Control of doubly-fed induction generator based wind turbine at synchronous operating point

By direct coupling with a hydro-dynamically controlled gearbox, wind turbine generators can work at a determined speed, e.g., the synchronous operating point, regardless of the rotor speed. The doubly-fed induction generators operate usually away from the synchronous point due to unbalanced thermal stress on the rotor windings and inverter half-bridges. However, at this operation mode, there is no active power flowing in its rotor and thus the converter rating is reduced considerably, especially in MW-turbines. This work introduces methods to increase transient and oscillatory stability of the generator at the operating point without any modification of the machine structure. Moreover, the ability to ride-through the temporary low voltage is provided with a novel positive and negative sequence separation. The control methods are presented with selected simulation and experimental results.