1.Center for Advanced Power-conversion Technology and Equipment, Southeast University, Nanjing 210096, China;2.NARI Group Corporation (State Grid Electric Power Research Institute), Nanjing 211106, China;3.Electric Power Research Institute of State Grid Jiangsu Electric Power Co., Ltd., Nanjing 211103, China;4.State Grid Jiangsu Electric Power Co., Ltd., Nanjing 211100, China

For the flexible ring network controller without the interface transformer, the transmission of the zero-sequence voltage component cannot be prevented when the fault occurs at the AC side, thus increasing the fault range. Therefore, this paper uses the classical circuit analysis method and positive and negative sequence analysis algorithm to explain the basic principles of the formation and transmission of zero-sequence voltage components. A topology of the flexible ring network controller without the interface transformer is proposed. The AC side converters are all modular multilevel converters with traditional half-bridge sub-modules. And the full-bridge sub-module valve strings are connected in series on the positive and negative polarity busbars. Utilizing the ability of the full-bridge sub-module outputing positive and negative voltages, the DC-side voltage fluctuations are suppressed, and the fault range expanding is prevented. By using the MATLAB/Simulink software, the characteristics of zero-sequence voltage suppressed during the fault are simulated and analyzed. The simulation results verify the correctness of the theoretical analysis and the effectiveness of the proposed topology.