Modular Multilevel Converters With Multilevel Submodules and Novel Capacitor Voltage Balancing Methods

ABSTRACT Traditional modular multilevel converters (MMCs) are widely used in high voltage applications that require a large number of submodules (SMs), which often results in considerable overall weight and cost of the MMC station. To address this issue, this paper introduces a multimodular gradationally controlled MMC (MGC‐MMC) by integrating the idea of gradationally controlled inverter into the traditional MMC, and develops a new capacitor voltage balancing control method. The salient feature of this method is a two‐round modulation scheme that aligns the arm output voltage with the arm reference voltage obtained from the modulation strategy. This balancing method based on the two‐round modulation is then combined with the nearest level pulse width modulation (NL‐PWM) strategy to modulate the MGC‐MMC. Simulation and experimental results indicate that the proposed two‐round modulation‐based balancing method is effective for the MGC‐MMC, and the MGC‐MMC with NL‐PWM achieves higher voltage quality than the traditional MMC under the same conditions. Economic analysis shows that the cost of MGC‐MMC is only 37.84% that of the traditional MMC, and the MGC‐MMC uses approximately two‐thirds fewer SMs, highlighting its significant advantages in both weight and cost.