Secondary control is a centralised automatic function to regulate the generation in a control area, based on secondary control reserves in order to maintain its interchange power flow at the control programme with all other control areas (and to correct the loss of capacity in a control area affected by a loss of production) and, at the same time, in the case of a major frequency deviation originating from the control area, particularly after the loss of a large generation unit, to restore the frequency to its set value in order to free the capacity engaged by the primary control (and to restore the primary control reserves). In order to fulfil these functions, secondary control operates by the Network Characteristic Method. Secondary control applies to selected generator sets in the power plants comprising this control loop. Secondary control operates for periods of several minutes, and is therefore dissociated from primary control. This behaviour over time is associated with the PI (proportional-integral) characteristic of the secondary controller.