Control of direct driven offshore wind turbines in a DC-collection grid within the wind farms

Recent research has been directed towards a DC-collection grid for offshore wind farms with an HVDC-connection to shore. The main motivation is to reduce the of number of conversion steps, and avoid the reactive power problem associated with AC-cables. However, this demands for changes in the control systems for the wind turbines, as DC-grids have different characteristics, both with regards to the grid itself, and the behavior of the grid-connected converters. The control of a direct driven wind turbine connected to a DC-collection grid through a voltage source converter is investigated in this paper. And more precisely, a control scheme for regulating the collection grid voltage is employed and evaluated. The behavior is documented through simulations in EMTDC/PSCAD and the consequences of the operation strategy are discussed.