The Microscopic Electrical Properties Of In-service Aged Sheds Of Composite Insulators

The use of silicone rubber composite insulators in transmission lines have significantly increased over the recent years thanks to their light weight,superior mechanical strength and excellent pollution performance.This tendency could especially be noticed with increasingly raising voltage level of high voltage direct current transmission(HVDC)lines in China,operating today up to ±800 kV.Except for the electrical stress,composite insulators applied in UHVDC transmission lines need to withstand harsh and varying climate conditions as UHVDC lines often cross areas of strong solar radiation and large temperature variations at high altitudes.All these factors contribute to ageing of the insulator housings and with the effect of sunlight combined with access to oxygen and moisture,the rate of material oxidation accelerates.In general,the ageing of insulator housing does not likely result in a loss of electric or mechanical withstand of composite insulators.However,if its effects accumulate,a leap from quantitative to qualitative change of insulator functionality cannot be excluded.The program of the study included measurements of DC conductivity and complex permittivity at frequency range 10-4 – 103 Hz at various temperatures in dry and wet(water immersion)conditions.The obtained results are analyzed by considering a combination of relations describing various dielectric relaxation modes in dielectrics,included three Cole-Cole types of responses and a hopping process.Based on the results of this study,it is claimed that dielectric response measurements on specimens of in-service aged composite insulator sheds can provide useful information on the degree of material degradation.A water immersion test in combination with such measurements may be a good way to enhancing the observed changes.Surface potential decay characteristics of specimens extracted from in-service aged and reference UHVDC composite insulator sheds are reported and analyzed in this paper.In the experiments,surfaces of the insulator samples were charged by dc corona and the decay was recorded utilizing a non-contact technique.Surface and bulk conductivities of the specimens were determined and used for analyzing by means of computer simulations their impact on the potential distribution profiles and its decay.Based on the performed experiments,trap density distributions,mobility of charge carriers and field dependent bulk conductivities are deduced for the investigated samples with the aim to evaluate the ageing severity.The hydrophobicity test,scanning electron microscope(SEM)detection and Fourier Transform Infrared Spectroscopy analysis(FTIR)were used to study the ageing characteristics of in-service aged sheds of HVDC silicone rubber composite insulators.SEM and FTIR provide an important reference for the evaluation of aging degree of composite insulator shed materials.