Ancillary services (electric power)

The United States Federal Energy Regulatory Commission (FERC) defines the ancillary services as: "those services necessary to support the transmission of electric power from seller to purchaser given the obligations of control areas and transmitting utilities within those control areas to maintain reliable operations of the interconnected transmission system."

Ancillary services are the specialty services and functions provided by the electric grid that facilitate and support the continuous flow of electricity so that supply will continually meet demand. 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, frequency control, spinning reserves and operating reserves. Traditionally ancillary services have been provided by generators, however, the integration of intermittent generation and the development of smart grid technologies have prompted a shift in the equipment that can be used to provide ancillary services.

FERC identifies six different kinds of ancillary services:

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.

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.

Frequency control refers to the need to ensure that the grid frequency stays within a specific range of the nominal frequency. Mismatch between electricity demand causes variations in frequency and control services are required to bring the frequency back to its nominal value and ensure it does not vary out of range.

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.

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Wikipedia