Closed-loop control design for a three-level three-phase neutral-point-clamped inverter using the optimized nearest-three virtual-space-vector modulation

This paper presents a closed-loop control scheme for the three-level three-phase neutral-point-clamped dc-ac converter using the optimized nearest-three virtual-space-vector pulsewidth modulation, a modulation that produces a low output voltage distortion with a significant reduction of the dc-link capacitance. A specific loop modifying the modulating waveforms is introduced to rapidly control possible perturbations in the neutral-point voltage balance. The remaining part of the control is analogous to the control for a two-level converter with an appropriate interfacing to the selected modulation, including an online estimation of the load displacement angle at no extra cost. The closed-loop control is designed for the case of a renewable energy source connected to the ac mains and its performance is analyzed through simulation and experiments.