Parallel strategy of direct-driven variable-speed constant-frequency wind turbine system with separate DC buses

To increase the power rating of the direct-driven variable-speed constant-frequency wind turbine system, the paper presented a new parallel strategy with multiple converters which are parallel with each other. Each converter had its own separate DC bus. The phenomenon of zero-sequence circulating-currents were analyzed in the three-phase stationary coordinates and the synchronous rotating coordinates respectively. The mathematical model and equivalent circuit were got. It could be seen that the zero-sequence circulating-currents were caused by the zero-axis component discrepancy of three-phase duties and the DC bus voltage discrepancy. Besides, the generator-side zero-sequence circulating-currents were coupled with the line-side ones. Then the zero-sequence circulating-current controllers were designed based on the above analysis. They suppressed the zero-sequence circulating-currents effectively and the problems of the current discrepancy and the three-phase current distortion were overcome well. At last, the simulation and experiment were done based on a direct-driven wind turbine system and the results verified the validity of the above analysis and control strategy.