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This paper studies the AC pollution breakdown characteristics of the EGLA and broaden current knowledge of pollution flashover. Compared with the flashover of the normal insulator, three kinds of specificity of the EGLA were found. Firstly, the pollution flashover process can be divided into three categories. Secondly, the wet flashover voltage, Uf _wet, of the novel insulator is higher than the dry flashover voltage, Uf _dry. Thirdly, the piecewise fitting methods with aESDD^-b+d and aESDD^-b+ cESDD^-d are more suitable for fitting the empirical formula of flashover voltage. Furthermore, to explain specificity, the electric potential and electric field distribution are calculated based on the finite element method. Calculated results indicate that when the EGLA is covered with a 0.5mm water film, the maximum electric field intensity decreased from 2.17 kV/cm to 1.99 kV/cm. Also, U_arrester (the withstand voltage of the insulation section) was obtained from the tests and it presents a discontinuous and random increasing trend until flashover occurs, which is considered to be one of the important reasons for the specificity. Besides, based on the Obenaus model and Hampton's criterion, we further analyzed the arc development process by a simplified plate model

In areas with heavy dust or continuous wind in a single direction, fan-shaped nonuniform contamination at the windward and leeward side of the insulator string may affect flashover performance. In this paper, dc pollution flashover tests of insulators under fan-shaped nonuniform pollution were presented, and a comparison of flashover performance among 7-unit suspension insulator strings with four different disk profiles was made. Then, the influence of fan-shaped nonuniform pollution on flashover voltage, arc propagation, and critical leakage distance were investigated. Research results indicate that the insulators with different types were influenced variously on their flashover stress, utilization rate of creepage distance, and critical leakage distance by fan-shaped nonuniform pollution. A reduction in the ratio W/L of windward to leeward side salt deposit density (SDD) from 1/1 to 1/15 gave a median 35% $\pm$ 4% decrease in flashover strength. The extrapolation of results for the seven-unit strings suggests that some increment in shed space or structure height of the insulator can be preferable in dc transmission lines where there is fan-shaped nonuniform pollution accumulating at the windward and leeward side of the insulator string.

Transmission-line icing presents a serious threat to the safe operation of power systems. Therefore, the icing condition of overhead transmission lines must be accurately calculated for online monitoring and prewarning systems. Based on the icing data obtained at Xuefeng Mountain Natural Icing Stations (XMNIS), the cross-sectional shape of glaze icing on conductors was studied, and the correction coefficient of the actual nonuniform ice was proposed to provide an accurate and convenient method for field icing measurement. In addition, by analyzing the characteristics of the tension and tilt of the insulator string at the iced tangent tower, a new mechanical calculation model considering the ice and wind loads was established. One-month field observations and experiments were conducted in XMNIS, and it is shown that the calculation results have good agreement with experiment results, which suggests that the model can be used for ice thickness calculation and online monitoring on transmission lines.

Pollution-induced flashover has a serious threat on the safe and reliable operation of power systems. It is known that the pollution flashover voltage is directly related to the wetting degree of the pollution layer. But the wetting performance characteristics and control method of pollution layer on insulators have not been properly specified in the existing pollution test standards. In this paper, the wetting processes of the pollution layer on different types of insulators have been studied in an artificial fog chamber. In our test studies, the leakage current on the surface of polluted insulators under a different test environment is measured and then the curves of leakage current varying with time are plotted. Furthermore, the main influencing factors of the wetting process of the pollution layer have been analyzed in this paper. Test results show that the wetting process of the pollution layer can be characterized well through the change of leakage current, and when the leakage current reaches the maximum value, the pollution layer is saturated with fog. Specifically, during the wetting process, the leakage current gradually increases until the maximum value and then decreases; the time for the pollution layer to become saturated is dependent on the salt-deposited density (SDD), the steam fog input rate, the temperature in the artificial fog chamber, and the type of insulators. It is found that with the increase of SDD and the decrease of the steam fog input rate and the temperature in the climate chamber, the necessary wetting time will increase, and the more complex the insulator structure is, the longer the wetting time will be.

In this paper, the dc pollution flashover performance of various types of porcelain, glass, and composite insulators is investigated. It also presents analysis of the dc flashover process of polluted insulator string at high altitude using insight from high-speed photography. The research results indicate that the relationship between the dc pollution flashover voltage and the string length of insulators is basically linear, the characteristic exponents describing the influence degree of air pressure on pollution flashover voltage vary between 0.35 and 0.77 and are related to the insulator types and pollution degree, etc., the characteristic exponents describing the influence degree of pollution on flashover voltage vary between 0.24 and 0.36 and are related to the insulator types and air pressure, etc. Based on the flashover phenomena using the insight from high-speed photography, a new physical model explaining the flashover mechanism for a polluted insulator string at high altitude is introduced, which can be expressed as an electrical circuit consisting of a surface arc of length x1 and air-gap arc of length x2 in series with a resistance representing the wet pollution layer. In addition, the exponent n describing the influence degree of air pressure on flashover voltage for the polluted insulator is discussed.

Ice melting with dc is one of the key technologies to prevent the Chinese power grid from ice storms. Especially after the severe ice storm in the southern part of China in early 2008, dc ice-melting technology has drawn more attention than ever before. However, there are few satisfactory methods to select correct parameters so the ice-melting project rarely achieves desired effects in some cases when applied in Hunan, Guizhou, and other provinces in China. Therefore, it is of great significance to develop a method to estimate parameters which is applicable in the practical situation for dc ice melting. To handle this, the factors, such as wind velocity, ambient temperature, current density and ice-layer thickness were analyzed and then a dc ice-melting model is put forward in this paper. Both the results of simulations and experiments show that the ice-melting process can be divided into three stages composed of temperature rising, ice melting, and ice shedding, among which the ice melting is the key stage consuming most of the ice-melting time. In this stage, an elliptic airgap is formed and widened gradually with an influence on the ice-melting time and the temperature of the conductor. The experiments in the artificial climate chamber demonstrate that the results of the presented model are consistent with those of the experiments generally, so it can be employed to estimate the parameters of ice melting in practical engineering as reference.

The external insulation of electric equipments will be affected by the atmospheric conditions in the high altitude regions. However, a few investigations at the high altitude sites of 3000 m and above have been conducted. This paper reports on the laboratory tests of artificially polluted cap-and-pin porcelain insulators performed at the high altitudes of 2800 m to 4500 m. The test results and analysis show that the pollution flashover voltages in high altitude regions are affected not only by the pollution grades but also by the atmospheric parameters. The exponent characterizing the influence of equivalent salt deposit density is 0.26 to 0.29 and the one characterizing the influence of nonsoluble deposit density is 0.09 to 0.15. The ac pollution flashover voltage U f will decrease with the decrease of air pressure P and increase with the decrease of ambient temperature t . U f is a power function of the ration of air pressure P/P 0 and also is a power function of the ambient temperature coefficient (1 + 0.02(t - 20)). The average exponent characterizing the influence of air pressure is about 0.56 and the exponent characterizing the influence of ambient temperature is about 0.21 for the three types of insulators in the paper.

Based on the artificial pollution tests, the flashover performance of various types of DC porcelain and glass suspension insulators as well as composite long-rod insulators were analyzed and compared. The test results show that there is a nearly linear relation between the pollution flashover voltage and the disc-type insulator string length. The flashover voltage gradients of the insulators are affected by their materials and shed shapes. The antipollution performances of glass insulators are superior to those of porcelain insulators with the same profile. The flashover voltage gradients of composite insulators are higher than those of porcelain or glass ones. The exponent characterizing the influence of salt deposit density on the pollution flashover voltage is dependent on the profile and the material of insulators, and the values of the composite insulators' exponents are smaller than those of porcelain or glass insulators, namely, the influence of the pollution on the composite insulators is relatively less. The effectiveness of leakage distances of porcelain or glass insulators is less than 0.9 while that of composite insulators is higher than 0.9.

Artificial pollution tests have been carried out to study AC flashover performances of various types of porcelain, glass, and polymeric insulators in this paper. The experimental results show that the polluted flashover voltage Uf is affected by equivalent salt deposit density rhoESDD and nonsoluble deposit density rhoNSDD, the influences of which are independent of each other. Based on the analyses of the flashover voltages, the correction formulas of the flashover voltages Uf of various insulators at various rhoESDD and rhoNSDD levels are pointed out. The experimental results also show that there is a distinct difference among the flashover voltages of various types of polluted insulators. At the same pollution degree, the antipollution performance of the fog-type glass insulator is better than those of the outer-rib porcelain insulators. In terms of the effectiveness of creepage distance of insulators, the antipollution performance of polymeric insulators is superior to those of porcelain and glass insulators.

Switching impulse discharge performance of air gaps at high altitudes, which is a fundamental in the extremely high voltage external insulation design, remains an unsolved key technology across the world. Due to the limited applicable range of "g" parameter method recommended by IEC Publication 60-1(1989), this paper aims to explore the relationship between positive switching impulse 50% discharge voltage U 50 of rod-plane air gaps and atmospheric parameters, and proposes two new voltage correction methods based on the parameter "g" and the atmospheric pressure ratio, respectively. The two methods have been proved by the experimental investigations carried out on 1-m rod-plane air gap both in the artificial climate chamber and at three spots of different high altitudes in this study. The results show that, if the additional influence of absolute humidity is considered, the positive switching impulse 50% discharge voltage U50 of rod-plane air gaps is a power function of relative pressure and relative air density; the "g" parameter method in IEC Publication 60-1(1989) is not applicable to the regions with an altitude above 2000 m or the condition of h/delta>15 g/m3; compared with the correction method based on the "g" parameter method, the correction method based on relative atmospheric pressure has a wider applicable range and less error; when the absolute humidity increases 1 g/m3 , U50 varies within the range of 1.0%-1.15%

Based on the results of experimental investigations performed on the polluted and iced IEC standard suspension insulator strings of 3~21 units, the ac flashover performance and discharge process are discussed in this paper. It is put forward that the icing stress product (ISP), defined as the product of the ice-layer conductivity and the ice layer weight per centimeter of dry arc distance can be taken as a characteristic parameter to indicate the effect of ice accretions and contaminants on the flashover voltage. The flashover stress of an ice-covered insulator string can be expressed as a power function of ISP and has a linear relationship with the length of the insulator string. The partial arc of an iced insulator string is initiated by the breakdown of the air gaps between the tips of icicles and iced insulator surface and the persistence is much longer than that of a polluted insulator string