The voltages in the power system are maintained at a technically and commercially optimal level during both normal and disturbance situations. The voltage level of the grid tends to change as the load situation of the grid changes.
The frequency of the grid is influenced through active power adjustment, whereas the voltage of the grid is influenced through reactive power adjustment. The objective of voltage level and reactive power adjustment is to prevent overvoltage and undervoltage, to achieve nominal voltages specified in agreements (110 kV grid) and to minimise the grid losses. The voltage level in the transmission grid is adjusted by using reactors and capacitors. The voltage ratio between different voltage steps is controlled with on-load tap changers of transformers.
Instantaneous reactive power reserve is also needed in order to secure the technical functioning of the power system and to restore the power system to the normal state after disturbance situations. The reactive power reserves of the main transmission grid are located in spinning generators. Reactors and capacitors also serve as reserves. Reactive power reserves are activated automatically when the frequency in the grid decreases as a result of a disturbance. The connecting parties are paid a compensation for reactive power reserves reserved in generators.