Dual HVDC system with line- and self-commutated converters for grid connection of offshore wind farms

The `Dual´ high voltage direct current (HVDC) transmission system is a new concept for the grid connection of large or several nearby-located offshore wind farms. The Dual HVDC system consists of one line-commutated converter (LCC) and one auxiliary self-commutated voltage source inverter (VSI) at both ends of a point-to-point DC transmission line. The paper begins with an introduction into the operating principle of this novel HVDC concept in conjunction with the control of the wind farm internal AC grid. This includes the highlighting of the features provided by the Dual HVDC as well as the challenges that come with that idea. In this context an important point is the supply of reactive power for the offshore-side LCC. In order to avoid the installation of bulk passive components on platforms in the wind farm, a solution will be proposed allowing reactive power compensation by the VSI in cooperation with the wind turbines with full power inverters. Simulation results for the power transmission with the Dual HVDC concept and the control of the wind farm AC grid will be given. A test-stand for the Dual HVDC has been commissioned, the co-working between the LCC-HVDC and the VSI-HVDC has been successfully tested by experiment.