Port reduction operation of a three‐port nonagonal modular multilevel converter as soft open point

Abstract

SummaryNonagonal modular multilevel converter (MMC) is a three‐port AC/AC converter promising as a soft open point (SOP) to connect different AC feeders or buses in distribution networks, owing to its flexible power control, transformerless structure, and low power component count and its capability of accommodating multiple AC ports of different frequencies. An SOP should be able to work under port reduction condition, which is often encountered when maintenance is required or the network structure is reconfigured. However, as a component reuse converter of circular structure, nonagonal MMC cannot turn off one or some of its branches to achieve this. To address this issue, a control scheme is proposed in this paper to enable nonagonal MMC to operate under port reduction condition by switching the nonagonal MMC to a hexagonal MMC, which is an AC/AC topology of a similar circular structure but has only two ports. Without inserting extra switches, the nine branches of nonagonal MMC is redistributed into six branches. By alternately operating the submodules, no submodules need to be shut down, reducing the submodule capacitor voltage fluctuation during port reduction operation. RTlab results verify the feasibility of the quasi‐hexagonal topology and the effectiveness of port reduction control.