A vector-controlled doubly-fed induction generator for a variable-speed wind turbine application

This paper examines the performance of a vector-controlled doubly-fed induction generator for wind turbine use. The scheme is considered for variable-speed-range applications to produce a greater wind energy capture from varying wind velocity. A major advantage of the doubly-fed scheme over the squirrel-cage generator is the reduced volt-ampere rating of the power converter used to control the rotor current. The singly-fed scheme requires that the power converter is able to carry the full generator output. In order to produce decoupled regulation of active and reactive output power, the rotor current is controlled using field orientation principles. The measurement of rotor position and the grid voltage vector position allows the rotor current to be controlled in a reference frame closely associated with the stator magnetising current. The stator active and reactive power are also measured and fed back into outer P s and Q s control loops. The P s and Qs controllers produce d and q axis rotor current demands for the inner rotor current vector controller. By control of the reactive power loop the generator can be made to produce power at unity power factor. The vector control of rotor current is carried out using a commercial PWM inverter equipped with vector control hardware as standard. The inverter also produces a unity power factor sinusoidal current interface to the grid network. The performance of the scheme is examined using both simulation and experimental results. The decoupling control of active and reactive power is demonstrated and shows the ability to generate power at unity power factor.