Analysis and Augmented Model-Based Control Design of Distributed Generation Converters With a Flexible Grid-Support Controller

Supporting the host grid during voltage dips has become a major connection requirement for large distributed generation units. Because most of the grid faults are unsymmetrical, the recently developed grid codes suggest the injection of a flexible positive- and negative-sequence reactive current components proportional to the magnitude of the voltage dip at the point of common coupling. However, detailed dynamic analysis of the augmented grid-connected converter with the flexible positive- and negative-sequence current injection function and the characterization of the impact of the grid strength, converter control parameters, and proportionality constants used in the reference current generation block are not reported in the literature. To fill in this gap, first, a multi-stage linear model of the augmented nonlinear system dynamics is developed, and the small-signal stability analysis is performed on the system dynamic behavior before, during, and after the fault. The effects of different system and control parameters are studied and characterized. Second, a new and effective model-based controller design method is proposed to maintain the system stability during and after the fault with the consideration of the mutual interaction among different system controllers. Finally, the time-domain simulations and laboratory experiments validate the accuracy and effectiveness of the proposed control method.