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Unbalanced control system design for DFIG-based wind turbines

To achieve high performance operation of doubly fed induction generator (DFIG) based wind turbines under unbalanced voltage conditions, a well designed control system is important, otherwise wind turbines will take a risk of being cut out of the power grid. Meanwhile, without proper designed control the power ripples generated from wind turbines will further aggravate the power grid. Under unbalanced conditions, DFIG was modeled in dual synchronous reference frame (SRF), namely the positive one and the negative one, based on which the dual PI current controllers were designed. To implement the dual current control, the sensing variables were divided into positive and negative sequence components with the designed generalized filters, and then the corresponding control in positive and negative SRF were designed respectively. At the same time, to get the dual SRFs oriented to the positive and negative sequence voltage components, a notch filter based phase latch loop (PLL) control was designed. Experimental results on 11 kW DFIG wind turbine test bed validated the control system design.