Research on interconnecting offshore wind farms based on multi-terminal VSC-HVDC

The paper describes the use of multi-terminal HVDC transmission system based on voltage source converter (VSC) (VSC-MTDC) for connecting doubly fed induction generator (DFIG) based wind farms over long distance. The operation principles between DFIG and VSC-MTDC of the proposed system are described. Stator flux oriented vector control technology is used to execute AC excitation for DFIG rotor, then independent control of active and reactive power is realized and electricity generation by capturing maximal wind energy is also obtained. The control objective of rectifiers is delivering the generated power from the wind farms to the DC network instantaneously and to keep AC voltage at wind farm bus constant. The control objective of inverters is receiving the power from the DC network and to keep DC voltage constant, and the multi VSCs are controlled coordinately, the relevant mathematical model was established. The parameters optimization based on simplex algorithm and objective function are used to get suitable PI control parameters of VSC-MTDC. Interactions between DFIG and VSC-MTDC link are investigated and simulations performed in PSCAD/EMTDC to verify the proposed control strategy.