This paper proposes a controller tuning methodology for voltage-current droop-based DC-DC converters in DC microgrids to reduce the output capacitance. This minimization is cost saving and implies lower fault currents. However, it leads to higher DC voltage variability during load transients, which requires an output impedance shaping by control means to reduce over or undershoot. The proposed control structure and problem definition simultaneously takes into account that the solution must achieve the impedance shaping, performance and stand-alone stability objectives. This comprises a multi-objective problem which is effectively formulated here and, then, solved by a non-smooth H ∞ optimization technique that tunes all free parameters. For comparison purposes, this tuning methodology is applied to several droop proposals, and the proposed droop is able to reduce the output capacitance of bidirectional buck-type and boost-type half-bridge converters by 37.5% and 23.08%, respectively, with respect to previous proposals. The designs are validated in time and frequency domains by means of theoretical analysis and experimental results on DC microgrid prototypes with bidirectional buck-type or boost-type half-bridge converters.INDEX TERMS DC-DC power conversion, DC microgrids, Multi-objective tuning, H-infinity control, Impedance shaping.
This paper presents a systematic robust current control design approach for three-phase voltage-source converters. Robustness is guaranteed by combining intrinsic passive properties of the impedance uncertainty at the point of common coupling together with stability results from Passivity-based Control theory. This approach ensures stability against typical uncertainty sources at mid and high-frequencies, such as cable resonances or other converters interaction, with significant less conservative performances than the obtained with traditional Robust Control theory. The approach uses multi-objective controller synthesis formulation that allows to logically combine robustness requirements with performance objectives avoiding heuristic iteration over the control structure and parameters. The controller synthesis is performed by means of a non-smooth H∞ optimization technique that tunes all free parameters of a vector-based controller function, which constraints its structure. This results in a synthesized controller with lower order than those obtained with convex optimization definitions of the H∞ control problem. The design methodology is validated in time and frequency domain by means of theoretical analysis and experimental results with three usual grid filters: L, LCL and LLCL.
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