The DC electric field analysis of valve side outlet device in converter transformer considering nonlinear and anisotropic characteristics

Valve side outlet device, which consists of bushing terminal and surrounding barrier system, is the insulation structure located in the connection part between valve winding lead and bushing. The insulation structure of outlet device is made up of oil and paperboard. It is the most complicated and the most important insulation part in converter transformer, and it not only withstands AC voltage, but also withstands DC voltage in operation stage. The electric field distribution is determined by the resistivity of each material under DC steady state voltage. Resistivity of oil and pressboard are affected by the electric field strength applied to them, so the resistivity of both insulation materials is nonlinear. Moreover, the resistivity of pressboard is anisotropic. There is a great difference value between the direction of through and along the paper layer. In this paper, the nonlinear and anisotropic DC electric field distribution of valve outlet device was calculated, and the result was compared with the results which not considering anisotropism or nonlinearity of material. The comparison revealed that anisotropic and nonlinear characteristics must be taken into account in the research and design process.