Primary control (frequency control, primary frequency control) maintains the balance between generation and demand in the network, using turbine speed governors. Primary control is an automatic decentralised function of the turbine governor to adjust the generator output of a unit as a consequence of a frequency deviation/offset in the synchronous area. Primary control should be distributed as evenly as possible over units in operation in the synchronous area. The global primary control behaviour of an interconnection partner (control area/block) may be assessed by the calculation of the equivalent fallout of the area (basically resulting from the fallout of all generators and the self-regulation of the total demand). By the joint action of all interconnected undertakings, primary control ensures the operational reliability for the power system of the synchronous area.