Optimized Design of Full-Bridge Modular Multilevel Converter with Low Energy Storage Requirements fo

Abstract

Modular multilevel converter (MMC) is a well proved circuit topology in voltage source converter based high voltage direct current (VSC-HVDC) transmission systems. As is known, the conventional half bridge submodule (HBSM) based MMC-HVDC is not suitable for overhead line transmission applications. In addition, high energy storage requirements, i.e. large capacitance is inevitable. The conventional design of the full-bridge submodule (FBSM) based MMC (F-MMC) usually does not utilize the negative voltage state of FBSM in normal operation. Considering the same dc voltage as with the HBSM case and utilizing the negative voltage state of the FBSM, this paper presents the design method of the power transmission capability of a single FBSM. Meanwhile, an optimized energy storage capacitance design method of the FBSM is proposed. With this method, the capacitance of FBSM can be reduced significantly.

Existing System

Current Injection Method.

Proposed System

This paper presents the analyses about the F-MMC utilizing the negative voltage state of FBSM. The analyses demonstrate that the power transmission capability of a single FBSM of the F-MMC with using negative voltage state is very close to that of the F-MMC without using the negative voltage state if the modulation index is high. The optimization of parameter for the power transmission capability of a single FBSM is also proposed under the same dc voltage and RMS of arm currents conditions. In the meantime, the fundamental frequency fluctuations of the FBSM voltage are inhibited obviously if the F-MMC is operated under pure active power conditions. The optimization of parameter presented in the paper indicates that the FBSM capacitance can be reduced to nearly half of the original. The F-MMC designed in this paper can provide a space-saving and cost-effective converter for HVDC transmission systems.

Architecture

The simplified schematic of F-MMC