Technical challenges of realising multi-terminal networking with VSC

Within the UK and other European countries, the renewable “green” energy assets near to the load centres are largely exploited. Both national governments and power utilities have now turned their attention off-shore with what promises to be a bountiful supply of wind and tidal energy. Until recently power transmission between a “wind farm” and the receiving ac system was by ac cable. However at distances of greater than a few tens of kilometres the use of ac connections becomes uneconomic and HVDC becomes the only practical means of transmission. Large scale point to point HVDC transmission systems have been in existence since the 1950s and could provide a means of transferring wind generated power to shore. However the local “stochastic” nature of wind power generation coupled with the large geographic scale of the proposed off-shore developments, has led to the proposal that “off-shore” DC grids should be developed utilizing Voltage Source Converter (VSC) technology. As such DC Grids offer opportunities for significant savings in both main plant equipment and undersea power cables whilst, at the same time, reducing the visual and environmental impact of the converter stations and transmission system. This paper outlines how DC Grids might come into being and what challenges this poses for the control and protection of such a system. Whilst the control of a VSC Grid system could be developed in a similar manner to that of a “classic” HVDC scheme an alternative approach is proposed which may better suit fluctuating renewable generation such as wind farms.