Predictive Nearest-Level Control Algorithm for Modular Multilevel Converters With Reduced Harmonic Distortion

Minh Hoang Nguyen,Sangshin Kwak

doi:10.1109/access.2020.3048156

Minh Hoang Nguyen, Sangshin Kwak

Open Access

https://doi.org/10.1109/access.2020.3048156

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Abstract

Owing to the low number of submodules (SMs) in modular multilevel converters (MMCs), especially in medium-voltage applications, the output current and voltage generated by conventional nearest-level control (NLC) methods contain evident distortions. Previously, various hybrid and level-increased NLC methods have been proposed to improve the output voltage quality, but such measures wither increased the complexity or did not regulate all the control objectives simultaneously. To further enhance the output performances of MMCs containing low numbers of SMs, an improved predictive NLC (I-PNLC) method combining NLC and model predictive control (MPC) is proposed, where the output and circulating currents are regulated with the corresponding predicted references, and the output voltage is controlled by the added voltage correction. The proposed I-PNLC not only reduces the output current and voltage total harmonic distortion (THD) considerably but also avoids additional complexity in the control system design. The results of simulations and an experiment are presented to verify the proposed approach, in addition to a comparison of the evaluations of conventional NLC methods.

Highlights

Modular multilevel converters (MMCs) have garnered extensive interest since their invention by Marquardt in 2003 [1]
Compared with the conventional nearest-level control (NLC) scheme, the total harmonic distortion (THD) of the output current and voltage yielded by the conventional predictive NLC (PNLC) are reduced to 1.21% and 8.7%, respectively
The conventional PNLC, the level-increased NLC with circulating harmonic current suppression based on deadbeat controller, the reduced computational burden model predictive control (MPC), and proposed improved predictive NLC (I-PNLC) show better dynamic performances, where the circulating currents are tracked rapidly with the changes to the reference circulating current without any overshoots

Summary

INTRODUCTION

Modular multilevel converters (MMCs) have garnered extensive interest since their invention by Marquardt in 2003 [1]. To achieve adequate output quality, the conventional NLC method is primarily used in high-voltage applications when the MMC contains a lot of SMs. In an effort to further enhance the output current and voltage harmonic characteristics, one upper SM and one lower SM were operated in the PWM condition in [34] to increase the resolution ratio of the staircase waveform. A recent NLC method proposed in [40] defined the number of inserted SMs according to the output voltage level selection condition and circulating current regulation, thereby significantly improving the output performance and circulating current controllability.

MATHEMATICAL MODEL OF MMC

CONVENTIONAL NLC AND PNLC METHODS

Findings

CONCLUSION