| This paper was supported by the major project “High Voltage Direct Current Transmission Flexible” of Zhuzhou Electric Locomotive Research Institude.Modular multilevel converter had been chosen as the research object.Moreover,the control algorithm and key technology were studied and anglyzed in detail when the mudular multilevel converter was applied in high voltage direct current transmission,which can provide theoretical support for engineering design of the high voltage direct current transmission based on modular multilevel converter.Firstly,the topological structure and working principle of modular multilevel converter were analyzed in detail in this paper.Then the equivalent circuit model of modular multilevel converter was built and it showed that modular multilevel converter has the same external circuit structure as two-level or three-level converter.Secondly,in order to ensure that the high voltage direct current transmission system based on modular multilevel converter start quickly and smoothly,the startup strategies applied to high voltage direct current transmission connecting both ends of the active network and high voltage direct current transmission supplying power to passive network was analyzed.To solve the critical unbalance problem of capacitor voltage of each sub-module,the capacitor voltage balancing control technology based on sort method was analyzed in detail.Moreover an optimized control technology was studied.The simulation results demonstrate that the optimized capacitor voltage balancing control technology can balance the capacitor voltage and reduce the average switching frequency of IGBT effectively.Thirdly,the modulation algorithms for modular multilevel converter were introduced.In addition,this paper focused on the nearest level modulation algorithm.In order to make sure the high voltage direct current transmission system based on modular multilevel converter has a good operating performance,the system control algorithm applied in high voltage direct current transmission connecting both ends of the active network and high voltage direct current transmission supplying power to passive network was studied.The the simulation results based on MATLAB/SIMULNK and PSCAD/EMTDC verified the validity and effectiveness of the control algorithm,the simulation results indicated that the control algorithm studied by this paper can realized fast decoupling control between active power and reactive power.Finally,experiments were performed based on the platform of the project “High Voltage Direct Current Transmission Flexible”,which verified the validity and effectiveness of the system control algorithm proposed in this paper further. |
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