A conventional cable comprises three conductive cores, each provided with a respective conductive sheath which are together housed within an outer insulating sheath. The three-core cable is used to transmit electric power in the form of dc current and single-phase ac current. A first one of the conductive cores is used to transmit dc current, while a second conductive core is used to transmit the return dc current. The third conductive core is used to transmit the single-phase ac current. The return ac current is transmitted along all three conductive outer sheaths in parallel. The cable is used to transmit dc current from a wind turbine generator to a substation where it is converted to ac before connecting to the mains electricity grid. The ac current is used to power auxiliary circuitry of the wind turbine generator when the output of the turbine generator falls below a give value.