Modeling and Proportional-Integral State Feedback Control of Fully Parallel Grid-Connected Inverters

A novel three-phase grid-connected inverter topology with a split dc link and LC filter is proposed. It allows for a full parallel connection of multiple inverters simultaneously on both the ac and dc sides, offering high modularity, redundancy, expandability, and overall system reliability. A generic dynamical system model is derived, considering the coupling effects between parallelized inverters, physical constraints, and varying grid impedance. A decentralized proportional- integral state feedback control (PI-SFC) with an extended Luenberger state observer is developed and compared with a conventional PI controller regulating inverter-side currents. A stability analysis shows that both closed-loop control systems remain stable even if an arbitrarily large number of inverters are connected in parallel. Simulations and experiments confirm the functionality and robustness of the closed-loop system under varying grid impedances and during grid faults. For the experimental results, the controllers were implemented on commercially available hardware of the proposed topology. In particular, the PI-SFC allows for better exploitation of the full power of the inverters due to its enhanced controller performance and damping ability. Besides, the exceptional match between simulation and experimental results proves the accuracy of the proposed system model as well.