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This paper presents a robust system-oriented control design approach for distributed generation (DG) converters in microgrids. The conceptual design of the proposed interface is to provide control system robustness against system-level interactions without strict knowledge of complete microgrid system dynamics. To increase the robustness against converter-microgrid interactions, the microgrid system is modeled by a dynamic equivalent circuit, which might include uncertainties induced due to microgrid impedance variation and interactions with the equivalent microgrid bus-voltage. The equivalent microgrid model along with local load interactions and uncertainties are augmented with the DG interface power circuit model to develop a robust H∞ voltage controller. To account for power angle interaction dynamics, an angle feed-forward control approach is adopted, where the angle of the equivalent microgrid bus, as seen by each DG unit, is estimated and used for feed-forward control. Unlike conventional droop controllers, the proposed scheme yields two-degree-of-freedom controller, resulting in stable and smooth power sharing performance over a wide range for the static droop gain and also at different loading conditions. A theoretical analysis and comparative simulation and experimental results are presented to demonstrate the robustness and effectiveness of the proposed control scheme.
Date of Publication: Dec. 2012