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The dielectric response of filled high temperature vulcanized (HTV) silicone rubber has been measured in the frequency range 10−4∼100 Hz and in the temperature range 20∼60°C. The complex relative susceptibility of filled HTV silicone rubber was calculated from the measured complex capacitance by removing the effect of infinity frequency capacitance and taking the geometrical sample factor into consideration, then a master curve of complex susceptibility was constructed. The complex susceptibility results and the master curve were found to have the frequency dependence characteristic of low frequency dispersion (LFD) process. The origination of the LFD process in the dielectric response of HTV silicone rubber was analyzed in depth by taking the microstructure of HTV silicone rubber into consideration.

The pollution flashover characteristics of insulators are tightly related to the salt dissolving out of pollution layer which makes contribution to surface conductivity of insulators in wetting process. For hydrophobic pollution layer on silicone rubber (SR) insulator, both the process of salt dissolving out and the process of salt running off are more complicated in wetting process. The concept of Effective Equivalent Salt Deposit Density (EESDD) is used to characterize the effective salt in wetting process, which is defined as the amount of salt dissolved in water on insulator surface minus the amount of salt ran off from insulator. Actual measurements indicate that EESDD of SR insulators increases in initial half of the wetting period and decreases in ending half of the wetting period. Hydrophobicity of pollution layer, NSDD, polluted manner all have significant influence on EESDD in wetting process. The wetting process of SR specimen keeps 6-9 times as long as that of glass specimen. For naturally polluted SR insulators, maximum percent of EESDD is about 15% of the total ESDD, which is another reason why SR insulators have higher pollution flashover voltage.

The development of composite insulators in China dates back to the early 1970's. In the 1990's they have become widely used in China. At present, up to 600000 composite insulators are operating on 35 to 500 kV ac overhead transmission lines. They are to be introduced into the 500 KV HVDC transmission schemes. In China all composite insulators in service are silicone rubber and are operating in various Contamination conditions. They have successfully and efficiently avoided serious pollution flashover, which occurred frequently when conventional ceramic insulators were used, and radically reduced the maintenance cost of transmission lines in pollution areas. Study of composite insulators is very active in China. The main aspects include properties of materials such as silicone rubber and fiberglass reinforced polymer (FRP) rods, electrical and mechanical performance, aging as well as evaluation and inspection of composite insulators. These fruitful works greatly promote the development of composite insulators in China.

The hydrophobicity and hydrophobicity transfer property of silicone rubber composite insulators not only prevents the formation of continuous water film, but also make the salt in the pollution layer more difficult to dissolve out. In order to study the regularity of salt dissolving process, a model based on mass transfer laws was established in this paper and the second order partial differential equation for describing the salt dissolving process was obtained. The correctness of this model was verified by measuring the salt dissolving process of a site contaminated composite insulators in our laboratory.

For long-term operating outdoor insulation, contamination accumulation has become an increasingly serious problem. In this paper, atomic force microscopy was used to measure the adhesion force of glass, silicon rubber, porcelain, stalinite samples. Test results showed that adhesion force of silicon rubber was the largest, approximately larger than the other three samples by one or two orders of magnitude. Furthermore, silicon rubber surface exhibited a long-range attractive force and multiple points contact, probably due to the electrostatic interaction and surface roughness. It might be a theoretical proof which accounts for the different contamination accumulation between composite insulator and porcelain or glass insulator.

It is well known that the excellent pollution performance of silicone rubber (SR) insulators originates from its hydrophobicity and hydrophobicity transfer properties. With the sustainable growth of composite insulator usage in China, a convenient method should be given to judge the hydrophobicity properties and to help the utilities choose proper products. In this paper, a four-parameter method used in the evaluation of hydrophobicity status was introduced. The four parameters are hydrophobicity on clean surface, hydrophobicity loss property, hydrophobicity recovery property and hydrophobicity transfer property named H/sub c,/ H/sub l,/ H/sub r/ and H/sub t,/ respectively. Relative values of the four parameters, which were based on the research work of hydrophobicity, were also given. This method has already been used in some parts of China for its easy operation and effectiveness regarded. In addition, the mechanism of hydrophobicity transfer property was further investigated and the difference of the results between contact angle and hydrophobicity classification was also discussed in detail.

Publisher Summary This chapter presents the influences of annealing method on the space charge properties in low-density polyethylene (LDPE). LDPE films are prepared by three kinds of annealing methods, which are different in cooling velocity. The crystallinity of the LDPE samples are measured and compared. Surfaces of samples are observed. Higher cooling velocity results in lower crystallinity. High DC negative voltage is applied on the samples and then samples are open-circuit. Space charge formed in samples is measured during the above procedure. The space charge properties based on crystallinity and morphology are discussed.

The usage of composite insulators in various contaminated areas is increasing prominently in recent years. Most of composite insulators are operating in heavy or medium pollution environment in China. As the exposure time in the field prolonging, the change of hydrophobicity and the build-up of pollution layer directly affect pollution performance of the composite insulators. This paper is based on the work of investigation on the pollution performance and aging of nearly 40 composite insulators operated in Jiamusi city, the north east area of China. These composite insulators were removed from 110 kV lines, and they are all made of HTV silicone rubber and mostly are in service for nine years. Four aspects of hydrophobicity of these insulators are studied after the measurement of pollution status.

The mechanisms of tracking and erosion failure of high temperature vulcanized HTV silicone rubber were studied by means of the inclined-plane test. Test conditions were varied by using different kinds of electrolytic contamination solutions instead of the stipulated NH/sub 4/Cl. Arc current I, arc burning, and extinguishing time t/sub a/ and t/sub 0/ were all recorded. Therefore, both the energy E/sub a/ dissipated on the sample surface in each t/sub a/ and the average energy E/sub t/ dissipated on the sample surface per minute could be calculated. Boiling temperatures T/sub b/ of different saturated contamination solutions were also measured. The tracking and erosion test results of silicone rubber showed a good correspondence with E/sub a/ and T/sub b/, but a poor correspondence with E/sub t/. The higher the E/sub a/ and T/sub b/, the severer the tracking and erosion damage of test samples. This means that the tracking and erosion failure of silicone rubber in the inclined-plane test is mainly caused by the heat absorbed on a simple surface in each arc burning time, and the actions of successively burned arcs are independent. >

The usage of silicone rubber insulators in various contaminated areas has increasesd prominently and still keeps rising. In the case of China, more than 90% of the 800000 composite insulators are operating in heavy or medium pollution environments. As the exposure time in the field is prolonged, the change of hydrophobicity and the build-up of pollution layer directly affect pollution performance of the composite insulator. The paper is based on part of the work of a system investigation aiming at inspection and evaluation on pollution performance and aging of the composite insulator in service. More than 40 composite insulators, all made of HTV silicone rubber, were removed from 110 kV-220 kV HV lines. The service time changes from 1 year to 10 years. Hydrophobicity status and pollution status of these insulators are reviewed in the paper. Data from the field provide important references for the research on aging and pollution property and for the build-up of relevant test methods.

Considering various contaminated environments in the field, research on the hydrophobicity transfer properties of different pollutants is necessary for the proper selection and maintenance of polymeric insulators. In the paper, hydrophobicity of SR polluted by several kinds of pollutants such as kieselguhr, kaolin, Tonoko, metal oxides and inorganic salts were investigated. Contact angle measurement and FTIR analysis have been used to trace the hydrophobicity transfer process. It is found that the inherent chemical properties and the physical morphology of pollutants play important roles on the hydrophobicity transfer property.