Study On Heat Equivalent Method For Discharging Arc In Short Gaps And Application In The Thermal-stress Characteristic Analysis For Dielectric Fitting

With the large-scale construction of national key projects such as high-voltage AC/DC transmission lines,high-speed railway,and oil and gas pipelines,the mutual interference between different systems has become increasingly prominent.Due to different reasons,AC and DC interferences in various systems are also becoming more and more serious,which causes discharging in the internal gap of equipment and devices.The discharging in devices will cause thermal shock and thermal ablation and damages the devices.Specially,in gas transmission systems,the discharging often occur in a dieletric fitting of the system,which can generate a large amount of heat.The overheating not only causes erosion to the dielectric fitting,but also beaks the insulation and fluid containment of the dielectric fitting.Therefore,it is important to characterize and analyze the thermal status of the dielectric fitting under discharging.However,the thermal analysis of dielectric fitting under discharging is a complex process involving electromagnetic field,temperature field,flow field and stress field.It is very difficult to analyze directly.Therefore,in order to analyze the multi-physics of dielectric fitting under discharging and study the thermal shock and thermal ablation of electric arc,the thesis carried out the thermal field analysis method of the dielectric fitting under discharging,and the main works include follows:(1)The MHD model for the arc in the short gaps was studied and the heat source distribution characteristics of the arc were determined.Firstly,the paper built the physical model and mathematical model for the ar,and analyzed the solution method for the multi-physical coupling field of arc.At the same time,the paper takes the pin-plate discharge model as an example to detemine the thermal behavior of the arc by analyzing the temperature field of the arc and the flow field distribution.The thermal effect of the arc can be divided into the thermal effect generated by the arc column and cathode surface.Based on the MHD model,the heat source distribution characteristics of the arc were determined.(2)According to the heat source distribution characteristics of the arc,an equivalent heat source model of the arc was proposed,and the ellipsoid heat source and the Gaussian heat source were used to be equivalent to thermal effects of the arc column and cathode surface respectively.Then,the tip-plate model under the equivalent heat source is established to calculate the temperature distribution and flow field distribution.By comparing with the results in the MHD model,the accuracy of the equivalent heat source model is verified.Finally,the paper analyzes the method to determine the equivalent heat source parameters,and determines the structural parameters of the equivalent heat source by the arc current.(3)The paper proposes a thermal-stress field calculation method for the dielectric fitting under discharging,which translates the complex multi-physics coupling problem into the heat conduction problem of the dielectric fitting under the equivalent thermal sources.A 3-D model the dielectric fitting under the equivalent heat source is established.Then,the thermal characteristics of the dielectric fitting under dischargingare analyzed by example calculations,and the insulation layer of the dielectric fittingis also determined.The study simplifies the complicated coupling relationship between multiple physical fields in the dielectric fitting under discharging,and provides a new method for solving the multi-physics coupling problem.(4)The experimental platformsof tip-plate discharging model and the dielectric fittingare set up.The accuracy of the MHD model and equivalent heat source model is verified by experimental analysis.In the experiment,the temperature distribution of arc between tip-plate electrodes can be abtained by an infrared imager.By comparing with the numerical results,the accuracy of the model and method proposed in the paper are verified.Furthermore,the repetitive results verify the reliability of the experimental results.(5)The effects of discharging on the cathode at different discharge times,arc currents,metal materials and the length of gap were studied.The distribution rule of temperature of arc under different conditions is analyzed.At the same time,by analyzing the ablation radius of the cathode surface,the ablation rule under different conditions is determined.Based on the rules,the method to reduce the temperature of the insulating layer of the insulating joint is analyzing.