Principles and dynamics of natural arm capacitor voltage balancing of a direct modulated modular multilevel converter

Natural capacitor voltage balancing of six arms of a direct modulated Modular Multilevel Converter (MMC) has been observed by several articles both by experiments and simulations. However, its principles have not been revealed clearly and its dynamics have not been analyzed analytically. In this paper it's shown that a DC component and a line frequency circulating current would be induced inside the converter inherently in case of arm capacitor voltage unbalance and the induced circulating current would transfer energy among six arms to balance arm capacitor voltage naturally without any closed-loop control. And the dynamics of natural arm capacitor voltage balancing have been analyzed mathematically and verified by both computer simulations and experiments.

I. Introduction

Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) transmission is a promising solution for future smart grid which would integrate a great amount of renewable energy sources to the existing grids. For VSC-HVDC, compared to the conventional two level or three level converters, a Modular Multilevel Converter (MMC) is a competitive candidate and is attracting worldwide attention [1]–[3]. MMC presents many advantages such as very low harmonics, low dv/dt, modularity and simple scaling, no necessity of series connection of power semiconductors, and DC bus capacitor elimination [4]–[6], etc.