Improved control strategy of full-bridge modular multilevel converter

Abstract

This paper describes a control approach that allows the cell capacitors of the full-bridge modular multilevel converter (FB-MMC) to be controlled independent of dc link voltage. Also the control approach offers the possibility of operating the FB-MMC from bi-polar dc link voltages; thus, creating new possibilities for building generic hybrid dc grids with reversible dc link voltage, where conventional line commutated current source converters can operate in conjunction with voltage source converters. Additionally the control approach improves dc fault ride-through of the FB-MMC compared with existing approaches. This is achieved by active control of the arm currents and cell capacitor voltages, and exploitation of the FB-MMC redundant switch states. FB-MMC operation with reversible DC link voltage and decoupled control of the cell capacitor voltages from the dc link voltage are demonstrated using simulations.