Influence Characteristics Of Electric-thermal Compound Field On Insulator Surface Charge Accumulation In SF₆

Gas-insulated metal-enclosed transmission lines(GIL)have natural advantages such as low loss,large capacity,and low radiation.They can be a good substitute for overhead lines and cables,and have high value and broad prospects.The temperature gradient of the GIL can cause severe distortion of the electric field.Therefore,to guide the design of novel DC GILs,it is particularly important to understand the charge accumulation and electric field distribution of insulators under temperature gradients.In this paper,a mathematical model for numerical analysis of GIL heat-flow-electricity three-dimensional coupling field is established on the background of the actual GIL pipe gallery,simulating the actual operation of the GIL pipe under the condition of no sunlight radiation in a large space,and conducting the resistivity measurement test of the insulator.Considering the variation law of insulator conductivity with temperature,the effects of different insulator temperature,voltage polarity and insulator conductivity caused by insulating gas pressure and ambient temperature on surface charge accumulation and surface electric field distribution were explored theoretically.Guide the design of insulator structure in DC GIL.Research shows that when the load current is 3150 A,the gas pressure is 0.5MPa and the ambient temperature is 25℃,the temperature of the upper part of the GIL as a whole is higher than the lower part,and the temperature of the concave surface of the insulator is lower than the temperature of the convex surface at the same position.According to the experiment,the body resistivity decreases with the increase of temperature and decreases with the increase of field strength;the greater the electric field strength,the lower the surface resistivity.From the conductor to the shell field strength gradually decreases,the maximum field strength on the convex surface is higher than the maximum field strength on the concave surface.The presence of surface charges on the basin insulator will significantly alter the distribution of the surface field strength,causing the field strength on the convex surface to increase significantly.Positive ions accumulate in the outer shell direction,while negative ions accumulate at the high voltage electrodes on the inner side of the insulator.Positive ions have a large concentration on the concave side,while negative ions accumulate mainly at the triple bond point between the basin insulator and the guide rod,and the ion accumulation is particularly severe at the high voltage triple bond point on the concave side;the overall positive ion concentration is higher than the negative ion concentration.The charge density distribution under the experimental data is significantly different from the charge distribution under the thesis data,so the experimentally obtained data was applied to the study in order to obtain a surface charge distribution closer to the actual one;the direction of the electric field under negative polarity voltage is opposite to that under positive polarity voltage.The maximum field strength under negative pressure on the convex surface is much lower than under positive pressure,and the maximum electric field strength under negative pressure is approximately 64% lower than under positive pressure,and the overall electric field is also lower;the difference between the electric fields on the concave surface is not significant.With the change in air pressure,there is little difference in the distribution of positive and negative charges.The higher the air pressure,the lower the insulator temperature,resulting in a lower electrical conductivity and thus a lower electric field strength.As the ambient temperature rises,the electric field changes in a non-linear fashion,so the change in charge is also non-linear and with a large difference.The insulator conductivity increases with increasing ambient temperature and the increase becomes progressively larger due to the non-linear effect of temperature on conductivity,which also explains the non-linearity of the electric field change with temperature.