Frequency control: specification requirements, practical problems and possible solutions

An individual machine will, in general, have very little influence on the frequency of a power system as a whole, which is determined at any instant by the net balance of power and demand. Due to variations in load, continuous random fluctuations in power system frequency always occur and the mean frequency will predictably drift away from the nominal value unless corrective action is taken. At a higher level therefore, automatic generation control (AGC) is utilised to control the total power in such a manner as to maintain the system frequency. The authors describe how, in order to do this, a secondary or load frequency control is established over a group of machines whereby a term related to frequency error is applied to all the machines simultaneously (but possibly with various weightings). Such a controller generally incorporates an integral action term. At a higher level, tertiary control allows connected groups of machines to be controlled so as to control the power flows between areas and so obtain the most economic overall operation