Study On Corona Onset Characteristics Of EHV AC Fittings At Different Altitudes

As a key component of EHV power transmission and transformation project,fittings have prominent corona problems due to their large curvature or lack of shielding of other metal conductors.However,the current research level and relevant design specifications can not meet the relevant requirements,and the corona problem becomes more obvious with the increase of altitude.Therefore,in order to effectively suppress the corona discharge on the surface of fittings at different altitudes,research on its corona onset characteristics is very important.Previous studies mainly focuse on the corona onset characteristics of transmission lines,but only few studies was conducted on the corona onset test and calculation method of corona onset field intensity of fittings.The corona discharge of 17 500 kV EHV AC substations in Inner Mongolia was observed by using the "solar-blind" UV imager.It was found that the corona discharge of shielding ball,grading ring and spacer in the substation is severe.Therefore,by using the finite element method(FEM)and COMSOL multiphysics simulation software,the 3-D finite element calculation models of ES field were established for the three typical fittings with serious corona discharge,and the surface electric field distribution was calculated.Then,by changing the structure and size of the fittings,the influence laws on the surface electric field intensity of the fittings were obtained.The experimental investigation of corona onset characteristics on fittings were carried out in high-voltage laboratory of CEPRI in Changping,Beijing(altitude 50m),and Qinghai electric power company in Chengdong District,Xining City,Qinghai Province(altitude 2250 m).The corona onset field intensity values of fittings having different structures and sizes were obtained at different altitudes.The influence laws of altitudes,structures and sizes parameters on the corona onset field intensity were analyzed.The empirical prediction formulas of corona onset field intensity of different types of fittings in different altitude areas were put forward.Based on the basic physical process of corona discharge,the photon geometric absorption area factors suitable for three typical structures of shielding ball,grading ring and spacer were proposed,and the 3-D calculation physical models of the corona onset field intensity were established.The experimental results are in good agreement with the calculated results,which verify the accuracy of the calculation model.According to the calculation results of electron impact ionization coefficient,electron adhesion coefficient and ionization integral,the photoelectron emission mechanism on the surface of negative conductor is quantitatively studied.In addition,combined with the ionospheric thickness and effective ionization coefficient,the influence mechanism of environmental parameters(atmospheric pressure,humidity and temperature)on the corona onset field intensity was analyzed.