Passivity-based Fixed-order H-infinity Controller Design for Grid-forming VSCs

This paper presents a systematic approach on voltage controller design for three-phase inverters with passivity-based robust stability capabilities. Such design involves several requirements as stand-alone stability margins and tracking behavior along with input impedance shaping for passivity-based robustness. Besides, symmetrical three-phase systems are expressed as multiple-input multiple-output systems, which makes controller synthesis intricate. Then, the proposed multi-objective approach synthesizes the controller by means of a non-smooth H-infinity optimization technique that tunes all free parameters of the corresponding vector-based controller function. We present the design methodology for the two most usual grid-forming strategies: single-loop and dual-loop (cascaded with inner current control). Both designs are validated in time and frequency domain by means of theoretical, simulation and experimental results.