Analysis of the control limit for rotor-side converter of doubly fed induction generator-based wind energy conversion system under various voltage dips

For the grid-connected doubly-fed induction generator (DFIG)-based wind energy conversion system (WECS), many improved control algorithms have been developed for the rotor-side converter (RSC) to suppress the overcurrents in the rotor-side under voltage dips. However, such objective can hardly be achieved under severe grid fault conditions because of the limitation of RSCs output voltage. An analysis tool is proposed to estimate the the theoretical control limit of the RSC in suppressing the short-circuit rotor currents during grid faults in this study. The tool is based on the optimisation theory and takes the practical constraints of the RSC into account. To execute the analysis, a simplified DFIG model with decoupled stator and rotor fluxes is presented, and the low-voltage ride through (LVRT) problem can be formulated as an optimisation problem, which intends to suppress the rotor winding currents with voltage constraints. The Pontryagin´s minimum principle is employed to solve the optimisation problem and the results can identify the control limit of the RSC. A case study based on a typical 1.5 MW DFIG-based WECS under various grid voltage dips is carried out to validate the analytical method. The proposed method is also further verified by experimental tests on a scaled 3 KW DFIG system. The results are expected to help the manufacturers to assess and improve their RSC controllers or LVRT measures.