Development of DC Magnetization Prevention Control for a Zero-sequence-cancelling Modular Multilevel Converter

This paper proposes a DC magnetization prevention control in the zero-sequence-cancelling modular multilevel converter (ZC-MMC) as a buffer reactor-less MMC circuit topology. Using the leakage inductance of a zig-zag transformer, the transformer can function as a buffer reactor. By applying a zig-zag connection, DC magnetic flux caused by the zero-sequence DC current flowing through the transformer can be canceled. On the contrary, the core of the transformer is DC-magnetized when a non-zero-sequence DC current is flowing through the transformer. The transformer abnormally heats due to the magnetic saturation of the core caused by DC magnetization. To prevent DC magnetization, DC magnetization prevention control in ZC-MMC was proposed. Using a simulation model and 1-kW experimental setup, DC magnetization prevention control was confirmed.