A Compensation-Based 3D-SVPWM Strategy Employing Positive and Negative DC-link Voltages Sampling for Three-Phase Four-wire Three-level Converters

Three-phase four-wire three-level (3P4W-3L) converters demonstrate broad application prospects in off-grid systems, while facing complex and variable operating conditions. Under asymmetrical loads, their DC-link neutral-point potential (NPP) exhibits fundamental-frequency oscillation proportional to the load unbalance factor. The adoption of traditional threedimensional space vector modulation (3D-SVPWM) based on the assumption of balanced positive and negative DC-link voltages leads to substantial output errors during large-amplitude NPP oscillation, ultimately causing three-phase voltage unbalance and elevated total harmonic distortion (THD). Further theoretical analysis confirms that such errors introduce third-harmonic components in the output voltages. A compensation-based 3DSVPWM strategy employing positive and negative DC-link voltages sampling is proposed in this paper, which constructs the 3D space vector diagram based on the real-time sampling values of positive and negative DC-link voltages. Using the real-time positions of basic vectors, subsectors in the vector diagram are optimally redivided and the vector dwell times are calculated accordingly. This strategy effectively suppresses third-harmonic components in the output voltages, ensuring that the system can still output high-quality voltage waveforms even when the NPP oscillates greatly. Finally, the effectiveness of the proposed strategy is verified through comparative experiments, showing a high balance degree and low THD of the output voltages.