Abstract
The modular multilevel matrix converter (M3C) is a promising topology for medium-voltage, high-power applications. Due to the modular structure, it is scalable and capable to produce high quality output waveforms and can be fault tolerant. However, the M3C suffers from low frequency capacitor voltage fluctuation if the output frequency is close to the input voltage frequency, which limits its application in adjustable speed drive fields. This paper presents a theoretical analysis in the phasor domain to find the branch-energy equilibrium point of the M3C when operating with equal input and output frequency first. Then, a branch energy balancing control method based on branch current reallocation is proposed to equalize the energy stored in the nine converter branches. With the proposed method, the M3C can effectively suppress the capacitor voltage fluctuation without injecting common-mode voltage or applying reactive power to the input side. Experimental results are presented to validate the proposed method.
Highlights
The modular multilevel matrix converter (M3C), shown in Fig. 1, connects two three-phase systems by nine branches
M3C is more suitable for lowspeed constant-torque motor drives compared with the modular multilevel converters (MMC) in back-to-back configuration [5], [6]
M3C suffers serious capacitor-voltage fluctuation if the output frequency gets closer to the input frequency
Summary
The modular multilevel matrix converter (M3C), shown in Fig. 1, connects two three-phase systems (input-side and output-side) by nine branches. M3C suffers serious capacitor-voltage fluctuation if the output frequency gets closer to the input frequency This is caused by a very low-frequency (the frequencies’ difference between two three-phase system) reactive power on the branch. As the reactive power at the input side is not allowed in some conditions and the common voltage may cause serious problems, this paper attempts to develop a control method only using circulating currents in the M3C to equalize the energy of the nine branches. It develops a strategy to design appropriate circulating currents With this control method, M3C can effectively overcome the capacitor voltage fluctuation with neither using common voltage nor applying reactive power at the input side. The proposed control strategy has been validated by simulation and experiment
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