Research And Application Of Modular Multilevel Converter Based On Zigzag Transformer

To implement the national "double carbon" policy,new energy generation and grid connection are receiving more and more attention.Flexible DC transmission technology can significantly improve the new energy acceptance capacity of the grid and has a broad development prospect.Modular multilevel converter(MMC)is the most efficient and widely used power electronic converter in the current flexible DC transmission system,which plays the role of AC and DC conversion.At present,there are still some problems in the flexible DC transmission system: under the single pole earth operation mode,the converter transformer is prone to the DC bias magnetization phenomenon,and the DC bias magnetization suppression device needs to be installed;for extensive MMC converter system,the number of sub-modules is large,the cost and the installation area of bridge arm reactor is high.All of the above will lead to an increase in the hardware investment cost of the converter station.To address this problem,the thesis researches modular multilevel converter based on zigzag transformer.The main research works are as follows:First,to address the problem that the converter transformer is prone to the DC bias phenomenon,a zigzag transformer is used to suppress DC bias.The winding structure,voltage relationship and the principle of DC bias suppression of the zigzag transformer are analyzed.It is applied to MMC as a converter transformer,and a simulation model is established to verify its feasibility and to solve the DC bias problem of converter transformer.Second,to address the problem of high hardware investment cost in converter stations,a modular multilevel converter based on zigzag transformer(ZT-MMC)is studied with the number of sub-modules halved and the bridge arm reactor removed.The ZT-MMC’s operating principle and operating characteristics are analyzed,and its mathematical models are derived in the abc and dq coordinate systems respectively.Third,the valve-level control strategy and system-level control strategy of ZTMMC are studied.The DC transmission control principle based on ZT-MMC is analyzed,the basic controller of ZT-MMC is derived using the idea of dq decoupling control strategy,and a single-ended ZT-MMC simulation model is built in Matlab/Simulink,the simulation results verify that the control method is effective and feasible.Meanwhile,a double-ended ZT-MMC DC transmission system model is built,and the simulation results show that the output waveform quality is good,the harmonic content is low,and the system can operate stably.The economic analysis is also conducted,and ZT-MMC can significantly reduce the construction cost of converter stations.Fourth,the detection and protection of DC line fault,internal fault of zigzag transformer and sub-module fault in the ZT-MMC DC transmission system are studied.For DC line fault,a fault detection method based on the DC current rate of change is adopted,and a full-bridge sub-module ZT-MMC with fault self-clearing capability is studied.For internal fault in zigzag transformer,an improved ratio differential protection method applicable to ZT-MMC is adopted.For sub-module fault,sub-module cold standby and hot standby redundant protection strategies are adopted.And the reliability of each method is verified by simulation respectively.