Series Hybrid Modular Multilevel Converter for HVDC Transmission Systems

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Abstract
In this paper, a multilevel Voltage Source Converter for HVDC transmission systems is proposed. The topology is based on a string of full-bridge submodules in addition to an unfolder on the converter’s AC-side. Compared to the modular multilevel converter, it has fewer components and has the advantage that almost two third of the switches operate in soft switching mode. Furthermore, for the same level of stress on converter and transformer, it provides a DC-link voltage 3.33 times higher than AC-side rms-voltage. A control strategy is proposed to regulate the active and reactive power exchanged with the grid while the capacitor voltage balancing is ensured. The viability of the proposed converter, as well as the effectiveness of the voltage balancing control strategy are confirmed by simulation and experiment.

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