Ancillary services (electric power)

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.

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.[2]

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.

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:

${\displaystyle slope=-R={\frac {\Delta f}{\Delta P_{m}}}}$

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:

${\displaystyle \beta ={\frac {1}{R_{1}}}+{\frac {1}{R_{2}}}+...+{\frac {1}{R_{n}}}}$

The change in frequency due to a change in power can be found with:

${\displaystyle \Delta f={\frac {\Delta P_{m}}{\beta }}}$

This simple equation can be rearranged to find the change in power that corresponds to a given change in frequency.[4]