Control of Multi-Terminal VSC-HVDC for Offshore Wind Power Integration

Abstract

Wind power evolution in Northern Europe is foreseen to continue in the future with development of large-scale wind power plants (WPPs) on far offshore. Integration scheme of these WPPs to the onshore grids would grow from point-to-point connection to a transnational multi-terminal network where the transmission capacity serves both to evacuate the wind power and to facilitate power trading between countries. In such a situation, application of multi-terminal VSC-HVDC transmission is considered the favorable technological solution. A control strategy which capable of accommodating different dispatch schemes is however required. This thesis presents a control strategy for dispatching power in the future transnational network situation. The control strategy is developed based on the voltage-margin method and is customized to comply with different dispatch schemes possibly be applied in the future transnational network. The control strategy is implemented on a multi-terminal VSC-HVDC network representing the future transnational network and its capabilities are confirmed through simulation studies for normal and abnormal operations. For normal operation, the capability to dispatch the offshore wind power as well as the traded power among the onshore grids is explored under wind speed changes. For abnormal operation, the capability to interchangeably control the direct voltage upon disconnection of a direct voltage controlling converter is examined. In addition, the compatibility of the control strategy to work with particular low voltage ride-through mechanisms is reported. The control strategy, the multi-terminal network and the simulation studies are implemented in MATLAB/Simulink.