A variable frequency injection method for modular multilevel converters in variable speed drives

Abstract

Modular multilevel converter (MMC) is a novel circuit that has appeared in the last few decades, and has been extensively applied to medium/high voltage scenarios, especially in motor drive owing to its scalability and easy to supply a higher number of output voltage levels. As theoretical analysis shows, the sub-module (SM) capacitor voltage magnitude is positively related to the converter output current amplitude and conversely connected to the fundamental frequency. Therefore, the prime challenge of applying MMC to variable speed drives is that there is a huge voltage oscillation in SM capacitor voltage during low-speed working. An innovative high-frequency voltage injection modality to diminish the fluctuation of the capacitor voltage in variable speed drives is proposed in this paper, in which the frequency of the injected component can be changed with the operating frequency of the converter and the low frequency component of power can be converted into high frequency component. The discussed measure not only significantly reduces the voltage float in capacitor, but also avoids the overmodulation of arm voltage modulation waveform caused by high frequency of injection component when converter works in low frequency. With the improved method, the performance of motor at low frequency is enhanced. Finally, the feasibility and effectiveness of the proposed improved high-frequency voltage injection method are verified by simulation and experiment.