General Model of Modular Multilevel Converter for Analyzing the Steady-State Performance Optimization

The steady-state performance of modular multilevel converter (MMC) can be optimized by the following methods: circulating current injection, third-harmonic signal injection, and average capacitor voltage control. However, most of the papers, due to the deficiencies of their MMC models, are limited to the study of adopting just one method and only optimizing a single-aspect of the performance. Thus, this article proposes a general MMC model for the comprehensive study of the steady-state performance optimization. The general model is built based on an equilibrium equation between the internal quantities and the external quantities of the MMC. This general model is capable of analyzing the optimization where the optimizing methods are combined and used to improve the multiaspect performance of the MMC. In addition, more influence factors are taken into account; hence, the calculation error can be smaller than 1% and even the minor effect of each optimizing method can be reflected. The proposed model is compared with traditional models; and it is found that both dc and fundamental components in modulation signals will be affected by the injected circulating current, rather than the constant values in the traditional models. Finally, the proposed MMC model is verified by both simulation and experiment.

I. Introduction

Modular multilevel converter (MMC) is considered a promising technology for various high power applications [1]–[3]. Much research has been conducted on it, such as parameter selection [4], [5], capacitor voltage balance [6], [7], modulation technique [8], submodule (SM) failure detection [9], and so on. These studies have contributed to the stable operation of the MMC. However, in addition to the stable operation, its steady-state performance should be further optimized. For example, if the steady-state capacitor voltage is able to be reduced, the MMC can use SM capacitors with smaller capacitance, which will decrease the project cost.