A Novel Multiport Embedded DC Current Flow Controller Based on a Cascaded Multilevel Inverter

The meshed high-voltage dc network is an effective solution for large-scale new energy consumption and long-distance power transmission. However, when the number of lines is larger than the number of controllable nodes, the converters cannot fully control all line currents. This article proposes a cascaded multilevel embedded dc current flow controller (DCCFC) to increase the dc current flow control freedom in the meshed high-voltage direct current transmission system. It is directly coupled with the main modular multilevel converter (MMC) and maintains power balance through collaborative control. The DCCFC achieves the bidirectional control of dc current flow without an additional power supply or isolation transformer. This article analyzes the working principle of the proposed embedded DCCFC. A power balancing control strategy is proposed with the main MMC and the DCCFC operating in concert. This article also proposes an internal power balancing control strategy considering two circulation optimization methods: common-mode dc voltage injection and q-axis ac voltage injection. The experimental results of the downscaled prototype validate the feasibility and effectiveness of the proposed embedded DCCFC and its control strategy.