Ancillary Services are support services in the power system, particularly, those which are necessary to support the transmission capacity and are essential in maintaining power quality, reliability and security of the grid. An ancillary service could be;
1. Load Following and Frequency Regulating, and contingency reserves.
Load following and frequency regulating ancillary service is offered by plants that can respond to the continuous variation of the power system demand. Usually, this is offered by power plants that are robust and can respond quickly to the variations of load and system frequency.
Contingency Reserve, on the other hand, are plants that are intended to take care of the loss of the synchronized generating unit, specially, the largest one. Contingency reserves can either be a spinning reserve (also called hot standby reserve) or a back-up reserve (also called cold standby reserve). Spinning reserve are generator/s which is synchronized to the grid and ready to take the load, in case of the loss of any synchronized generator. Back-up reserve, on the other hand, refers to a generating unit that has fast start capability and synchronizes to the grid to provide its declared capability.
Interruptible Load can also be offered an ancillary service since it can take care of the loss of a generating unit by dropping itself from the transmission system.
2. Reactive Power Support.
Reactive Power Support is ancillary service that takes care of the supply of inductive and reactive load in the transmission system. It could be an inductive shunt reactor, shut capacitor or synchronous condenser, or a generating unit that can absorb or supply Mvar from and to the system.
3. Black Start Capability
Black Start Capability is the ancillary service that is offered by power plants that has a capability to start-up by itself and power-up the associated sub-grid in the event of total grid black-out.
Like any other generating unit that sell its output energy to the consumer and Spot Market, Ancillary Services are also offered and traded for a fee.