Analysis On Aging Properties Of Composite Insulators In Ice-covered Environment Of Xuefeng Mountain

Composite insulators with shed of silicone rubber and mandrel of oxy resin have been widely used in AC and DC transmission in China for 25 years for their excellent electrical and mechanical properties.In long-term operating process,composite insulators are aging to varying degree in harsh environments,especially high altitude area.Due to the long sunshine time,large temperature difference between day and night,intense ultraviolet radiation and long icing period,superficial structures of silicone rubber could be destroyed more easiliy and more chemical reactions such as degradation and oxidation occur in them.In this case,more cracks would appear in the superificial material of silicone rubber and hydrophobicity of silicone rubber would reduce,resulting in the decrease of insulation and stability of power system.In this paper,aging properties including physical,chemical and electrical of composite insulators which have operated in Xuefeng mountain for different years were studied.Based on the test results,the aging characterization method of composite insulators aged in high altitude area was proposed.The main conclusions are as follows:(1)With the increase of operating time,the change regularity of aging properties incluiding physical,chemical and electrical of composite insulators is similar.At the early stage of aging,characterization parameters used to evaluate aging degree of composite insulators change slowly;When aging reaches a certain stage,the change rate of parameters accelerate.Besides,there are some differences of change rate and values of parameters among composite insulators from different factories.(2)With the increase of operating time,the hydrophobicity in the superifial material of silicone rubber decreases gradually,and the hardness and water absorption increase gradually.Combining the microstructure of composite insulators with the Wenzel model of static contact angle,it is found that the surface roughness increases slightly and the hydrophobicity decreases gradually with the gradual appearance of holes and cracks on the surface of composite insulators in the early stage of aging.And influenced by the capillary effect on the surface of silicone rubber,the water absorption and hardness of the composite insulator increase slowly.When aging reaches a certain stage,more rough protrusions appear in the superficial material of composite insulator,resulting in the great increase of surface roughness of silicone rubber.In this case,the surface water droplets of the composite insulator collapse gradually,the hydrophobicity decreases rapidly and the rate of the increase of water absorption and hardness rises substantially.(3)With the increase of operating time,the chemical properties of composite insulators change obviously.Due to the long-term exposure of ultraviolet radiation at high altitude area and the hydrolysis of water molecules in air,the main chain of silicone rubber surface molecules is destroyed,resulting in the increase of the polarity degree.In this case,the polarity degree,the hydrophilic groups in superificial material,the relative content of O elements,the relative dielectric constant of materials,the polarization loss of silicone rubber increase,and the relative content of Si and C elements and the volume resistivity under alternating electric field decrease.(4)Based on the test results of aging properties of composite insulators from four factories,13 aging parameters were proposed.Through correlation analysis independent calculation among these parameters,it is found that the water absorption,the relative content of Si element and the relative content of O element have significant correlation with operating time,which can be used to characterize aging degree of composite insulators.(5)In the process of analyzing aging degree of composite insulators quantitatively,the change percentage of aging characterization parameter was proposed to characterize aging degree of composite insulators quantitatively.Then he fitting relationship between the aging age and the change percentage of saturated water absorption ratio,the relative content of Si and O elements was obtained respectively.Finally,the testing results are substituted for the characterization parameters of four manufacturers,and the theoretical calculation is obtained.The error between the calculated value and the actual value is less than 15%,which is within the allowable range of engineering,and it can be used to guide the life prediction of composite insulators in future research.