Laboratory demonstration of an offshore grid in the North Sea with DC droop control

The system studied in this article is a multi-terminal HVDC (MTDC) network composed by four voltage-source converter terminals. It is intended to be a model of the future “super-grid” in the North Sea, where Norway, Germany and the UK are interconnected together with offshore wind farms on the east coast of England. The main challenges related with multi-terminal HVDC grids are market and regulatory issues, control and coordination of the grid, and protection and circuit-breakers. This article focuses on the technical challenges, especially those in relation with DC voltage control and power flow balancing. Several studies have proposed different methods for controlling the DC voltage in MTDC grids. This article uses a DC voltage droop control scheme. The system is analysed by means of simulations and experimentally in a down-scaled laboratory model with ratings in the range of 60 kVA. In order to evaluate the robustness of the droop-control strategy, several large disturbances are emulated: abrupt variation in wind production, sudden change in droop line set-points and loss of two converter terminals during full wind production. Finally, the similarities and differences between the experimental set-up and a real system are discussed.