Variable rotor-resistance control of wind turbine generators

By adding a variable external resistance to the rotor of an induction generator used in a wind turbine, it is possible to manipulate the torque-speed curve and control the output power. The hardware implementation is accomplished by a set of three-phase external resistors connected in series with the rotor windings. To vary the effective value of the external resistance, a three-phase diode bridge and a DC chopper with a variable duty cycle is commonly used. Thus, the wind turbine generator operates at variable slip above synchronous speed. This method is less flexible than a doubly-fed induction generator, but does not require the use of slip rings to access the rotor circuit. This paper describes the construction of a time-domain electromechanical model for a wind turbine. The model is used to evaluate control strategies for the output power based on stator power, the equivalent circuit, or aerodynamic torque. All are capable of maintaining a constant power beyond rated wind speed. The transient performance of the controllers is also compared and control based on the aerodynamic torque is found to be the fastest, albeit with high transient power deviations.