Superimposing AC on HVDC cable to improve dielectric strength

In order to improve the transmissivity characteristics of an electric cable (1), particularly a submarine cable, transmitting power by means of a DC voltage, an AC voltage lower than the DC voltage (E1, E2) is superimposed on the DC voltage to improve dielectric strength. The AC voltage may be generated permanently, intermittently for periods of time, or prior to installation of the cable. Converter stations (4, 5) are provided at each end of the cable for transmitting and receiving power, and one station contains a first AC generator (E1) while the other contains a passive or active AC source (E2) which provides a voltage equal and in phase with the first generator (E1) to cancel out the alternating current flow in the loop comprising the cable and converter stations. The AC generators may be separate voltage generators or may be provided by modulation of the control system controlling the power conversion means in the generator.