| Since the DC transmission technology based on voltage source converter(VSC) came in, converter components in direct-voltage transmission are being carried out constantly. Early HVDC system uses the two-level or three-level converter, which made up of a large number of IGBTs connected in series, while the circuit structure is simple, but the control technology is very difficult, and the loss is very high.Modular multilevel converter(MMC) is a new type of voltage source inverter, which has a high modular structure, so that extensible to creat redundancy control system and implementation,it also has low switching frequency and good output of voltage waveform, in recent years it is widely used in VSC-HVDC system. First this article introduces the research background meaning of the VSC-HVDC technology and engineering application, and introduces several kinds of typical multilevel inverter and its advantages and disadvantages,then analyzes the current research status of modular multilevel converter, what's more,it analyzes the advantages VSC-HVDC and technical characteristics of MMC-HVDC.According to the characteristics of MMC, and combined with MMC mathematical model of topological structure, This article studied the working principle of MMC and vector control method of the MMC. Several kinds of commonly used modulation strategies of MMC are studied, and analyzed the application of carrier phase shift pulse width modulation(CPS-PWM) strategy in detail. Because of DC side voltage of MMC is mainly supported by capacitor voltage of MMC sub modules, MMC state determined by the switching of sub modules. In order to prevent failure modules from affecting the normal operation of MMC, the system must set up some Redundancy modules, and a range of research on Redundancy submodule can make the Redundancy system more effictive.Several kinds of and in order to prevent failure module from affecting the system operation, the system must be set up some Redundancy sub-modules. Redundancy topology structure of modular multilevel inverter is studied in this paper, which also analyses the mathematical model of MMC which has Redundancy SMs. Method for Redundancy sub-module number configuration is studied. Several kinds of fault conditions and protection switches are studyed, common redundancy protection scheme of MMC are also introduced. Further more, a novel dynamic redundancy scheme of MMC is come forward. Redundancy control strategy and optimization modulation method based on the conditions of MMC redundancy run, In order to suppress circulating current and capacitance voltage fluctuation, circulating current restraining method and energy balancing control strategy are applied.Finally,in order to verify the correctness of the above control strategy and redundancy scheme,simulation models are built in Matlab/simulink simulation software,the simulation results show that energy balancing control strategy and circulating current restraining method are effective in improving the performance of MMC, and the dynamic redundancy control strategy could ensure the normal running of the MMC which has Redundancy SM,and keep the capacitor voltage stability, output voltage of rectifier side is also verified by the experimental results. When a SM malfunctioned, it will be byipassed, and redundancy SM goes into running mode, what's more, the MMC system system is not adversely affected. |
PDF Full Text Request