A low voltage ride through technology for direct-drive wind turbines under unbalanced voltage dips

The response of direct-drive wind turbine converters under unbalanced grid conditions is analyzed. A control strategy in this condition is proposed, which coordinates the converter with the DC side Crowbar circuit during unbalanced voltage dips. Since the direct-drive wind turbines exchange full power with grid via converters, the fluctuation of active power caused by unbalanced voltages will lead to DC-link voltage ripples. The control strategy aims to stabilize the active power outputs. The control model is based on the rotating frame, decomposition of positive and negative sequence vectors. The current limit and complexity of the control system are also taken into account. When operating in the vicinity of rated power, wind turbines can pass through unbalanced voltage dips. Simulation results using MATLAB verify the correctness of the proposed control strategy.