Hybrid Voltage Balancing Control for Four-Level Hybrid-Clamped Converters With Low Switching Frequency

In this article, a hybrid capacitor voltage balancing control based on selective harmonic elimination pulsewidth modulation (SHE-PWM) is proposed for four-level hybrid-clamped (4L-HC) converters that operates at very low switching frequency. In low and medium modulation index (
m_a
) range, a novel SHE formulation is introduced, where the objective function for natural capacitor balancing is added into the nonlinear SHE equations. With the optimal SHE patterns and switching angles obtained from this formulation, all capacitors can be balanced naturally without additional switching actions for swapping the redundant states to regulate the capacitor voltages, which greatly reduces the switching frequency compared with other active capacitor voltage balancing methods and modulations. In high
m_a
range, SHE model predictive control (SHE-MPC) is adopted, where the output voltages follow SHE-PWM patterns but with MPC used for capacitor balancing and switching frequency regulations. Simulation and experimental results, as well as detailed numerical analysis and comparisons with other PWM and capacitor voltage balancing methods for 4L-HC converters are provided to demonstrate the good voltage regulation performance and superiorities of the proposed method.