Robust Single-Loop Direct Current Control of LCL-Filtered Converter-Based DG Units in Grid-Connected and Autonomous Microgrid Modes

This paper presents a robust direct single-loop current control scheme based on structured singular value (mu) minimization approach for induced-capacitor-inductor (LCL)-filtered distributed generation converters in grid-connected and isolated microgrid modes. Unlike the conventional H-infinity-based approach, the proposed interface maintains perturbed system stability, under wide range of grid (or microgrid) impedance variation, without the application of any additional damping loops. Moreover, the performance of the perturbed system in terms of grid-voltage and harmonic disturbance rejection can be improved significantly by the adopted method. This is due to the less conservative nature of the mu-synthesis-based solution as it takes advantage of the additional structure introduced to the uncertainty block by the performance criteria. The salient features of the proposed controller are 1) single-loop direct current control of LCL-filtered converters with inherent damping of the LCL filter resonance without any need for additional damping loops; 2) robust stability and active damping performances by mitigating the LCL resonance under wide range of grid (or microgrid) impedance variation; 3) improving the performance of the current controller by removing its dependency on the grid-voltage feed-forward loop by providing high disturbance rejection feature against fundamental and harmonic voltage disturbances; 4) computationally efficient fixed-order structure with minimum sensor requirements (only grid-side currents are needed for feedback control). A comparative theoretical analysis, time-domain simulation results, and experimental test results are presented to show the effectiveness of the proposed control scheme.