Voltage Source Inverter Control for Variable speed wind generator

Isn't the usual way to do a wind turbine to run an induction generator at constant speed plus slip? An induction generator is essentially an induction motor excited by the mains but driven higher than synchronous speed by geared wind torque from the prop. An induction motor becomes a generator when mechanically driven at a speed higher than the synchronous speed. It then delivers power into the mains instead of taking power as it does when used as a motor. The frequency is the same as the mains and controlled by them. The waveform is sinusoidal as in other rotating machinery.

The local controller controls the prop pitch and adjusts the blades to keep the slip speed and thus the torque in bounds. A cutout is used to keep generator from motoring in low wind velocity. The blades are feathered and locked when the wind is out of bounds, too high or too low.

Such a scheme doesn't require an inverter or other high power electronics.

True. But it is probably less efficient. In order to assert this I need to design and optimise a system to see what type of efficiency I can get. Variable speed is more efficient because it is possible to optimise the RPM to maximum torque. This is done by adjusting the load as seen from the wind generator via PWM. It is also possible to feedback energy at lower wind speeds, when Vout is below the grid level. This is done with a boost PWM.