Study On The Hydrophobicity And Pollution Flashover Properties Of Silicone Rubber Composite Insulators

Silicone rubber (SIR) composite insulators have been widely used in electric power systems to combat pollution flashover problems of transmission lines due to the unique hydrophobic properties of SIR materials. However, the hydrophobic and pollution flashover properties of SIR insulators and the relevant mechanisms have not been thoroughly understood by people. In this dissertation, four aspects of research work on the hydrophobic and pollution flashover properties of SIR insulators were carried out with the consideration of the typical operation environments and the surface status of the operated SIR insulators. The relevant mechanisms were also explained in detail.In order to better master the hydrophobic properties of polluted SIR insulators under actual operating environmental conditions, an on-line hydrophobicity inspection technology was developed. Firstly, based on the spray method and the digital image process technique, a new hydrohobicity class evaluation method was brought forward, in which two parameters K (the ratio of the area of the maximal water droplet or trace in a image of a water sprayed insulator surface to the gross area of the analyzed region in the image) and fc (the shape factor of the maximal water droplet or trace) are calculated. Also, a new on-line hydrophobicity inspection device was designed, which consists of an automatically electric sprayer, a mini digital video camera and a hydrophobcity analyzing software. Then, on-line hydrophobicity inspection was conducted for the polluted SIR insulators on four AC transmission lines in the north of China for more than one year by using the device. It is found from the inspection results that the surface hydrophobcity of the SIR insulators varies remarkably with seasons. On the Basis of the inspection results, an on-line dynamic inspection method on the surface hydrophobicity of SIR insulators is proposed, in which the surface hydrophobicity decrease in winter and its recovery in summer are considered.Considering the fact that the surface of the operated SIR insulators often appear the weak hydrophobic status from November a year to April of the coming year during which period more than 90% pollution flashover accidents happened in China, the pollution flashover properties of kaolin-polluted SIR insulators with weak surface hydrophobicity were studied by using the up and down method. It is found that although the pollution layer of the samples can be completed wetted by fog, the discrete water droplets still can be formed on the sample surfaces, resulting in their 50% withstand voltage higher about 40% than that of the hydrophobic samples. Based on the Obennaus theory, it is believed that the uneven distribution of the surface conductivity, caused by the diffusion of soluble salt from the pollution layer to the discrete water droplets, decreases the effective surface conductivity of the samples, and increases the pollution flashover voltage.Because the long-time wetting stresses, i.e., high relative humidity and fog, are considered as the important factors heavily influencing the outdoor insulation properties of the operated SIR insulators, the hydrophobic change properties and pollution flashover properties of kaolin-polluted SIR insulators with weak surface hydrophobicity and kieselguhr-polluted SIR insulators with high surface hydrophobcity were studies under the long-time light-fog conditions. The test results show that both two kinds of samples exhibit the good anti-hydrophobicity-lose capability. However, after subjected to the long-time wetting stress, the pollution flashover voltage of most kieselguhr-polluted SIR insulators heavily decrease to the case of the kaolin-polluted SIR insulators with weak surface hydrophobicity. According to the Obennaus theory and the discharge theory under uneven electric field, it is believed that the phenomena of the low flashover voltage on the high hydrophobic samples is caused by the combination of the decrease of the surface resistance and the intensification of the partial electric field between the water droplets and along the mold junction areas on the samples.According to the typical ambient temperature characteristics of the pollution flashover accidences of insulators, the pollution flashover properties of kieselguhr-polluted SIR insulators with high surface hydrophobcity were studied by using a cold fog test method we developed, in which the ambient temperature is controlled to cycle in the range of -2°C~2°C during testing. The test results show that under the cold fog conditions, the existence of a thin ice layer on the sample surface forces the melting ice water to diffuse into the pollution layer and to spread along the pollution layer surface, causing the distinct decrease of the surface hydrophobicity on the samples, and further resulting in the decrease of the flashover voltage of the samples.Finally, on the basis of the whole experimental results mentioned above, it is suggested that the outdoor insulation parameters of SIR insulators intended for use in pollution areas should be designed by using the artificial pollution test results of the weak hydrophobic SIR insulator samples.