Research On Modular Multilevel Converter-high Voltage Direct Current

This paper focuses on the start-up strategy in MMC-HVDC and the voltage equalization strategy between sub-modules.Firstly,the history of the development of flexible DC transmission technology is briefly introduced.Secondly,the mathematical model,modulation strategy and control mode of MMC are introduced.Then,the starting control of MMC is studied deeply.And a fast self-excited charging strategy is proposed.Finally,the voltage equalization strategy among the sub-modules of MMC is discussed in depth,and the hierarchical sub-leasing method is improved.In the MMC start-up control,the traditional method of starting the heuristic is studied,and it is found that the charging is not up to the rated value and the charging rate is too slow,and the problem is improved to solve the traditional And then the traditional self-excitation start method is studied.On the basis of it,a method is proposed to compare the maximum and minimum thresholds of the bridge arm to obtain the number and time of the cut-off sub-module,which greatly reduces the charging start time.And the sub-module prioritized the law has been improved,thus avoiding the entire charging process of the bridge over the current phenomenon.The validity of the simulation is verified by simulation.In the study of voltage equalization strategy of MMC sub-module,the traditional voltage equalization method,the double-holding factor method and the hierarchical grouping method are introduced in principle.Because hierarchical grouping method has a great advantage in terms of sorting computation and hardware requirements,it is more advantageous than other methods,so we focus on hierarchical grouping method with factorization.In the study,it was found that in the event of a sub-module failure,especially when the input sub-module charge,there will be three phase cell voltage is not equal,so that the flow between the bridge arm increases,and because of the equivalent resistance of the bridge very small,loop flow attenuation is very slow,may lead to MMC accident.In this paper,an anti-accident strategy is proposed,which is based on the combination of the pre-estimation method and the adjacent sub-module instead of the conduction method,which avoids the occurrence of the above phenomena.The correctness and validity of this method are verified by simulation.