A novel control strategy for operating grid connected Doubly Fed Induction Generators under unbalanced grid voltage

The wind turbine using doubly fed induction generators (DFIGs) is becoming a popular concept because of its improved power quality, high energy efficiency, and controllability. In DFIGs unbalanced three phase stator voltage causes a number of problems such as over current, unbalanced current, reactive power pulsation, and stress on mechanical components from torque pulsations. In this work we are proposing a technique that has the objective to keep the generator connected to the grid in the case of a grid failure. Here a three phase voltage is injected to the rotor circuit with the help of a sinusoidal PWM inverter there by the air gap flux can maintain a constant value at fault condition. This control strategy enhances the standard speed and reactive power control with controller that can compensate for the problems caused by unbalanced grid by balancing the stator current and eliminating the torque and reactive power pulsations. Thus DFIGs can stay connected to the grid under fault conditions also.