Multi-terminal DC (MTDC) system for wind farms powered by doubly-fed induction generators (DFIGs)

Of the environmentally friendly, renewable energy sources of the future, wind turbines are, cost-wise, amongst the most competitive. Doubly-fed induction generators (DFIGs) are ideal for wind turbines because both their real and reactive powers can be controlled on the rotor side using back-to-back converters at slip (reduced) MVA ratings. This paper shows that there can be a further reduction in component count and total MVA by organizing the back-to-back converters in a multi-terminal DC (MTDC) system. In the MTDC system, the many AC grid-side VSCs (and transformers needed for voltage matching) are merged into a single or a few VSC(s). The MVA of the large VSC is smaller than the sum of the MVAs of VSCs serving individual DFIGs because the wind power is never evenly distributed across the wind farm. Simulations show that optimal capture of wind power and AC voltage regulation are achievable under MTDC control.