Solution to nonlinear electric field of HVDC at ends of valve-side winding in converter transformer

To study the serious nonlinearity of electric field at ends of valve-side winding in converter transformer HVDC, this paper presents and develops a finite element method calculation model for nonlinear electric field based on theoretical analyses. Considering the nonlinearity of the insulation materials, the validity of the FEM model is proved with iterative solution by illustration examples, the electric field intensity at the part concerned by engineer is obtained as a result. Finally, the FEM model is employed to calculate the DC electric field distribution in a typical insulation structure of valve-side Winding in converter transformer. The results show that the maximum electric field intensity among the elements in both the insulation papers and transformer oil have drops down obviously, especially in the insulation papers which are very beneficial for the design of the insulation structure. This change tendency indicates that the traditional calculation method for the electric field is conservative and the insulation design is forced to be more difficult and complicated, meanwhile the FEM nonlinear calculation method can provide a theoretical guide for the insulation design and manufacture.