Power system operations and control

The term power system control describes actions taken in response to unplanned disturbances (e.g., changes in demand or equipment failures) in order to provide reliable electric supply of acceptable quality.

This variability, coupled with uncertain future power demand and the need to accommodate possible generation and transmission failures requires scheduling of operating reserves that are not expected to produce electricity, but can be dispatched on a very short notice.

[1] Some units have unique features that require their commitment much earlier: for example, the nuclear power stations take a very long time to start, while hydroelectric plants require planning of water resources usage way in advance, therefore commitment decisions for these are made weeks or even months before prior to the delivery.

[2] The decisions ("economic dispatch") are based on the dispatch curve, where the X-axis constitutes the system power, intervals for the generation units are placed on this axis in the merit order with the interval length corresponding to the maximum power of the unit, Y-axis values represent the marginal cost (per-MWh of electricity, ignoring the startup costs).

This system redispatch change is controlled in real-time by the central operator issuing directives to market participants that submit in advance bids for the increase/decrease in the power levels.

Due to the centralized nature of redispatch, there is no delay to negotiate terms of contracts; the cost incurred are allocated either to participants responsible for the disruption based on preestablished tariffs or in equal shares.

[12] In the minutes prior to the delivery, a system operator is using the power-flow study algorithms in order to find the optimal power flow.

Small mismatches between the total demand and total load are typical and initially are taken care of by the kinetic energy of the rotating machinery (mostly synchronous generators): when there is too much supply, the devices absorb the excess, and frequency goes above the scheduled rate, conversely, too much demand causes the generator to deliver extra electricity through slowing down, with frequency slightly decreasing,[13] not requiring an intervention from the operator.

Secondary control uses both the spinning and non-spinning reserves, with balancing services deployed within minutes after disturbance (hydropower plants are capable of an even faster reaction).