Sliding-Mode Control Algorithm for DFIG Synchronization to Unbalanced and Harmonically Distorted Grids

In order to smoothly connect to permanently disturbed grids, DFIG-based wind turbines must precisely synchronize the voltage induced at their open stator with that of the grid. Hence, aiming at addressing the still unpublished task of synchronizing DFIGs to simultaneously unbalanced and harmonically distorted grids, a phase-locked loop (PLL)-less and naturally chatter-free sliding-mode control (SMC) algorithm is proposed. The strategies developed so far were formulated considering synchronous reference frames. However, by designing a stationary reference frame-based solution, decomposition into positive- and negative-sequences and harmonic components is avoided. As a result, a relatively straightforward control structure with strong potential for industrialization is obtained, consisting of a single voltage loop per component with just one parameter to be tuned. The stability and robustness resulting from its application are analytically studied under both uncertainties and disturbances. Additionally, a simple method for rotor positioning, independent of grid disturbances, is provided. Simulation over a 2-MW DFIG model and rapid control prototyping (RCP) over a 7-kW DFIG experimental rig are carried out. In this way, the performance and robustness of the suggested control scheme are validated under both unbalanced and harmonically distorted grid voltage, substantial parameter deviations, and varying wind speed and grid frequency profiles.