Analysis and Modeling of the Nonlinear Characteristics of the Insulation Medium in Capacitor Cores and the Electric Thermal Coupling Mechanism of the Main Insulation Structure of High-Voltage Bushings

The valve side bushing of ultra-high voltage converter transformers is used in multiple fields. Due to the temperature gradient distribution inside the capacitor core, classical theoretical formulas cannot quantitatively describe the influence of the nonlinear characteristics of the insulation material in the capacitor core on the distribution of electric field strength. Therefore, it is urgent to introduce the nonlinear characteristics of insulating media into the optimization design of valve side bushings, and improve their operational reliability on the basis of fully considering the thermal coupling mechanism of electrical factors. In order to solve the above problems, this article first conducts a discrete spectral analysis of the rheological values of typical valve side bushings, introduces the physical structure, constituent materials, and operating conditions of the ultra-high voltage side bushing, analyzes the influence of temperature field on the nonlinear characteristics of the bushing capacitor core, and elucidates the mechanism of space charge electric field at the interface of the converter. Secondly, an optimal isochoric design method that is more suitable for the working environment of the epoxy impregnated dry-type sleeve valve side shell of $\pm \; 800$ kV converter transformers was proposed, achieving uniform distribution of electrical performance parameters in the core of the valve side shell and improving its operational reliability. Finally, the proposed nonlinear model of electric thermal coupling was validated in practical scenarios, and a rheological value side shrinking sleeve was manufactured based on it. The results showed that the proposed nonlinear mathematical model of electric thermal coupling had little impact on the geometric dimensions and layout of sleeve capacitors, and had good theoretical and engineering application value.