Zhijin Zhang

Study on AC Artificial-Contaminated Flashover Performance of Various Types of Insulators

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.

Study on AC Flashover Performance for Different Types of Porcelain and Glass Insulators With Non-Uniform Pollution

The ratio <i>T</i>/<i>B</i> of top to bottom surface salt deposit density (SDD) affects the ac pollution flashover performance of disk insulator strings. The ac pollution flashover stress was established for fifteen different combinations of SDD and T/B in a systematic study, making use of seven-unit suspension strings with six different disk profiles in this paper. Then a comparison was made of pollution performance for glass and porcelain disks with the same profile. The observed relation of ac flashover stress <i>EL</i> to SDD and <i>T</i>/<i>B</i> followed an equation of the form <i>EL</i>=<i>c</i>·SDD^-<i>b</i>·(1-<i>A</i>·log(<i>T</i>/<i>B</i>)) . The values of <i>c</i> , <i>b</i> and <i>A</i> were fitted to test results for glass and porcelain disks of identical bottom-rib profile, and to four other bottom-rib and external-rib profiles. A reduction in the ratio <i>T</i>/<i>B</i> from 1/1 to 1/15 gave a median 26% ± 8% increase in flashover strength, corresponding to the calculated increase in overall pollution layer resistance. Extrapolation of results for the seven-unit strings to UHV dimensions suggests that some reduction in leakage distance can be accepted in areas where there is frequent natural washing of the top surfaces of disk insulators.

Investigation of flashover voltage and non-uniform pollution correction coefficient of short samples of composite insulator intended for ±800kV UHVDC

Differences in the amount of pollution on the top and bottom surfaces of an insulator have a great influence on the DC pollution flashover voltage. Up to now, many investigations have been carried out on non-uniform pollution flashover performance of porcelain and glass insulators, but very few on composite insulators. In this paper the influence of non-uniform pollution distribution on DC composite insulators on their flashover performance was analyzed, and a correction formula for the non-uniform pollution flashover voltage was proposed, based on artificial pollution tests carried out on short samples of a composite insulator intended for ±800 kV UHVDC. The test results showed that, when the ratio (T/B) of the salt deposit density (SDD) on the top surface to that on the bottom surface is between 1/1 and 1/10, the value of the characteristic exponent a indicating the effect of SDD on the flashover voltage ranges from 0.22 to 0.255. The value of a is nearly independent of T/B. However, the flashover voltage U50 decreases with the increase in T/B. The correction formula indicating the influence of non-uniform pollution distribution on the flashover voltage could be expressed as K=1 - b×log(T/B). For the tested composite insulators, the factor b was 0.141 to 0.156, which is smaller than that of porcelain or glass insulator. That is to say, the influence of T/B on the flashover voltage of the composite insulator is weaker than those on the porcelain and glass insulators.

Effects of shed configuration on AC flashover performance of ice-covered composite long-rod insulators

The flashover tests of artificially ice-covered composite long-rod insulators, including four types with a nominal voltage of 110 kV, four types of 220 kV and four types of 500 kV, were carried out in the multi-function artificial climate chamber. The result indicates that, firstly, icing flashover performance of the improved composite insulators for icing regions is not always superior to that of the standard composite insulators when icing is light. Meanwhile, Eh (flashover voltage gradient of dry arcing distance) first increases and then decreases with the increase of CF (creepage factor, namely the ratio of creepage to dry arcing distance). Secondly, when icing is moderate, the icing flashover performance is influenced by such main factors as shed diameter, shed spacing and the configuration of sheds with different diameters. In this situation, Eh of the improved composite insulators is higher than that of the standard composite insulators, and ice-covered composite insulator with three or four different shed diameters has better flashover performance than that with two different shed diameters. Thirdly, when icing is heavy, compared with the moderate icing condition, the difference of Eh between the improved composite insulators and the standard composite insulators diminishes. Finally, when icing is moderate or heavy, there is no obvious regular relation between CF and Eh.

DC flashover performance and effect of sheds configuration on polluted and ice–covered composite insulators at low atmospheric pressure

How to choose external insulation under high altitude, pollution and icing conditions for UHVDC transmission project is a complicated problem. To provide technical reference to this engineering roadblock, the dc icing flashover performance of seven types of pre-polluted short samples of composite insulators at low air pressure simulated by the multifunction artificial climate chamber was researched in this paper. The influences of ice thickness, pollution and air pressure on icing flashover performance were analyzed. The correct formula of dc icing flashover gradient of seven types of pre-polluted specimens at high altitude was presented. The influence of the sheds configuration parameters on the flashover gradient was discussed. And the selection of sheds configuration was suggested.

Equivalence of Influence of Pollution Simulating Methods on DC Flashover Stress of Ice-Covered Insulators

The solid-layer method (SLM) and the icing-water- conductivity method (IWCM) are widely used in the insulator icing tests, but little research has been dedicated to the equivalence of these two methods. Based on the research on dc flashover performance of ice-covered insulators in the artificial climate chamber, this paper studied the equivalent relation between SLM and IWCM according to the equivalent principle of the flashover stress of ice-covered insulators. The results show that the influence of these two methods on flashover stress of iced insulator is equivalent, and this equivalence is determined by the configuration of insulator as well as the icing degree, but the icing flashover stress of the same type of insulators which are on the same pollution level simulated by the two different methods is consistent.

Influence of low atmospheric pressure on AC pollution flashover performance of various types of insulators

The electrical performance of outdoor insulators decreases under low air pressure conditions. The ac pollution flashover performance of various types of insulators are investigated in the multifunction artificial climate chamber in the State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University. The field investigation of the ac pollution flashover voltage performance are carried out at three different high altitudes including Wangkun station (altitude of 4484 m), Nachitai station (altitude of 3575 m) along the Qingzang railway and Geermu urban (altitude of 2820 m). The test results indicate that the characteristic exponent n describing the influence degree of air pressure on pollution flashover voltage, which has influence on the pollution flashover voltage of insulators, is variable, and the value of n is related to the pollution, insulator type etc. Under ac voltage, n is 0.45-0.85. The composite insulators are superior in external insulation selection in high altitude and heavy polluted areas. The relative error between the field test results and laboratory test results is less than 8%, so the research results obtained in the laboratory can be applied to engineering design.

Study of the influence of test methods on DC pollution flashover voltage of insulator strings and its flashover process

The influence of different test methods on dc pollution flashover voltage of insulator strings is presented in this paper. In addition, the relationship among the flashover voltages obtained by different voltage applied methods is put forward. Subsequently, the pollution flashover phenomenon is investigated using the insight from high-speed photography, and the comprehensive magnitude and orientation of forces on partial arcs is analyzed during the propagation. Thus, a new electrical circuit model consisting of a surface arc of length x1 and air arc of length x2 in series with a resistance representing the wet pollution layer supplied by a constant voltage is built. The results obtained from the model indicate that the main reason that makes the exponent b describing the influence of pollution on flashover voltage varying with the configuration and material of insulators is the difference of partial arc fluttering degree for insulators.

Study on the Wetting Process and Its Influencing Factors of Pollution Deposited on Different Insulators Based on Leakage Current

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.

Study on AC pollution flashover performance of composite insulators at high altitude sites of 2800-4500 m

The newly constructed Qinghai-Tibet Railway connecting Golumd and Lhasa is the highest railway in the world, which is located on the hinterland of Qinghai- Tibet Plateau with the average altitude of above 4000 m. Therefore, the external insulation of electric equipments will be affected by the atmospheric conditions in the high altitude regions. In contrast to many test investigations in the artificial climate chamber, there are few ones at the high altitude sites of 3000 m and above. In the paper, the AC artificial pollution tests of four types of composite insulators are carried out at three high altitude sites of 2820 m, 3575 m and 4484 m along the Qinghai-Tibet Railway. The test results show that the AC pollution flashover voltages in high altitude regions are affected not only by the pollution grades but also by the atmospheric parameters. The pollution flashover voltages will decrease with the increase of air pressure but increase with the decreases of ambient temperature. The exponent characterizing the influence of equivalent salt deposit density is 0.26 to 0.32 and the exponent characterizing the influence of non-soluble deposit density is 0.12 to 0.16, the influence of non-soluble deposit density cannot be neglected. The exponent characterizing the influence of air pressure is 0.507 to 0.587. The exponent characterizing the influence of ambient temperature is about 0.20. Considering the combined influence of the air pressure and ambient temperature, the AC flashover voltage of the polluted composite insulators will decrease by 4.0% to 6.4% for the altitude increased per 1000 m.