Abstract
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 a 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. In addition, it allows voltage-source-converter (VSC)-based HVDC links that employ FB-MMC to operate with both positive and negative dc negative dc-link voltages. This feature is well suited for hybrid HVDC networks, where the VSCs operate 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 the full-scale model of the HVDC link that uses FB-MMC with 101 cells/arm, considering the cases of pole-to-ground and pole-to-pole dc faults.
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