Robust and generic control of full-bridge modular multilevel converter high-voltage DC transmission systems

This paper presents the theoretical basis of the control strategy that allows the cell capacitor voltage regulation of the full-bridge modular multilevel converter (FB-MMC) to be controlled independent of its dc link voltage. The presented control strategy permits operation with reduced dc link voltage during permanent pole-to-ground dc fault, and controlled discharge and recharge of the HVDC links during shutdown and restart following clearance of temporary pole-to-pole dc faults. Additionally, it allows voltage source converter based HVDC links that employ FB-MMC to be operated with both positive and negative dc negative dc link voltages. This feature is well suited for hybrid HVDC networks, where the voltage source converters are operated alongside the line commutating current source converters, without any compromise to the power reversal at any terminals. The usefulness of the presented control strategy is demonstrated on full-scale model of HVDC link that uses FB-MMC with 101 cells per arm, considering the cases of pole-to-ground and pole-to-pole dc faults