Decomposed Nearest Level PWM Method With Reduced Switching Frequency for MMC

Abstract

For modular multilevel converters in medium-voltage applications, the nearest level pulse width modulation (NL-PWM), which combines the nearest level modulation (NLM) with pulse width modulation (PWM), has the advantage of better harmonic characteristics over conventional NLM. However, the introduction of high-frequency PWM also results in a significant increase in switching frequency. To solve this issue, a decomposed NL-PWM method is proposed in this article. By properly allocating the rising edge and falling edge of PWM to two different SMs, the capacitor voltage differences can be decreased more efficiently. On this basis, the allocation priority of different switching transitions is quantitatively analyzed, and then a new voltage-balancing strategy involving five switching modes is proposed. Moreover, to achieve a good tradeoff between voltage-balancing effect and switching loss, the relationship between voltage threshold and switching frequency under the proposed method is also derived. Finally, the comparative simulation and experimental results demonstrate the superiority of the proposed method in different aspects of performance.