The present invention relates to a plant for transmitting electric power between a direct voltage network for High Voltage Direct Current (HVDC), and at least two alternating voltage networks connected thereto through a station with each of the stations being adapted to perform transmission of electric power between the direct voltage network and a respective one of the alternating voltage networks.
In such plants, according to the prior art, primarily uninsulated overhead lines have been used to implement a direct voltage network interconnecting stations which may be located very large distances apart, for example, 1000 km. However, such overhead lines have a significant disturbing influence upon the country and the living things where they exist. The alternative has been to use cables having an inner conductor surrounded by a thick insulating layer formed by a paper impregnated by oil, but such cables are so costly that they are not a realistic alternative to overhead lines. Experiments have been carried out with cables having a conductor and an insulating layer on a polymer base surrounding the conductor for transmitting High Voltage Direct Current (HVDC), but these experiments have revealed great problems in using this type of cable for transmitting High Voltage Direct Current (HVDC), since space charges are created in the insulating layer which may cause breakthroughs and breakdowns in the cables. The space charges are caused by the changes of polarity which the conductor is subjected to when the feeding direction of the power transmitted between the direct voltage network and an alternating voltage network is changed. A result of these problems is that overhead lines have been used for transmitting High Voltage Direct Current (HVDC) over land, and cables having tailor made insulating sheets have been used as sea cables for transmitting between land areas separated by water.