Performance analysis of off shore wind energy systems using HVDC transmission

The application of high voltage dc (HVDC) transmission for integrating large scale and/or off-shore wind generation systems with the electric grid is attractive in comparison to extra high voltage (EHV) ac transmission due to a variety of reasons. Power electronic converters play a major role. More recently, a more modular scheme that may be termed ´bridge of bridge´ converters (BoBC) has been introduced to realize HVDC systems. The paper gives an overview of HVAC and HVDC connection of wind farm to the grid, with an emphasis on voltage source converter (VSC) based HVDC and BoBC for large wind farms requiring long distance cable connection. Flexible control capabilities of a VSC-based HVDC system enables smooth integration of wind farm into the power grid network while meeting the Grid Code Requirements (GCR). The paper also deals with the control flexibility and fault ride through capability of the bi-polar multi-terminal DC transmission system based on voltage source converters and BoBC used for integration of large-scale offshore wind farms. Issues such as voltage support and short-term frequency stabilization of the AC network utilizing the stored energy in the wind turbine inertia and DC link capacitors are discussed.