Hydroelectric generating sets controlled by conventional, mechanical, temporary-droop governors respond rather slowly to changes in system frequency and do not, therefore, realise their full potential as spinning reserves. Because of this, the electrohydraulic governor system described in this paper was developed for on-site evaluation of control strategies on a 32.5 MW set at Sloy power station, and to complement earlier work on a hybrid simulation of the plant. It is arranged so that it can be used as an alternative to the mechanical governor, and changeover from one to the other can be effected in a few minutes. Frequency-response test data are presented. Also shown are the results of a series of system-splitting tests in each of which a substantial area of West Scotland was supplied by the Sloy set when isolated from the Grid system. An electronic double-derivative governor is shown to improve greatly the response of the generator to frequency changes while at the same time preserving operational stability