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The pollution on insulators is composed of various chemical components, which threaten the stability of external insulation. However, the existing insulation designs have not taken this issue into consideration. This work aimed to study the influence of pollution chemical components on flashover performance of insulators with various materials. Flashover tests of porcelain, glass and silicone rubber insulators polluted by eight types of common contaminants were conducted. Based on the test data, the influence of chemical components and their solubility on equivalent salt deposit density (ESDD) and insulator flashover performance were analysed; and the electrical properties of different types of insulators were compared. Results show that the ESDD contribution rate changes with chemical components. The dissolution characteristic has a significant influence on the measurement of ESDD; slightly soluble contaminants need to be considered separately. The corresponding insulator flashover gradient for ions can be summarised as Cl^−

Insulator contamination directly threatens the safe and stable operation of the overhead line. Through spatial electric field monitoring, it is expected to realize insulator pollution condition monitoring and contamination flashover warning in a non-contact way. So in this paper, the spatial electric field distribution characteristics of 110 kV composite insulators are simulated, where the effects of different surface states and their discharge levels on the spatial electric field of insulators are analyzed. On this basis, a non-contact monitoring method for composite insulator pollution based on the spatial electric field is proposed. The results show that there are significant differences in the spatial electric field of the composite insulator among three conditions of clean, pollution layer wetting, and dry band arcing. With the increase of pollution layer wetting and dry band arcing, it would lead to an increase of the amplitude of the spatial electric field of the insulator. It is feasible to identify the degree of pollution layer wetting as well as dry band arcing of the insulator string, by fixed-point monitoring the spatial electric field signal at the cross-strand of d = 0.5 m and directly opposite the last three positions. Research results can provide references for the online monitoring of overhead line pollution insulators and its flashover warning.

Pollution-induced flashover is one of the most serious power accidents, and the pollution degree of insulators depends on atmospheric environmental parameters. The pollution models used in the power system research are usually static, but the environmental parameters are dynamic. Therefore, the study on the dynamic pollution prediction model is of great importance. In this paper, the dynamic pollution prediction model of insulators based on atmospheric environmental parameters was built, and insulators’ structure coefficients were proposed based on the model. Firstly, the insulator dynamic pollution model based on meteorological data (PM2.5, PM10, TSP (total suspended particulate), and wind speed) was proposed, and natural pollution tests were also conducted as verification tests. Furthermore, insulator structure coefficients c1, c2 (c1: pollution ratio of U210BP/170 to XP-160; c2: calculated pollution ratio of U210BP/170T to XP-160) were then obtained, and their influence factors were discussed. At last, insulator structure coefficients were calculated, and it can be seen that the calculated error of insulator structure coefficients was acceptable, with the average re (relative errors) at 9.0% (c1) and 13.5% (c2), which verifies the feasibility of the model. Based on the results in this paper, the NSDD (non-soluble deposit density) of insulators with different structures can be obtained using the insulators’ structure coefficient and the reference XP-160 insulator’s NSDD.

Through spatial electric field monitoring, it is expected to realize insulator pollution condition monitoring and contamination flashover warning in a non-contact way. Therefore, in this paper, the spatial electric field distribution characteristics of 110 kV composite insulators are simulated, where the effects of different surface states and their discharge levels on the spatial electric field of insulators are analyzed. On this basis, a non-contact monitoring method for composite insulator pollution based on the spatial electric field is proposed. The results show that there are significant differences in the spatial electric field of the composite insulator among three conditions, namely cleaning, pollution layer wetting, and dry band arcing. Increases of pollution layer wetting and dry band arcing would lead to an increase of the amplitude of the spatial electric field of the insulator. Verification experiments well indicated that it is feasible to identify the degree of pollution layer wetting as well as dry band arcing of the insulator string by fixed-point monitoring, the spatial electric field signal at the cross-strand of d = 0.5 m and directly opposite the last three positions. Research results can provide references for the online monitoring of overhead line polluted insulators and its flashover warning.

Pollution materials affect insulator flashover performance, and there is long-term significance in researching it. In this paper, common kinds of nitrates were used as the soluble pollution, and the ac flashover performance of four types of insulators under different nitrates were investigated. Basing on a large amount of primary data, the calculation of flashover gradient in specific to different pollution materials was discussed, and the effects of nitrates on it were analyzed. Research results indicate that, flashover gradient of insulator under nitrates is commonly higher than that under NaCl; among the nitrates, KNO3 corresponds to lower insulator flashover gradient while Ca(NO3)2 corresponds to a higher one; the exponent b, which characterizing the influence of ESDD on flashover gradient, is lower under nitrites than that under NaCl; for the four insulator samples polluted by nitrates, the flashover gradient of the porcelain type can be expressed by A×ESDD−0.21×NSDD−0.16, the flashover gradient EL of the glass type can be expressed by A×ESDD−0.22×NSDD−0.14, while that of the composite samples can be expressed by A×ESDD−0.19×NSDD−0.13. Research data may provide references for the better prediction of flashover voltage as well as for the better design of external insulation.

Affected by long-term crosswind, insulator surface will get coated with fan-shaped non-uniform pollution, which lowers insulator flashover voltage as previous study revealed. In this study, a further research in specific to HVDC line insulation is presented. Taking typical dc insulator as the sample, its dc flashover performance under fan-shaped non-uniform pollution was tested. Then the influence of that kind non-uniformity was investigated through analysing the pollution layer residual resistance, the flashover process and critical flashover voltage. Finally the relevant HVDC insulation selection was made. Research results indicate that: under fan-shaped non-uniform pollution, insulator flashover voltage ( U 50) gets impaired, and is more obviously impaired by ratio J of windward to leeward side salt deposit density (SDD) than leeward side area proportion T ; under various T and J , there is still an exponential relationship between U 50 and SDD; under fan-shaped non-uniform pollution the pollution layer residual resistance decreases, which theoretically promotes the arc propagation and flashover process, and thus finally lowers flashover voltage; for the transmission line in area with fan-shaped non-uniform pollution, adding insulator units to strengthen the withstand property are needed.

Pollution-induced flashover is a serious threat to the safe operation of power systems. With the development of High Voltage Direct Current (HVDC), it is necessary to study insulator contamination in DC electric fields. In this paper, the energized wind tunnel contamination test was conducted in order to systematically study the pollution ratio, k (ratio of non-soluble deposit density (NSDD) of a DC-energized condition to a non-energized condition), under different environmental parameters. Later, a two-dimensional contamination model of short samples of an HVDC composite insulator was established. The particle motion characteristics under different environmental parameters were then analyzed by the finite element method (FEM). The research results showed that—the DC electric field had an influence on particle motion but in different environments, the degree of influence was different. In addition, k was found to largely vary, with a variation in the environmental parameters. When the electrical stress (Es) increased from 0 to 70 kV/m, k increased gradually. However, when the wind speed (ws) increased, k experienced a decreasing trend. Finally, as the particle diameter (dp) decreased, k increased at first, followed by a decrease, and then again showed an increase. The results of the pollution ratio, k, for different environmental parameters are of great importance for guiding anti-pollution work in power systems.

Currently, it is founded that under unidirectional wind and certain landform, composite insulators are always fan-shaped non-uniformly polluted. In this paper, artificial pollution tests on three types of composite insulators with different shed configurations and under various fan-shaped non-uniform pollution conditions were carried out. Then the flashover performances of composite insulators, porcelain insulators and glass insulators were compared. Results indicate that there is a big difference between ac flashover performance of composite insulators under non-uninform pollution and uninform pollution. The flashover voltage of composite insulator is largely influenced by salt deposit density (SDD), the ratio of SDD of windward side to leeward side (W/L), the occupation ratio of leeward side k and the shed configurations. The relationship between SDD and U 50 still meets negative power function when composite insulator is fan-shaped non-uninform polluted. There is a reduction of 17.8–27.6% in the flashover strength when the ratio W/L of SDD decreases from 1/1 to 1/15. The shed configurations of composite insulators have great effects on the flashover performance. Composite insulators always have better withstand property compared to glass and porcelain insulators under either uniform pollution or fan-shaped non-uniform pollution.

Previous simulation and observations showed that, the air flow field at windward and leeward side of insulator string will probably be different if the wind velocity is over a certain value, and thus causing the imparity of contamination degree at the two sides. In this paper, fan-shaped non-uniform polluting method was used to artificially simulate the different pollution severity at windward and leeward side of insulator string. Then, the effects of fan-shaped non-uniform pollution on ac flashover performance of 5 types of insulators were discussed. Test results indicate that, SDD, the ratio SDD of windward to leeward side (W/L), the occupation ratio of leeward side (k), will affect the ac flashover performance of insulator string. U 50 and SDD still can be expressed by the negative power function. A reduction in W/L from 1/1 to 1/15 would give a decrease about 23% in a value, which causes U 50 reducing about 25%, but n value changes little. The correction method proposed by EPRI is also applicable for fan-shaped non-uniform pollution, but has difference. A larger creepage distance of insulator does not mean that the creepage distance utilization rate is higher. Due to the influence of fan-shaped non-uniform pollution, the existing insulation configuration of transmission lines in heavily polluted areas may not satisfy the required insulation margin. The results have certain reference significance for the design and selection of transmission line external insulation.

Observations of contamination tests showed that, the contamination distribution of insulators is usually uneven. In this study, the solid layer method was used to imitate different non-uniform pollution conditions and the AC pollution flashover tests of 7-unit insulators were carried out. The results indicate that the increase of uneven pollution degree J from 1 to 15 will give a 39% raise of U 50 under non-uniform pollution on top and bottom surfaces, a 23% decrease for fan-shaped non-uniform pollution and a 21% higher U 50 with ring-shaped non-uniform pollution. A higher occupation ratio (k) of leeward side will reduce U 50 under fan-shaped non-uniform pollution and a bigger radius R of heavy polluted area has certain effect on U 50 for ring-shaped non-uniform pollution. The U 50 and salt deposit density still can be expressed by the negative power function, and non-uniform pollution conditions and J have great effect on a, but less impact on n. The correction equations of different non-uniform pollution conditions are obtained and the relative errors are acceptable. The research has certain reference significance for the design and selection of AC transmission line external insulation.

The non-uniform air flow field and electrical field distributing around insulators cause uneven deposition of pollution particles on different positions and parts of the insulator surfaces. Pollution-induced flashover is a serious threat to the safe operation of the power system, and non-uniform pollution distribution has great influence on flashover voltage of insulators. Towards this, in this study, the solid layer method was used to simulate different non-uniform pollution conditions. Then, DC pollution flashover tests of composite insulators were carried out and the flashover performances were analyzed. The results indicated that the increase of KL/W (from 1 to 15) decreased U50 by 27–32%. While the increase of KT/B (from 1 to 10) increased U50 by 35%, and the increase of KH/M (from 1 to 4) increased U50 by 25%. Where KL/W, KT/B and KH/M are the SDD (salt deposit density) ratio of windward to leeward side, top to bottom surface and high voltage to middle group respectively. Besides, higher proportion of leeward side area will reduce U50. U50 and SDD are still in agreement with the negative power function relationship, non-uniform pollution conditions have great effect on a (constant of power function), but less impact on n (characteristic index of power function). Finally, equations for calculating different non-uniform pollution flashover voltages were established and validated, also the relative errors are acceptable. The results are very important for guiding insulation design and anti-pollution works of operating transmission lines.

Pollution-induced flashover is critically affecting the safety operation of the power system so as to puts social production at risk. However, few literatures have predicted insulator pollution levels. Therefore, in this paper grey theory is introduced and GM (1, N) method for the prediction of pollution on insulator surface was proposed. Further, a pollution test lasting for 9 months was conducted to build the GM (1, N) model. The test of another 4 months was also conducted to contrast the test results (ΔESDD (Equivalent salt deposit density)) and predicted results. The research results have shown that: GM (1, N) was applicable to predict the pollution on insulators at the cases of poor information with less data for building model. Besides, considering more environmental factors is conducive to improving the prediction accuracy of the GM (1, N) model. However, wind speed that varied very little during the 13 months' nature contamination periods in Chongqing can be neglected, which may improve the accuracy of the model (with its relative error decreasing from 7.9% to 6.3%). Next, the application scenario of the model is proposed. In addition, GM (1, N) method has potential applications in other field, and the prediction accuracy of GM (1, N) model can also be improved by increasing or decreasing the number of influencing variables as well as the number of samples. The proposed GM (1, N) method provides guidance for anti-pollution work in power systems, which promotes the reduction of flashover accidents and maintain social production safety.

Under dc electric field, pollution may be more easily deposited around insulator string caps and pins, distributing non-uniformly on its inner and outer parts, and presenting ring-like shape. dc Flashover tests of 7-unit XHP-160 insulator string were carried out. During the tests, the ring-shaped non-uniform pollution was simulated using solid-layer method by changing the ratio I/O of inner-to-outer surface salt deposit density (SDD) and radius (r) of the heavily polluted area (the inner part). On the basis of the test data, the effects of ring-shaped non-uniform pollution on dc flashover characteristics were studied. Then, the relative variation trend of flashover voltage, surface pollution layer resistance, and leakage current were discussed. Research results show that the ratio I/O affects the dc flashover performance of the insulator string. The flashover voltage increases with the increase of I/O. With the increase of the radius r, the flashover voltage will increase at first and then decrease. Under ring-shaped non-uniform pollution, surface pollution layer resistance of insulator will increase, which results in lower leakage current and higher flashover voltage. The results will be of certain value in providing references for outdoor insulation selection.

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.

The type of pollution materials may influence the flashover performance of an insulator. In this paper the pollution flashover performance of a 7-unit insulator string, consisting of typical discs, is determined by laboratory tests while varying the weight percentage of soluble (NaCl and CaSO4) or non-soluble (SiO2, Al2O3 and Fe2O3) materials. When studying the effect of pollution materials on flashover performance, the influence of volume conductivity and equivalent salt deposit density (ESDD) under different weight percentages of soluble materials is firstly investigated, and then the underlying factor, the thermal ionization of pollution materials, is considered. Finally, the measurement results of leakage current under different pollution materials are presented. The research results indicate that the effect by weight of CaSO4 is not as severe as that of NaCl, and the reason is not just its effect on the layer conductivity and ESDD, but its effect on the thermal ionization during the arc propagation process may also be one of the key factors. The fact that a small amount of Al2O3 and Fe2O3 causes the flashover voltage to decrease by 20%, may result from the lower ionization potentials and the higher leakage currents caused by non-soluble materials containing Al2O3 and Fe2O3.

Under continuous wind of single direction, the leeward area of insulator string may be polluted more seriously than windward area, making the pollution layer distributes like a fan-shape. In this paper, dc pollution flashover tests of 7-unit XP-70 insulator string with fan-shaped non-uniform pollution were carried out. Then the influence of fan-shaped non-uniform pollution on flashover voltage, flashover process, leeward side surface layer conductivity and surface pollution layer resistance were discussed. Research results indicate that, the ratio W/L of windward to leeward side salt deposit density (SDD) affects the dc flashover performance of insulator string. A reduction in W/L from 1/1 to 1/15 would give a decrease of 28% in flashover strength. Flashover voltage may also decrease when the occupation ratio (T) of leeward side increases from 10% to 30%. Under fan-shaped non-uniform pollution, U50 can still be expressed by a negative exponential function of SDD, and the effect of fan-shaped non-uniform pollution on U50-SDD relationship may mainly lie in changing the proportionality coefficient a rather than the exponent n; discharge path was restricted along the leeward side, which may make the air-gap arc easily formed and benefit the flashover process.

DC flashover performance of ice-covered composite insulators with a parallel air gap (CI/PAG) is an important technical consideration when such insulators are used to isolate ground wires for the purpose of DC ice-melting. Tests on tension and suspension types of CI/PAG were thus carried out in the artificial climate chamber to investigate their DC icing flashover performance. The influences of parallel air gap, ice thickness, pollution severity and air pressure on DC negative 50% flashover voltage (U50%) of CI/PAG were investigated. Test results show that the parallel air gap affected both the discharge path and U50%. With increasing ice thickness, U50% declined by up to 52%, this effect was more evident when the breakdown occurred in the air gap. The pollution severity affected U50% only when the flashover happened along insulator surface. With a decrease of atmospheric pressure, U50% decreased. U50% and the ratio of air pressure were in a power function relationship with a positive characteristic exponent which was relevant to the discharge path.

As natural contamination of insulators may lead to flashover, its determining factors should be studied to prevent the occurrence of pollution flashover. In this paper, the fluid mechanics models of insulators with typical shed types were established to simulate their flow field distribution and particle collision coefficient (F). At the same time, a natural contamination test lasting for 8 months was carried out in order to contrast the experimental data and simulation results. The research results have shown that: wind velocity and shed type affect the flow field near the insulators; F is affected by particle diameter and wind velocity; under certain particle diameter and wind velocity, F reaches the maximum value; rainfall has different effects on contamination of insulators with different shed types; wind causes non-uniformly distribution of pollution on the surfaces; the simulation test and the experimental test are comparable. The research method and results can provide not only reference to establishing natural contaminating model but also significant meaning for the external insulation design and selection.

The ac pollution flashover performance and flashover process of LXHY 3 -I6O insulators was investigated at a high altitude site (altitude of 1400m) in this paper. The solid layer method was used, and even-rising voltage method was adopted. According to the test results, the average pollution flashover voltage ( U av ) decreased with the increase of pollution, and there was no obvious relationship between the exponent characterising the influence of pollution and the length of the insulator strings. The relationship between U av and the length of the insulator strings was nearly linear. Subsequently, the effect of the pollution and the length of insulator strings on the critical flashover arc trajectory were analysed. It concluded that the critical flashover distance ( l c ) decreased with the increase of pollution, and it was shorter than the leakage distance ( L 0 ), whereas there was no significant relationship between l c / L 0 and the length of insulator strings. The arc appearance at the high altitude site was very irregular with obvious bending and rocking; the local arcs deviated from the surface of the polluted insulators even more seriously and the diameter was thicker than those obtained in the normal environment. Moreover, the brightness and diameter of local arcs was related to pollution.

The effects of tumble combined with EGR (exhaust gas recirculation) on the combustion and emissions were experimentally investigated in a spark ignition engine in this study. A low pressure loop EGR system and a variable tumble valve in intake ports were employed, by which wide range of EGR ratio and tumble ratio could be achieved. All the tests were performed at part loads, and the equivalence ratio was kept constant at 1.0. The results show that the combination of EGR and enhanced tumble motion produces a significant improvement in fuel economy (13.1–19.5%) and a reduction in the coefficient of variations of indicated mean effective pressure (CoV IMEP ) due to the significantly shortened combustion duration. By optimizing the combustion process with EGR and enhanced tumble, NO x (nitrogen oxides) emissions drastically decrease, while HC (hydrocarbons) emissions slightly rise. For CO (carbon monoxide) emissions, the enhanced tumble takes a positive effect.

Natural polluted insulator surfaces are always coated with various kinds of soluble constituents, and those constituents affect flashover performance differentially. Currently, this fact is not considered either in laboratory experiments or field pollution degree measurements, causing the existing insulation selection method to be deficient. In this paper, a systematic study on insulator flashover voltage gradient correction involving different types of soluble pollution constituents is presented. Using a typical type glass insulator as the sample, its flashover tests, polluted by typical soluble chemicals (NaCl, NaNO3, KNO3, NH4NO3, MgSO4, Ca(NO3)2 and CaSO4), were carried out. Then, the flashover gradient correction was made by combining the flashover performance of each soluble constituent, the equivalent salt deposit density (ESDD) contribution of the seven constituents, and the saturation performance of CaSO4. The correction agreed with the flashover test results of insulator polluted by three types of soluble mixture. Research results indicate that the flashover gradient correction method proposed in this paper performs well in reducing the calculating error. It is recommended to carry out component measurements and flashover gradient correction to better select outdoor insulation configuration.

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.

The external insulation contamination on real transmission line is mainly composed by the mixture of NaCl, CaSO4 and other various kinds of constituents. Different constituents have effects on the flashover performance of various types insulator, but it is still not considered by the existing standard about external insulation design in polluted region. This paper investigated the electrical property of different types of insulator string under typical pollution constituents. During the tests, the insulator samples were polluted by NaCl, CaSO4 and mixture pollution constituents in representative region respectively. The flashover data was obtained and then the waste rate of insulation were calculated based on pollution withstand voltage method. Research results indicate that, The characteristic b of ceramic, glass and composite insulator under mixture constituents pollution is obviously lower than that under NaCl, traditional design method overrates the influence of ESDD on flashover performance; the electrical property of applied insulator under mixture pollution constituents should be studied to provide better reference for the external insulation design; When using the pollution withstand method to design the external insulation, the leakage distance of insulation should be chosen basing on the mixture pollution constituents in practical region.

Pollution materials affect insulator flashover performance, and there is long-term significance in researching it. In this paper, common kinds of nitrates were used as the soluble pollution, and the ac flashover tests of typical glass insulator were carried out. During the tests, the samples were polluted by dipping method. Also, up-and-down method was used to obtain the insulator flashover gradients under different pollutants and ESDD values. Basing on the test data, the effects of nitrates on insulator flashover performance were analyzed. Research results indicate that, using the current method to calculate the flashover gradient of insulator polluted with nitrates, the relative error will reach 24%; Flashover gradient of insulator under nitrates is higher than that under NaCl; KNO3 corresponds to lower flashover gradient while Ca(NO3)2 corresponds to a higher one; The pollution characteristic b of glass insulator under nitrates is within 0.18 – 0.25; The flashover gradient EL of glass insulator polluted by single or mixed nitrate can be expressed by 25.6×ESDD−0.21.

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.

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.

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.

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.

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.

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.

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.

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.

Based on the artificial pollution tests, the effects of pollution and high altitude on the flashover performance of short samples of five kinds of UHV/EHV dc composite long rod insulators are analyzed. The exponent characterizing the influence of salt deposit density on the flashover voltage is related with the profile and the material of the insulator shed. The values of the samples' exponents vary between 0.24 and 0.30, which are smaller than those of porcelain or glass cap-and-pin insulators, namely, the influence of the pollution on the composite long rod insulators is less, relatively. Thus, the composite insulators have certain advantages in severe pollution regions. The best ratio of the leakage distance to the arcing distance is about 3.35. The exponent characterizing the influence of air pressure on the flashover voltage is related with the profile and the material of the insulator shed and the pollution severity, the values of the samples' exponents vary between 0.6 and 0.8, which are larger than those of porcelain or glass cap-and-pin insulators. Therefore, the dc composite insulator used in high altitude regions should have enough arcing distance. Based on the test results, if insulator sample Type E is selected for the plusmn800 kV UHV dc transmission lines, the basic arcing distance should be no less than 8.16 m and the basic leakage distance no less than 30.2 m.

Under drizzle, light rain, heavy fog and other adverse weather conditions, polluted insulators may flashover under an operating voltage which serious threat to reliable operation of power systems. Therefore, it is very important to understand the natural pollution accumulating rule of insulator energized in the operating voltage. This paper takes the XP-70 insulator string being suspended without voltage in a certain power network in Shantou as the sample, tests and analyzes the pollution depositing rules of it. Test results indicate that: The performance of natural pollution deposit is related to geographic location and voltage class. The accumulation rate of non-soluble salt deposit density (NSDD) on the surface of insulator is more serious than that of equivalent salt deposit density (ESDD). The pollution depositing situation of insulator is different under different voltage classes, the pollution accumulating situation of the 110kV line is the most severe, while that of the 220kV line is a bit lighter and the 500 kV line is the lightest. The pollution levels of transmission lines in Shantou area are mainly c and d. The test results can be used to draw the map of pollution areas, help workers to clean insulators and direct external insulation coordination and design in the pollution area.

It is important to study the natural characteristic of pollution accumulation and flashover performance of insulator energized the operating voltage. This paper mainly focus on the insulator strings operating in a 500kV power network in Chongqing, tests and analyzes the pollution depositing rules and flashover performance of them. Test results indicate that: the value of equivalent salt deposit density (ESDD) on the top surface of insulator is lower than that on the bottom, as well as the value of non-soluble deposit density (NSDD);the accumulation rate of NSDD is heavier than that of ESDD; electric field distribution and precipitation exert a strong influence on the insulator obviously; at the same pollution level, the flashover voltage of natural contaminated insulator is 2 times as high as that of artificial polluted insulator, which mainly caused by the difference of pollution materials and non-uniform pollution distribution on the insulator surface. The results of this paper may be the reference for external insulation coordination and design in the pollution area.

In this study, icing flashover performance tests of typical DC porcelain, glass, and composite insulators are systematically carried out in a multifunction artificial climate chamber. The DC icing flashover voltages of seven typical insulators under various conditions of icing thickness, pollution severity before icing, string length, and atmospheric pressure are obtained. The relationships between icing thickness, salt deposit density as well as atmospheric pressure and the 50% icing flashover voltage are analyzed, and the formulas are obtained by regression method. In addition, the DC icing flashover voltage correction method of typical porcelain, glass, and composite insulator in the coexisting condition of high altitude, contamination, and icing is proposed.

This paper takes the composite insulator being operated in a certain 110kV power network in China as the sample, tests and analyzes the pollution depositing rules and flashover performance of it. Test results indicate that the equivalent salt deposit density (ESDD) and non-soluble deposit density (NSDD) on the upper and lower surface of composite insulators energized running voltage is different. The ratio of the ESDD and NSDD of upper surface to that of lower surface is 0.76 and 0.46 separately. The structure and shape of polymeric insulators have influence on the pollution deposit levels and non-uniform degree. The dispersion of the pollution flashover voltage of naturally polluted composite insulators is great. Under the same condition, the pollution flashover voltage of naturally polluted composite insulators is higher about 50% than that of artificially polluted composite insulators. The difference of hydrophobicity, component of contamination and non-uniform degree of pollution are the main reason that caused the pollution flashover voltage of naturally polluted composite insulators is higher than that of artificially polluted composite insulators.

The electric performance of polluted insulator string may reduce under high altitude. The paper studies the AC pollution flashover performance of insulators at different height of altitude field and chamber climate separately. The test results show that: The deviation between test results with the artificial and field experiments is less than 8%, when the test carried out in climate chamber under low air pressure conditions, 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 mutative, and the value of n is related to the pollution, insulator type etc.. Under AC voltage, n is 0.50~0.60. The composite insulator is superior in external insulation selection in high altitude and heavy polluted areas. The results obtained by the test which carried out in climate chamber can guide the designing of the external insulation.

Non-uniform distribution on an insulator has a great influence on the DC pollution flashover voltage. a lot of investigations have been carried out on the non-uniform pollution flashover performance of the porcelain and glass insulators, but the research on the composite insulators is still very few. In this paper, based on the artificial pollution tests carried on the DC composite insulator, the influence of non-uniform pollution distribution of the DC composite insulator on the flashover performance is analyzed, and the correction formula of the non-uniform pollution flashover voltage is proposed. The test results show that, 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 insulator, the factor b is 0.141~0.156, which is smaller than those of porcelain and glass insulators.