On the Bipolar MMC-HVDC Topology Suitable for Bulk Power Overhead Line Transmission: Configuration, Control, and DC Fault Analysis

This paper focuses on a bipolar modular multilevel converter (MMC)-based HVDC system, which is suitable for bulk power overhead line transmission. In this topology, each pole consists of several basic converter units (BCUs) in series or in parallel, and the BCU is the recently developed MMC adopting clamp double submodules (CDSM-MMC). Between the positive pole and the negative pole is the neutral bus which is connected to the ground electrode. To manage the power distribution among each BCU, three control layers in hierarchy are designed. Based on the equivalent circuit operations, the fault clearance principle is presented, including the calculation of the maximum inrush current. The entire fault ride-through process is described by the converter-states changing method. Detailed control sequences that cope with the temporary and permanent dc faults are proposed. Three main factors affecting the blocking time and inrush current are investigated. The feasibility and validity of the suggested topology are verified by the EMTDC simulations.