Abstract: A machine set (1) consists of a water-driven turbine (11) and a generator (12) coupled thereto. The generator feeds a network of constant frequency (3) via a frequency-controlled converter (2). The speed reference value (n*) of the machine set is given by means simulating turbine characteristics (4) in dependence on two characteristic quantities (a.sub.o or .PHI.) with optimum efficiency and is finely optimized (n) preferably by a maximum power-point control (5). The upper and lower water level (L.sub.u, L.sub.H) can be preset, in a single or double-action controlled turbine (11) additionally by a control device (6) if one of the two characteristic quantities (a.sub.o or .PHI., respectively) is used as a control signal for the nozzle orifice or the aperture at the guide wheel or rotor.

Abstract: A control circuit for an a-c control element consisting of controlled rectifier elements in conjunction with a transformer in which a separate current transformer is provided for each of the transformer windings and all current transformers of the same respective phase have their secondaries connected together in a bucking circuit, to which a measuring arrangement is connected which measures the peak value of the difference value formed during each half-wave, and the output signal of which is introduced, depending on the polarity of the difference value, into the control circuit of the one or the other branch of the a-c control element.