Deterioration Characteristics And Mechanism Of Materials In Composite Insulator Under High Humidity

With the popularity of infrared temperature measurement technology in transmission line inspection,a large number of composite insulators,running under high humidity in south China,were found abnormal heating in recent years.The phenomenon of abnormal heating is closely related to the deterioration of the materials in composite insulator,and may further develop into decay-like fracture.However,the main mechanism of abnormal heating and the relationship between the abnormal heating and deterioration of the materials are not clear.This thesis focuses on the deterioration characteristics and mechanism of silicone rubber and glass fiber reinforced epoxy resin polymer(GRP)in composite insulator under high humidity in order to obtain the relationship between the abnormal heating and deterioration of the materials,and to reveal the development process of composite insulator abnormal heating.The main advances are given as follows.The composite insulator with point temperature rise only showed temperature rise in high humidity environment and the area of temperature rise concentrated in the housing between the hardware and the first big umbrella in the high voltage end The heating source was the dielectric loss of the surface layer of the housing after moisture absorption under alternating electric field.Through the analysis of the layered characteristics of the silicone rubber at the temperature rise position,the dielectric loss after saturated hygroscopicity of silicone rubber was proposed as the index to evaluate the deterioration status.Corona discharge under high humidity was thought as the main cause of housing and umbrella deterioration in composite insulator.The segmental temperature rise of composite insulator was induced by the deteriorated GRP core.The temperature rise was larger than the point temperature rise,and the area of temperature rise extended from the high-voltage end to several umbrella units.It showed significant temperature rise both in high and low humidity environment.The GRP core in the composite insulator was found corroded.Erroded cavity and shadow on the cross section were the two kinds of deterioration forms.The chemical reaction was analyzed by SEM,TGA,XPS and FTIR.The eroded cavity was caused by hydrolysis.The shadow was caused by both hydrolysis and partial discharge.An artificial aging test for composite insulators was proposed,which took the factors into account such as high humidity environment,pollution and ultraviolet radiation.The generation and development of abnormal heating of composite insulator induced by housing and umbrella were reproduced.The relationship between the temperature rise and the saturated hygroscopic dielectric loss of silicone rubber,and the characteristics of saturated hygroscopic dielectric loss of silicone rubber under corona discharge in high humidity enviroment were obtained.The surface microstructure of silicone rubber after the treatment of corona discharge was obtained by SEM and the test results of TGA told that the decomposition of organosilicon compounds and aluminum hydroxide was the main cause of the cavities in the surface of silicone rubber.The relationship between the deterioration of silicone rubber and the content of free and bound water was studied by means of Langmuir diffusion model.Free water was the root cause of the abnormal heating induced by umbrella and housing,which increased the dielectric loss of silicone rubber.The saturated hygroscopic dielectric losses of silicone rubber under different corona discharge intensity have been obtained experimentally.A time-corona strength equivalent model to predict the saturated hygroscopic dielectric loss of silicone rubber was proposed.Considering the relationship between the temperature rise of and the saturated hygroscopic dielectric loss of the umbrella and housing in composite insulator,a prediction method of the abnormal heating was proposed.The water absorption characteristics of GRP at different humidities and temperatures were obtained.It was found that water invasion along the fiberglass orientation would result in weight loss of GRP due to hydrolysis and physical dissolution.The higher the temperature is,the faster the reaction velocity will be.A temperature dependent Langmuir diffusion model for GRP was proposed.The deterioration rules of electrical and mechanical properties of GRP under under hygrothermal environment were obtained.The deterioration rate of hygroscopicity,the properties of temperature rise,conductance,polarization loss,partial discharge of GRP under high humidity increased with aging time.The tensile and bending strength experienced a process of rapid decrease,stable and decrease with aging time.It was found by SEM that the damp-heat effect would hydrolyze the epoxy resin into clastic form,cause the interface failure of glass fiber/epoxy resin,forming the interface cavities,and break the glass fiber successively.The above changes would directly lead to the decline the mechanical properties,and the rise of hygroscopicity,which would cause the deterioration of the electrical properties and further induce the temperature rise of GRP.When water invaded into the micropores in GRP,water tree would be induced under local high electric field.The glass fiber showed an"attractive" effect on the growth of water tree.When the water tree reached to the glass fiber/epoxy resin interface,electric tree would be initiated.Water tree would also form micropores in GRP,and under dry condition,the micropores would intensify the concentration of electric field.The electric tree test was carried on the GRP samples with water tree under different water content conditions.It was found that water tree was more easily converted into electrical tree under dry condition.The growth characteristics of electric trees in GRP were studied experimentally.The growth of the electric tree was "attracted" by the glass fiber and the growth pattern near the glass fiber/epoxy resin interface varied ith the interface state.The increase of water content in the material would change the electric tree from dendritic to tussocky.The critical breakdown field strength was proposed as the index to evaluate the state of glass fiber/epoxy resin interface and a simulation model of electric tree in glass fiber/epoxy resin composites,which took the state of the glass fiber/epoxy resin interface into account,was established.The effects of dielectric constant and orientation of glass fiber,and the state of glass fiber/epoxy resin interface on the growth of electric tree were studied.The influence mechanism of these factors on the growth of electric tree in GRP was revealed.The dielectric constant of glass fiber determined the growth direction of electric tree.When the dielectric constant of glass fiber was higher than that of epoxy resin,the glass fiber showed "attraction" to the electric tree,and on the contrary,it was manifested as"exclusion".The high critical breakdown field strength of glass fiber made the electric tree unable to enter and detour.The interface state of glass fiber/epoxy resin determined the growth pattern of electric tree near the interface.When the critical breakdown field strength of the interface was lower than that of the epoxy resin,the electric tree tended to grow along the interface area straightly.When electric trees contacted the interface of higher critical breakdown field strength,they would stop growing and form a new electric tree growth channel in the adjacent branches.Based on the characteristics and mechanism of electric tree in GRP,the suggestions for optimal design of GRP were put forward from three aspects:decreasing the dielectric constant of glass fiber,reducing the diameter of glass fiber and increasing the surface area of glass fiber appropriately.The development and evolution of the GRP core with abnormal heating in the composite insulator was finally concluded and divided into 4 stages:1.Deterioration of terminal interface;2.Hydrolysis of GRP core;3.Oxidation of epoxy resin;4.Prefracture of GRP core.