Improved Virtual Space Vector Modulation for Four-Level Nested Neutral Point Clamped Inverters

Abstract

The four-level diode-clamped NNPC (Nested Neutral Point Clamped) inverter configuration is employed in high-power electrical drive systems or medium-voltage applications due to its ability to ensure excellent output voltage quality while requiring fewer components compared to traditional four-level inverter configurations. The Sinusoidal Pulse Width Modulation (SPWM) technique applied to this converter is straightforward to implement but has a limited output voltage range. This paper presents a Novel and improved Virtual Space Vector Pulse Width Modulation (VSVPWM) with an average common-mode voltage. Compared to the sine-wave pulse width modulation method, this technique offers several advantages, such as enabling control of the entire space vector diagram and achieving low harmonic distortion in the output waveform. Control and balancing of the flying capacitors are achieved by utilizing phase-leg redundant states. The results of the control algorithm analysis for this inverter are validated through simulations using MATLAB/Simulink software and experiments conducted on a real-world model. The evaluations of the Total Harmonic Distortion (THD%), output current, and voltage across the flying capacitors demonstrate the high-quality performance of the converter within the full voltage  range.