An apparatus for regulating electrical voltage in multiphase power mains has a control transformer having for each phase of the power mains a primary winding and a control winding having three taps. The second tap is connected centrally between the first and third taps and to one of the ends of the primary winding. This control winding has winding lengths between the first and second taps and second and third taps equal to a whole-number multiple of a winding length of the primary winding. Three circuit elements have inputs connected to the taps. A reactor winding has ends connected to outputs of the first and second circuit elements, while the output of the third circuit element electrically connected to the output of the first circuit element. A further circuit element is connected across the reactor winding, and an output is connected to a center of the reactor winding.