Abstract
The two-port full-bridge sub-modules (TP-FBSM) based modular multilevel converter (MMC) has more degrees of freedom for control as compared to the traditional FB-MMC, with DC fault current blocking capability preserved. In order to enhance the applicability of two-port FB-MMC (TP-MMC) in HVDC transmission, this paper proposes a specified open-loop voltage balancing algorithm. The algorithm minimizes the number of high-speed, high-accuracy capacitor voltage sensors and the controller hardware as compared to the traditional topology. This is realized by purposely controlling the TP-FBSM switches to parallel the adjacent capacitors for equal voltages. The current spikes during the paralleling are effectively suppressed by the distributed arm inductors within each voltage balancing circuit. The proposed open-loop method is validated by electromagnetic transient (EMT) simulations on PSCAD/EMTDC and by comparing these results with the experimental waveforms obtained from a down-scaled FB-MMC setup.
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