Novel Carrier Redistribution Pulsewidth Modulation and Active Voltage Balancing for Five-Level SM Converter with Low-Frequency Voltage Ripple Suppression

This study proposes a novel carrier redistribution pulse-width modulation (CRPWM) technique and its associated active voltage balancing method with low-frequency voltage ripple suppression for the five-level stacked multicell (SM) converter. While CRPWM offers simplicity and ease of implementation with optimal harmonics, it encounters significant neutral point (NP) voltage imbalance issues, particularly at low power factors. NP voltage imbalance is a primary cause of low-frequency voltage ripples in DC-link capacitors. Therefore, the proposed CRPWM utilizes new switching states to minimize NP current generation and ensures the natural voltage balance of floating capacitors to minimize voltage ripples. Consequently, NP current significantly diminishes at low power factors, and unbalanced NP current is almost completely suppressed under the active voltage balancing method theoretically. The proposed CRPWM method, with lower unbalanced NP current and equivalent line voltage harmonic performance compared with other CRPWM methods, exhibits the lowest low-frequency voltage ripples with optimal output performance. Experimental results reveal about 88% and 65% reduction in low-frequency voltage ripples, and about 0.5% and 0.02% reduction in line voltage total harmonic distortion (THD), particularly at lower power factors and highest modulation index with fundamental frequencies of 10 and 50 Hz, respectively.