The system frequency at 50 hertz (Hz) has to be controlled within the specified limits of +0.5Hz by balancing power supply and demand. Exact balancing is not possible at all times, leading to changes in frequency as variations in the total energy flow are absorbed and returned by the large reservoir of energy afforded by the combined rotating masses of the large turbine-generating plant in the conventional power stations. This plant, locked together in synchronism, rotates at 3000 revolutions per minute (rpm) +30rpm and provides the flexibility necessary for the instantaneous balancing of supply and demand. Unlike other interconnected continental power systems, the British system is an island system connected only by high voltage direct current (HVDC) links to neighbouring systems. HVDC links do not provide frequency control; thus, frequency control has to be exercised entirely within the national system.
Frequency control is but one aspect of guaranteeing power supply quality and is an important but seldom debated aspect of the power supply security question. Increasing levels of variable renewable generation capacity will bring both advantages and disadvantages for the maintenance of good power quality, although much remains to be learned in this respect. The importance of maintaining good power supply quality without voltage dips, surges, harmonics, frequency variations or interruptions in supply, even for milliseconds, does not feature in debates on the future of the industry; yet, without high-quality electrical power supplies, the operation of a modern industrialized society is not possible (Stones, 2003). Poor power quality can have large detrimental effects on industrial processes and in the commercial sector, with substantial costs associated with machine downtime, clean-up costs, product quality and equipment failure. Since this is a subject of specialist interest, however, it is not included for further discussion here.
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