Upon the occurrence of a change of an a.c. voltage amplitude of an a.c. power network line in one station A, a computing element generates a first pilot control variable by means of which the control angle of the respective static converter is shifted in terms of a reduction of its d.c. output voltage. In the other station a voltage correction regulator generates a correction variable by compensating the voltage drop in the HVDC transmission line, by means of which the control voltage of the respective static converter is corrected, which, for example, is generated by a pilot control voltage and the output signal of a marginal current or extinction angle regulator. Thus, even in case of an a.c. power network fault-induced voltage dip of the HVDC voltage, a preset value can be maintained for the HVDC current and the voltage dip in the power transmission be reduced. This procedure is independent of which station is operated as rectifier and which as inverter.