Grid-connected bi-directional modular multilevel converter with D-Σ digital control

Abstract

This paper presents a grid-connected bi-directional modular multilevel converter (MMC) with division-summation (D-Σ) digital control to achieve power injection and rectification functions. The D-Σ digital control is adopted to track current reference, regulate dc-bus voltage and cell voltage, and balance the upper and lower dc-bus voltages. It can accommodate wide filter inductance variation and take care of source voltage harmonics to achieve fast tracking response and low-distortion output current. Based on power-balance principle, the current command, including dc-bus and ac-arm currents can be determined. In this paper, the MMC configuration is introduced first, and then the control laws for current tracking and voltage regulation are derived in detail. Experimental results obtained from a 20 kW MMC have verified the analysis and discussion.