivery of electric energy

The electric power industry covers the generation, transmission, distribution and sale of electric power to the general public and industry. The commercial distribution of electric power started in 1882 when electricity was produced for electric lighting. In the 1880s and 1890s, growing economic and safety concerns lead to the regulation of the industry. What was once an expensive novelty limited to the most densely populated areas, reliable and economical electric power has become an essential aspect for normal operation of all elements of developed economies.

<span class="mw-page-title-main">Vehicle-to-grid</span> Vehicle charging system that allows discharge and storage of electricity

Vehicle-to-grid (V2G) describes a system in which plug-in electric vehicles (PEVs) sell demand response services to the grid. Demand services are either delivering electricity to the grid or reducing the rate of charge from the grid. Demand services reduce the peaks in demand for grid supply, and hence reduce the probability of disruption from load variations. Vehicle-to-load (V2L) and Vehicle-to-vehicle (V2V) are related, but the AC phase is not synchronised with the grid, so the power is only available to "off-grid" load.

A microgrid is a local electrical grid with defined electrical boundaries, acting as a single and controllable entity. It is able to operate in grid-connected and in island mode. A 'stand-alone microgrid' or 'isolated microgrid' only operates off-the-grid and cannot be connected to a wider electric power system. Very small microgrids are called nanogrids.

<span class="mw-page-title-main">Synchronous condenser</span> Machinery used to adjust conditions on the electric power transmission grid

In electrical engineering, a synchronous condenser is a DC-excited synchronous motor, whose shaft is not connected to anything but spins freely. Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid. Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to adjust the grid's voltage, or to improve power factor. The condenser’s installation and operation are identical to large electric motors and generators.

Dynamic Demand is the name of a semi-passive technology to support demand response by adjusting the load demand on an electrical power grid. It is also the name of an independent not-for-profit organization in the UK supported by a charitable grant from the Esmée Fairbairn Foundation, dedicated to promoting this technology. The concept is that by monitoring the frequency of the power grid, as well as their own controls, intermittent domestic and industrial loads switch themselves on/off at optimal moments to balance the overall grid load with generation, reducing critical power mismatches. As this switching would only advance or delay the appliance operating cycle by a few seconds, it would be unnoticeable to the end user. This is the foundation of dynamic demand control. In the United States, in 1982, a (now-lapsed) patent for this idea was issued to power systems engineer Fred Schweppe. Other patents have been issued based on this idea.

Doubly fed electric machines, also slip-ring generators, are electric motors or electric generators, where both the field magnet windings and armature windings are separately connected to equipment outside the machine.

<span class="mw-page-title-main">Dispatchable generation</span> Sources of electricity that can be used on demand

Dispatchable generation refers to sources of electricity that can be programmed on demand at the request of power grid operators, according to market needs. Dispatchable generators may adjust their power output according to an order. Non-dispatchable renewable energy sources such as wind power and solar photovoltaic (PV) power cannot be controlled by operators. Other types of renewable energy that are dispatchable without separate energy storage are hydroelectric, biomass, geothermal and ocean thermal energy conversion.

<span class="mw-page-title-main">Electric power system</span> Network of electrical component deployed to generate, transmit & distribute electricity

An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the electrical grid that provides power to homes and industries within an extended area. The electrical grid can be broadly divided into the generators that supply the power, the transmission system that carries the power from the generating centers to the load centers, and the distribution system that feeds the power to nearby homes and industries.

<span class="mw-page-title-main">Availability-based tariff</span> Pricing system used in India for unscheduled electric power transactions

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 term network stability as well as incentives and dis-incentives to grid participants against deviations in committed supplies as the case may be.

Droop speed control is a control mode used for AC electrical power generators, whereby the power output of a generator reduces as the line frequency increases. It is commonly used as the speed control mode of the governor of a prime mover driving a synchronous generator connected to an electrical grid. It works by controlling the rate of power produced by the prime mover according to the grid frequency. With droop speed control, when the grid is operating at maximum operating frequency, the prime mover's power is reduced to zero, and when the grid is at minimum operating frequency, the power is set to 100%, and intermediate values at other operating frequencies.

<span class="mw-page-title-main">Electrical grid</span> Interconnected network for delivering electricity from suppliers to consumers

An electrical grid is an interconnected network for electricity delivery from producers to consumers. Electrical grids consist of power stations, electrical substations to step voltage up or down, electric power transmission to carry power over long distances, and finally electric power distribution to customers. In that last step, voltage is stepped down again to the required service voltage. Power stations are typically built close to energy sources and far from densely populated areas. Electrical grids vary in size and can cover whole countries or continents. From small to large there are microgrids, wide area synchronous grids, and super grids.

<span class="mw-page-title-main">Wide area synchronous grid</span> Regional electrical grid

A wide area synchronous grid is a three-phase electric power grid that has regional scale or greater that operates at a synchronized utility frequency and is electrically tied together during normal system conditions. Also known as synchronous zones, the most powerful is the Northern Chinese State Grid with 1,700 gigawatts (GW) of generation capacity, while the widest region served is that of the IPS/UPS system serving most countries of the former Soviet Union. Synchronous grids with ample capacity facilitate electricity trading across wide areas. In the ENTSO-E in 2008, over 350,000 megawatt hours were sold per day on the European Energy Exchange (EEX).

<span class="mw-page-title-main">Automatic generation control</span> System adjusting the output of electric generators

In an electric power system, automatic generation control (AGC) is a system for adjusting the power output of multiple generators at different power plants, in response to changes in the load. Since a power grid requires that generation and load closely balance moment by moment, frequent adjustments to the output of generators are necessary. The balance can be judged by measuring the system frequency; if it is increasing, more power is being generated than used, which causes all the machines in the system to accelerate. If the system frequency is decreasing, more load is on the system than the instantaneous generation can provide, which causes all generators to slow down.

Inertial response is a property of large synchronous generators, which contain large synchronous rotating masses, and which acts to overcome any immediate imbalance between power supply and demand for electric power systems, typically the electrical grid. Due to the ever existing power imbalance between mechanical power supply and electric power demand the rotational frequency of the rotating masses in all synchronous generators in the grid either speed up and thus absorb the extra power in case of an excess power supply, or slow down and provide additional power in case of an excess power demand. This response in case of a synchronous generator is built-in into the design and happens without any external intervention or coordination, providing the automatic generation control and the grid operator with valuable time to rebalance the system The grid frequency is the combined result of the detailed motions of all individual synchronous rotors in the grid, which are modeled by a general equation of motion called the swing equation.

<span class="mw-page-title-main">Variable speed wind turbine</span> Type of wind turbine

A variable speed wind turbine is one which is specifically designed to operate over a wide range of rotor speeds. It is in direct contrast to fixed speed wind turbine where the rotor speed is approximately constant. The reason to vary the rotor speed is to capture the maximum aerodynamic power in the wind, as the wind speed varies. The aerodynamic efficiency, or coefficient of power, for a fixed blade pitch angle is obtained by operating the wind turbine at the optimal tip-speed ratio as shown in the following graph.

<span class="mw-page-title-main">Synchronverter</span> Type of electrical power inverter

Synchronverters or virtual synchronous generators are inverters which mimic synchronous generators (SG) to provide "synthetic inertia" for ancillary services in electric power systems. Inertia is a property of standard synchronous generators associated with the rotating physical mass of the system spinning at a frequency proportional to the electricity being generated. Inertia has implications towards grid stability as work is required to alter the kinetic energy of the spinning physical mass and therefore opposes changes in grid frequency. Inverter-based generation inherently lacks this property as the waveform is being created artificially via power electronics.

Wide-area damping control (WADC) is a class of automatic control systems used to provide stability augmentation to modern electrical power systems known as smart grids. Actuation for the controller is provided via modulation of capable active or reactive power devices throughout the grid. Such actuators are most commonly previously-existing power system devices, such as high-voltage direct current (HVDC) transmission lines and static VAR compensators (SVCs) which serve primary purposes not directly related to the WADC application. However, damping may be achieved with the utilization of other devices installed with the express purpose of stability augmentation, including energy storage technologies. Wide-area instability of a large electrical grid unequipped with a WADC is the result of the loss of generator rotor synchronicity, and is typically envisioned as a generator oscillating with an undamped exponential trajectory as the result of insufficient damping torque.

<span class="mw-page-title-main">Tehachapi Energy Storage Project</span> Utility-scale battery energy storage system in California, U.S.

The Tehachapi Energy Storage Project (TSP) is a 8MW/32MWh lithium-ion battery-based grid energy storage system at the Monolith Substation of Southern California Edison (SCE) in Tehachapi, California, sufficient to power between 1,600 and 2,400 homes for four hours. At the time of commissioning in 2014, it was the largest lithium-ion battery system operating in North America and one of the largest in the world. TSP is considered to be a modern-day energy storage pioneer with significant accomplishments that have proven the viability of utility-scale energy storage using lithium-ion technology. While originally envisioned as a research and development project, TSP operated as a distribution-level resource for SCE and for calendar year 2020, SCE reported that TSP operated in the wholesale energy market with revenue exceeding operating and maintenance costs. In 2021, SCE began the decommissioning of TSP, which was followed by formal decommissioning by state regulators in 2022. The physical dismantlement of TSP is expected to be completed by the end of 2022.

Power system operations is a term used in electricity generation to describe the process of decision-making on the timescale from one day to minutes prior to the power delivery. The term power system control describes actions taken in response to unplanned disturbances in order to provide reliable electric supply of acceptable quality. The corresponding engineering branch is called Power System Operations and Control. Electricity is hard to store, so at any moment the supply (generation) shall be balanced with demand. In an electrical grid the task of real-time balancing is performed by a regional-based control center, run by an electric utility in the traditional electricity market. In the restructured North American power transmission grid, these centers belong to balancing authorities numbered 74 in 2016, the entities responsible for operations are also called independent system operators, transmission system operators. The other form of balancing resources of multiple power plants is a power pool. The balancing authorities are overseen by reliability coordinators.

References

  1. ↑ Thermal Working Group .(2004). Ancillary Services Unbundling Electricity Products – an Emerging Market. retrieved form
  2. ↑ Ribó-Pérez, David; Larrosa-López, Luis; Pecondón-Tricas, David; Alcázar-Ortega, Manuel (January 2021). "A Critical Review of Demand Response Products as Resource for Ancillary Services: International Experience and Policy Recommendations". Energies. 14 (4): 846. doi: 10.3390/en14040846 . hdl: 10251/176164 .
  3. ↑ Rebours, Yann G., et al. "A survey of frequency and voltage control ancillary services—Part I: Technical features." Power Systems, IEEE Transactions on 22.1 (2007): 350-357.
  4. ↑ Glover, J. Duncan, et al. Power System Analysis & Design. 6th ed., Cengage Learning, 2017.
  5. ↑ Ahmadimanesh, A., and M. Kalantar. "A novel cost reducing reactive power market structure for modifying mandatory generation regions of producers." Energy Policy 108 (2017): 702-711.
  6. ↑ Sortomme, Eric, and Mohamed A. El-Sharkawi. "Optimal scheduling of vehicle-to-grid energy and ancillary services." Smart Grid, IEEE Transactions on 3.1 (2012): 351-359.
  7. ↑ Lalor, Gillian, Alan Mullane, and Mark O'Malley. "Frequency control and wind turbine technologies." Power Systems, IEEE Transactions on 20.4 (2005): p. 1905-1913.
  8. ↑ "The utilization of synthetic inertia from wind farms and its impact on existing speed governors and system performance". ELFORSK. 2013. p. 6 (Summary). Archived from the original on 21 April 2017. Retrieved 18 April 2017. Installing wind turbines with synthetic inertia is a way of preventing this deterioration.
  9. ↑ Early, Catherine (30 September 2020). "How a Single UK Turbine Could Prove a New Use Case for Wind Power". www.greentechmedia.com. Archived from the original on 3 October 2020.
  10. ↑ Balaraman, Kavya (13 March 2020). "Wind plants can provide grid services similar to gas, hydro, easing renewables integration: CAISO". Utility Dive. Archived from the original on 2020-03-16.
  11. ↑ Tuel Wind Power Plant Test Results CAISO , 2019
  12. ↑ Fairley, Peter (7 November 2016). "Can Synthetic Inertia from Wind Power Stabilize Grids?". IEEE. Retrieved 29 March 2017.
  13. ↑ "Network Code on Requirements for Grid Connection Applicable to all Generators (RfG)". ENTSO-E. April 2016. Retrieved 29 March 2017.
  14. ↑ Chandra Mouli, Gautham Ram; Kefayati, Mahdi; Baldick, Ross; Bauer, Pavol (16 October 2017). "Integrated PV Charging of EV Fleet Based on Energy Prices, V2G, and Offer of Reserves". IEEE Transactions on Smart Grid. 10 (2): 1313–1325. doi:10.1109/TSG.2017.2763683. ISSN   1949-3061. S2CID   3503020.
  15. ↑ Joos, G., et al. "The potential of distributed generation to provide ancillary services." Power Engineering Society Summer Meeting, 2000. IEEE. Vol. 3. IEEE, 2000.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.