Optimal Design of a Submodule Capacitor in a Modular Multilevel Converter for Medium Voltage Motor Drives

Abstract

This paper proposes an algorithm for determining the optimal capacitance by utilizing a mathematical model of a submodule (SM) capacitor in a modular multilevel converter (MMC) specifically for medium voltage motor drives (MVDs). By approximating the voltage fluctuation of the SM capacitor during low-frequency operation, it is feasible to ascertain the minimum capacitance required for the SM capacitor, ensuring that its voltage fluctuations remain within an acceptable limit that is predefined as a specified value. Moreover, the study considered the injection of both a high-frequency common-mode voltage (CMV) and a circulating current to alleviate the SM voltage fluctuation during the acceleration of motor drives. The effectiveness of the proposed method is validated through verification using time-domain simulation results obtained using the MATLAB/SIMULINK software and real-time simulation results acquired using the OPAL-RT simulator platform.

Keywords

• modular multilevel converter (MMC)
• optimal sizing of submodule (SM) capacitor
• SM capacitor voltage ripple limit
• high frequency injection (HFI)
• variable speed motor drives (VMD)