These plants are operated in a frequency sensitive mode, which will result in active power output changes, in response to a change in system frequency, to recover the target frequency. Reserves, regulated by balancing markets, play their role as part of the “solidarity principle” within the whole interconnected area (with many control areas combined)7 and participate in the primary adjustment, regardless of the area where the fault originated. The Union for the Coordination of the Transmission of Electricity (UCTE) recommends a primary control reserve margin of about 2.5% of the total installed capacity in an electricity generation system and that primary reserves are distributed homogeneously over the synchronous area.