Ancillary services
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.
"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".
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 synchronous 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.
Types of ancillary services
There are two broad categories of ancillary services:- Frequency related: Inertia, Primary frequency control such as Frequency Containment Reserve, secondary frequency control such as and Frequency Restoration Reserve
- Non-frequency related: reactive power and voltage control and congestion management
- system restart
- scheduling and dispatch
- loss compensation
- load following
- system protection
- energy imbalance
Frequency control
During a sudden change of the frequency will initially change according to the following equation:
The equation can be rewritten using the normalized inertial constant :
Where is the total exchange of active power for the entire grid and is the normalized inertia constant for the system.
A system with a small amount of total rotating kinetic energy will be more sensitive to deviations in, as the frequency's derivative will be more significantly affected.
As the total inertia of synchronous grids decreases due to the increasing integration of renewable generation sources, the methodologies and requirements for enabling converter-based generation units to control grid frequency with greater speed and precision are becoming more important.
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 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.
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.Reactive power can be used to compensate the voltage drops, but must be provided closer to the loads than real power needs. Notice that voltage can be controlled also using transformer taps and voltage regulators.
[|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 must equal the power out of the system. 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, while dispatch refers to the real-time control of the available resources.
Operating reserves
Since production and demand must match so perfectly, 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.