Research on Model of Electro-Thermal Coupling in Valve Side Bushing Considering the Non-linear Characteristics of SF6 and Core Insulation Medium

The valve side bushing of converter transformer is important component of the converter transformer, and its safe and reliable operation is of great significance for ensuring the normal operation of the DC transmission system. Constructing an accurate calculation model for the internal temperature and electric field of valve side bushing, and optimizing the bushing structure design under the complex operating conditions based on mathematical calculation models, is an important prerequisite for ensuring the reliable operation of actual bushing equipment. However, existing calculation models do not sufficiently consider the material non-linearity during the operation of the valve side bushing, resulting in differences between the calculation results and the actual results during the operation of the bushing, and the designed bushing has safety hazards. In response to this, this article starts from structure of the valve side bushing, considers nonlinear relationship of the insulation material of the capacitor core, and innovatively proposes the valve side bushing electric thermal coupling nonlinear model. Furthermore, it compares and calculates the distribution of the electric field of the valve side bushing under different methods: traditional equal margin design method, full model equal temperature condition, and full model unequal temperature. Calculation results using nonlinear electric thermal coupling model show that radial field strength near the central conductor of the bushing core decreases, while the field strength near the outermost electrode plate shows an upward trend, resulting in a field strength reversal phenomenon. Under the actual operating conditions of the full voltage and full current, there will be a temperature gradient distribution inside the capacitor core, and the field strength distribution will show significant changes. The traditional theoretical formulas cannot quantitatively describe this. Only by considering the nonlinear characteristics of materials in the process of the electric thermal coupling can the uniform distribution of axial field strength and partial discharge margin of the sleeve capacitor core be truly achieved.