Grid integration of offshore wind farms using multi-terminal DC transmission systems (MTDC)

This paper discusses the control flexibility and fault ride- through capability of the bi-polar multi-terminal DC transmission system based on voltage source converters 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. Regarding control flexibility, two aspects are presented: power sharing between the grid-side converters by any ratio, and provision of an alternative path for the power flow in case of a permanent DC fault or loss of one grid-side converter. In this investigation, the wind farms are based on fixed-speed wind generators, while the converters are modelled as a neutral-point clamped converter using the universal bridge. The investigation was conducted in Matlab/Simulink.