Generalized Low Switching Frequency Modulation for Neutral-Point-Clamped and Flying-Capacitor Four-Level Converters

Four-level neutral-point-clamped (NPC) and flying capacitor (FC) converters are suitable candidates for high-power medium-voltage applications due to their desired output performance and high power density. The major challenge on their successful applications lies in the dc-link capacitor and FC imbalance issue, especially for converters without redundant states that result in a significantly reduced control flexibility. In this article, a generalized low switching frequency modulation scheme based on selective harmonic elimination pulsewidth modulation (SHE-PWM) is proposed that can be applied to all types of 4L-NPC and 4L-FC converters. To address the dc-link capacitor and FC imbalance problems, the capacitor charge objective is formulated and incorporated into the nonlinear equations of SHE-PWM in solving the switching angles, which would then possess natural capacitor balancing ability. Four effective solutions are provided to address the reduced solution space issue with this formulation in high modulation index range for natural capacitor balancing, thus the full range operation can be guaranteed. Simulation and experimental results are presented to verify the effectiveness of this generalized modulation for 4L-NPC and 4L-FC converters.