The secondary power control (SC) of a generating unit concerns a change in the power output of a regulated generating unit as requested by the load frequency controller. The quality of this service is determined by the amount of power offered and its enabling rate.
This type of AnS is provided by changing the set value of the generating unit controller.
A certain amount of power output – the secondary control reserve – must be available at the respective generating unit at all times in order to provide this service; the amount depends on the generating unit’s technological parameters. The secondary control reserve must be enabled in full within 10 minutes and must be symmetrical about a base point. The following three arrangements are possible:
The secondary controller sends the required value of active power directly to the regulated generating unit.
The secondary controller sends the total value of the required amount of active power to a fictitious generating unit where it is subsequently dispatched by the control system to individual real generating units (all or certain units in operation).
The secondary controller sends the total value of the required amount of active power to a group of hydro power stations where it is subsequently dispatched by their common control centre to individual generating units.
Rules for this AnS are specified in the Agreement for AnS Provision.