Control of DFIG under Unsymmetrical Voltage Dip

Today, doubly fed induction generator (DFIG) gains the dominant market power in the wind energy industry. It provides the benefits of variable speed operation cost-effectively, and can control its active and reactive power independently. Crowbar protection is often adopted to protect the rotor side voltage source converter (VSC) from transient over current during grid voltage dip. But under unsymmetrical voltage dip, the severe problems are not the transient over current, but the electric torque pulsation and DC voltage ripple in the back to back VSCs. This paper develops dynamic models in Matlab/Simulink, investigates the behavior of DFIG during an unsymmetrical voltage dip, and proposes new controllers in separated positive and negative sequence. Methods to separate positive and negative sequence components in real time are also developed, and their responses to unsymmetrical voltage dip are compared. Simulation results prove that the separated positive and negative sequence controller limit the torque pulsation and DC voltage ripple effectively.