Nearest-Level Control Method With Improved Output Quality for Modular Multilevel Converters

Nearest-level control (NLC) is a popular technique used in modular multilevel converters (MMCs) with a large number of submodules (SMs) owing to the NLC's flexibility and ease of implementation. However, in medium-voltage applications, MMCs contain a relatively low number of SMs, and the drawba...

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Bibliographic Details
Published in:IEEE access 2020, Vol.8, p.110237-110250
Main Authors: Nguyen, Minh Hoang, Kwak, Sangshin
Format: Article
Language:English
Subjects:
Capacitors
circulating current
Control methods
Control systems
Converters
Current control
Electric potential
Harmonic distortion
increase level number
Mathematical model
Metal matrix composites
modular multilevel converter
Multilevel converters
Nearest-level control
output voltage quality
Topology
Voltage
Voltage control
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Summary:Nearest-level control (NLC) is a popular technique used in modular multilevel converters (MMCs) with a large number of submodules (SMs) owing to the NLC's flexibility and ease of implementation. However, in medium-voltage applications, MMCs contain a relatively low number of SMs, and the drawbacks of the NLC methods emerge, wherein the poor quality of output voltages and currents result in high total harmonic distortion, large ripples in SM capacitor voltages, and unsuppressed circulating currents. Several NLC methods have been proposed to handle these problems, but they do not satisfy all the control objectives simultaneously. This paper proposed a modified NLC capable of enhancing the output quality of MMCs with low number of SMs without deteriorating the control objectives. Unlike previously reported NLC methods, instead of directly calculating the numbers of SMs from the upper and lower arm voltage references, the difference and total number of SMs are obtained from the output voltage reference and circulating current control, respectively. Hence, the numbers of SMs in the upper and lower arms are acquired by simply solving a system of first-order two-variable equations. The simulated and experimental results for a single-phase MMC system were used to verify the appropriateness and effectiveness of the proposed modified NLC method.
ISSN:2169-3536
2169-3536
DOI:10.1109/ACCESS.2020.3001587