Differential and common-mode model-based controller for a grid-tied three-phase NPC converter

This paper introduces a controller designed for transformerless inverters connected to the grid via an inductor-capacitor-inductor (LCL) output filter. The controller incorporates a virtual ground (VG) connection, providing a straightforward solution to mitigate common-mode current (CMC), i.e., to reduce the leakage ground current issue. Specifically, the focus is on a three-level, three-phase NPC inverter with split DC-link capacitors. The VG is established by connecting the star point of the LCL filter capacitors to the midpoint of the DC-link. This topology presents specific challenges, including voltage imbalances in the DC-link capacitors and resonances in the homopolar components of the signals within the LCL filter. The proposed controller not only addresses the goal of current injection but also ensures, on average, the balance of DC-link capacitor voltages (or equivalently, the regulation of the neutral point). Additionally, it provides the necessary damping for the homopolar components of the LCL filter signals. The performance of the proposed control scheme has been experimentally tested in a 2 kW three-phase NPC inverter prototype.