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Stable control of the frequency in an electrical power system is essential to ensure continuity of supply. It is determined by the combined effect of the speed governors fitted to the generating sets throughout the system. The power output response of a steam-driven set to sudden loss in generating capacity is initially fast, but, after a few seconds, loss of boiler pressure reduces the output of the set. On the other hand, the response of a hydroelectric generator, although initially less rapid (due to the need to accelerate the water column), can be maintained at a high level for as long as storage water is available. Hydrosets are therefore particularly useful for system-frequency control provided they are fitted with governors that ensure a fast, but stable, response to a sudden change of load. In practice, governor settings are usually made during commissioning on the basis that the set will be stable when fully loaded and electrically isolated from the rest of the system. Attention, in the paper, is therefore given to the transient speed response of a single, isolated, governed hydrogenerator operating at, or near, full load.