A capacitor voltage ripple suppression method employing bidirectional‐switching channel for the MMC driven pumped storage system operating at low‐speed stage

Abstract

SummaryThe Modular Multilevel Converter (MMC) has gained the widespread application in the DC power transmission, owing to the advantages such as modularity and low output current harmonic content. However, its adoption in variable‐speed pumped storage systems remains limited due to the significant voltage fluctuations in its sub‐module capacitors during the low‐speed operation. In this paper, a capacitor voltage ripple suppression method employing bidirectional‐switching channel is proposed. It aims to improve the driving performance of MMC when the variable‐speed pumped storage motor operates at low speed. The proposed method creates bidirectional‐switching channels between the sub‐modules of the upper and lower bridge arms and moves the sub‐module capacitors that originally fixed to the upper and lower bridge arms into the channels. By controlling the bidirectional‐switching channel, the same capacitor can be put into the sub‐modules of different bridge arms. At the same time, a threshold switching control method is proposed to reduce the switching loss. Without introducing the common mode voltage, the capacitor voltage fluctuation can be substantially suppressed when the pumped storage motor operates at low speed. Experimental and simulation results verify the effectiveness of the proposed method.