| The electromagnetic environment in the valve hall is complicated.Due to a large number of power electronic devices are used to achieve convert,the requirements for electromagnetic environment become more stringent to ensure safe and stable operation.Therefore,the shield system must be designed reasonably to control the surface electric field,so as to ensure no corona occupancy.In the design process,numerical analysis is needed to calculate the electric field of the converter valve.The converter valve is large in scale,complex in structure,coexistence of multiple medium and enclosed in the overall environment.Traditional algorithms are slow in calculation and large in memory,thus low in efficiency.Therefore,an efficient numerical algorithm is needed,which provides conditions for designing the structure of the converter valve.The main contents and results achieved are as follows.Boundary element method(BEM)is not suitalbe for large scale problems.In order to speed up,save memory and expand the solution scale,the fast multipole boundary element method(FMBEM)was formed by using the fast multipole method(FMM)to accelerate Galerkin BEM,the process and storage structure were optimized.According to the characteristics of FMBEM,an adaptive generalized minimal residual method(GMRES)was proposed,and the sparse approximate inverse(SAI)preconditioner was improved.Combining adaptive GMRES and improved SAI preconditioner as solver,the convergence rate can be accelerated,and the reliability can be increased,which was verified by examples.It was verified by different examples that FMBEM can achieve high accuracy,significantly reduce time and memory usage compared with BEM.The effects of multipole expansion order,tree structure level and mesh density on accuracy and efficiency were studied,which provides a basis for the selection of parameters in practical application.Converter valve has complex structure which contains many kinds of curved surface.The mesh formed by plane element is difficult to match the actual model surface.Therefore,the curved element based on coordinate transformation was used to increase the accuracy.There are many kinds of media in the converter valve,such as air,insulator and so on.The traditional BEM is suitable for the single medium problem.For multi medium problem,the multi medium indirect integral equation was used,and the multipole expansion for this equation was derived.The fast multipole curved boundary element method(FMCBEM)was prpposed by using improved FMM to accelerate curved BEM(CBEM).In order to adapt to the characteristics of CBEM,the partition method of oc-tree structure was improved to reduce the calculation error.It was verified by different examples that FMCBEM can deal with multi medium problem.Compared with FMBEM,FMCBEM can improve the accuracy and efficiency.The influence of insulators on electric field was studied by using the actual converter valve tower model.The closed valve hall problem can be tranformed to multi plane symmetry roblem by multi image method,and the symmetry fast multipole curved boundary element method(SFMCBEM)was proposed based on FMCBEM.The symmetrical relationship of multipole expansion coefficients was derived.In the calculation,only the real structures need to be modeled and meshed.and the multipole expansion coefficients of image structures are obtained by symmetrical relationship,which reduces the memory usage.Using binary tree with tight bounds and optimizing algorithm steps,the image structures were divided into near and far parts according to the distance.The far parts complete the calculation by one transformation,which reduces the calculation time.The accuracy of SFMCBEM was verified by different examples.Compared with FMCBEM,SFMCBEM can significantly improve the computational efficiency.The influence of mirror order on the accuracy was studied.which provides a reference for practical application.The effect of wall distance on the surface electric field was studied by using the actual converter valve tower model.FMCBEM was successfully applied to analyze the electric field of ± 160 kV HVDC-flexible single arm models,which verifies the ability of FMCBEM to deal with large scale problems.The influence of surface electric field between valve towers was studied.For the accurate calculation of converter valve electric field need to analyze the valve hall full model.By using SFMCBEM,the electric field of two±160 kV HVDC-flexible valve hall full model in different moments were successfully analyzed.The number of nodes is 670,000 and 133,000.Closed valve hall was considered in the calculation,and all bridge arms were modeled.The shield system,valve module and supporting insulator are considered.This method can quickly and accurately solve the electric field of converter valve.It provides an effective tool for the optimization design of converter valve. |
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