A Voltage Balancing Scheme for Modular Multilevel Converter Based on Charge Variation in Each Cycle

Abstract

This article presents a voltage balancing scheme for a modular multilevel converter (MMC) based on capacitor charge variation in each fundamental cycle. An in-depth theoretical analysis of stored charge variation for the concept is described. A new hybrid pulsewidth modulation (PWM) sequence is proposed based on a rotation strategy to achieve capacitor voltage balancing without sensing the capacitor voltages or arm currents. The rotation strategy provides the freedom of finding the best possible sequence to achieve capacitor voltage balancing. This reduces hardware, software burden and overall cost of the design implementation. The functionality and performance of the balancing strategy with new hybrid PWM sequence are carried out over wide range of modulation index (M) for three-phase five-level MMC in MATLAB/Simulink. A hardware prototype of single-phase five-level MMC is designed for experimental validation. The proposed hybrid PWM sequence is implemented on an Altera/Cyclone I series (EP1C12Q240C8N) field-programmable gate array using alternate phase opposition disposition PWM method. Simulation and experimental results are presented.