Bipolar DC/DC Converter for HVDC to MVDC Connection with MMC and Zig-Zag Transformer

This paper proposes a ±20kV/20MW bipolar DC/DC converter for interconnection of HVDC to MVDC in the MTDC pilot project. The proposed converter consists of two three phase monopole modular multi-level converters (MMC), which are connected through three phase zigzag transformer. By using a zigzag transformer in the ac-side of monopole MMC topology, we can easily operate it as like a bipolar MMC topology. In the proposed monopole MMC, the upper three arms can control the positive DC bus, while the lower three arms can control the negative DC bus, respectively. This kind of independent control is possible if we allow the equal DC current injection into the zigzag transformer. Even though the line currents of the zigzag transformer have same DC current level, the zigzag transformer will not be saturated because the total flux generated by two DC components are cancelled in each leg of the transformer. In this paper, the control methods for proposed DC/DC converter are proposed and verified by computer simulation.