Modeling and slip control of a doubly fed induction wind turbine generator

Doubly fed induction generators (DFIGs) are widely used in wind generation. The possibility of getting a constant frequency AC output from a DFIG while driven by a variable speed prime mover improves the efficacy of energy harvest from wind. In the power lab at North Dakota State University (NDSU), the wind turbine is substituted by a variable-speed DC motor which dives a DFIG. A wound rotor induction motor is converted to a DFIG by injecting a three-phase voltage to the rotor at various frequencies. The stator is connected to a three-phase resistance load. Both computer simulation and experiments are performed to demonstrate the frequency, voltage and power relationships between the rotor and the stator. Using the relationship between the stator and rotor voltages, a PWM based slip control scheme together with volts/Hz control for the rotor side converter is developed and the performance verified in PSIM. The DFIG with the proposed control scheme generates a constant voltage with constant frequency at the stator. The experiments, simulation and analysis help students understand DFIG operation and PWM control.