Rotor speed control plays a crucial role in power regulation
At low and medium wind speeds, when the wind turbine is already producing electricity but is still below it's rated output, the aim of power regulation is to get as much energy as possible from the wind. Therefore, the rotor of the wind turbine should be operated at optimal aerodynamic conditions and the optimal tip speed ratio (TSR). The TSR is the speed of the blade at it's tip to the speed of the wind. The rotor is designed and built for a certain TSR. When the rotor rotates at the designed TSR, the airflow onto the rotor blade profile is always optimal. Three-blade rotors are designed with TSR from 8 to 9, i.e. the rotor has its highest efficiency when the tip speed is about 8 times the wind speed. At 6 [m/s] wind speed, the rotor-blade tip moves by 48 [m/s]. If wind speed changes, the tip speed must also change.
To achieve this, a generator with variable speed will be required. The speed of the generator is coupled to the rotor speed via gear reduction (gearbox). The rotor speed is ultimately controlled based on the generator. If one looks closely at the gearbox system, the rotor does not produce speed but a torque (mechanical power), the speed is the result of the mechanical power at the generator. The torque has to be applied in such a way that the desired speed is achieved. Fixed-speed, pitch-controlled wind turbines have been optimizing their performance in the partial load range for years by adjusting the pitch angle to the wind. However, this does not function as effective as speed regulation, because a rotor blade has a twisted profile and the entire blade cannot be exposed to an optimal wind profile, but only a part of it.