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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.

The electrical characteristics of ice-covered insulators in coexisting conditions of high altitude, icing and pollution are a basis for external insulation selection of transmission lines in icing regions. Ice accretion on insulators has been studied for decades, while few studies focus on the flashover performance of ice-covered insulators polluted by two types of contamination, that is the solid layer method and icing water conductivity method, simultaneously. On the basis of the investigations on DC flashover performance of ice-covered insulators in the artificial climate chamber, the influence of different parameters, such as insulator string length, icing water conductivity, salt deposit density as well as atmospheric pressure on DC icing flashover voltage of insulator strings is studied. According to the experimental results, the relationships between the flashover voltage and each of such factors are determined and analysed. Moreover, the correction formula for DC flashover voltage of ice-covered insulators in complex ambient conditions is proposed.

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.

Air pressure affects the electrical performance of polluted insulator. The dc pollution flashover process of 7-unit XP-160 insulator string under low air pressure was presented by laboratory tests in this paper. During the tests, the flashover voltage and the critical flashover current were measured. Also, based on the insight of insulator flashover process, the length of discharge path, the arc propagation velocity and arc radius under different air pressure were investigated and measured. The research results indicate that, the position of arcing point is stochastic and multipoint arcing phenomenon exists. Surface arc will levitate and develop into air-gap arc under low air pressure. The length of discharge path is related to air pressure and pollution degree, and it is always shorter than the total creepage of insulator string. The leakage current, the arc shape and arc propagation velocity, are also related to air pressure. In this study, it was measured that the arc radius (ra) varied from 1.5 to 3.5 mm, and the characteristic exponent describing the influence of air pressure on arc radius (m) varied from -0.35 to -0.65. The research results are of some significance for revealing the mechanism of flashover and discharge performance of arcs.

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.

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.

This paper examines the direct current (DC) icing flashover performance of typical DC porcelain, glass, and composite insulators in a complex environment of high altitude, icing, and pollution. The effects of the insulator string length, ice thickness, pollution level, freezing water conductivity, air pressure, and arrangement of insulator strings on the DC icing flashover performance are analyzed, and the correction formula for the DC icing flashover voltage is obtained. The DC flashover process of ice-covered insulators at high altitude is classified into dynamic changes of surface arcs, air-gap arcs, and residual resistance of the ice layer, as observed through a high-speed camera, and a DC flashover circuit model explaining the flashover performance for ice-covered insulators at high altitude is established.

This paper presents the relationship between ac flashover voltage and the varying positioning of iced insulator strings under different icing condition. The results were obtained through laboratory test in multifunction artificial climate chamber located at Chongqing University, China. Also difference of the ac icing flashover performance of single insulator string, double insulator strings, inverted v-type insulator strings and insulators connected with alternately large and small diameter sheds (CALSDS) were analyzed in this paper. According to test results, it is found that the distorted electric field distribution on iced insulator string is the main reason that results in the decrease of flashover voltage. With the same icing degree, the icing flashover voltage of either double insulator strings or as an inverted v-type is lower than that of single insulator string. If insulators are arranged with the type of CALSDS, the speed of icicles to bridge the insulators will decrease. To ensure the height of conductor from the earth remaining unchanged, the ac icing flashover voltage of insulator string can be increased by arranging the insulator strings as an inverted v-type or arranging the insulators with the types of CALSDS with the pattern of "3+1" or "4+1".

SUMMARY Transmission line is the primary facility for power delivery and is mainly composed of twisted aluminum wires. During the long-term outer door field service, overhead transmission lines will encounter strikes of lightning, corrosion of chemical contaminants, ice shedding, wind vibration of conductors, lines galloping, damage due to external forces and so on which will generally bring a series of latent faults such as aluminum strands fracture. It will induce some fatal accidents such as broken transmission lines which have a very strong impact on the safe operation of power grids if the latent faults can not be recognized and restored as early as possible. The method of detecting broken strands in transmission line by inspection robot with detectors is a method with good prospect. In this paper, a kind of eddy current transducer (ECT) for detecting broken strands in transmission line is developed. The developed ECT can be integrated to the transmission line inspection robot. Simulation and field experiment results show that the magnitude field induced by transmission current has no influence on the detection of broken strands in transmission line. Hence, the developed ECT carried by robot can identify broken strands with high reliability and sensitivity. At the same time, the proposed structural style and design method of the ECT can be applied to different types of transmission line. Copyright © 2012 John Wiley & Sons, Ltd.

In the current study, a multi-arc predicting model for DC critical flashover voltage of iced and pre-contaminated long insulator strings under low atmospheric pressure is developed. The model is composed of a series of different polarity surface arcs, icicle-icicle air gap arcs, and residual layer resistance. The calculation method of the residual resistance of the ice layer under DC multi-arc condition is established. To validate the model, 7-unit and 15-unit insulator strings were tested in a multi-function artificial climate chamber under the coexistent conditions of low air pressure, pollution, and icing. The test results showed that the values calculated by the model satisfactorily agreed with those experimentally measured, with the errors within the range of 10%, validating the rationality of the model.

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.

A method is introduced for testing insulators under icing conditions. The method is based on and developed from those described in the standard of IEC60-1, IEC 60507 and IEEE Task Force on Insulator Icing Test Methods, which give instructions to test insulators under pollution. Then, the paper explores the effects of various factors, including ice thickness, pollution severity on the surface of insulators before ice accretion, atmospheric pressure, and shed profiles, on the flashover performance of short samples of two different types of silicone rubber (SIR) composite long rod insulators intended for ultra high voltage (UHV) ac transmission lines. The experiments were carried out in the multi-function artificial climate chamber in the High Voltage and Insulation Technological Laboratory of Chongqing University, China. The experimental results reveal the regularity of the effects of ice thickness, pollution severity and atmospheric pressure on the average flashover voltage of two types of composite insulators. Those can provide reference for the outdoor insulation design for UHV ac transmission lines.

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

By comparing the differences of electric field distribution and 50% flashover voltage between common arrangement insulator string and adding booster shed every four insulators (ABSEFI) arrangement insulator string during the melting period, this study explores the influence of booster sheds on ice-covered 750kV class tension insulator strings. Computer simulations and artificial icing flashover tests were used for the comparative analysis. Numerical simulations were done by the use of the finite element method (FEM). The simulation results show that the electric field strength at insulator steel-feet, which booster sheds have little impact on, is much higher than that in other places. Therefore, compared with common arrangement, there is no obvious improvement in the potential distribution after booster sheds being added on the tension string. Artificial icing flashover tests were carried out to compare the 50% icing flashover voltage of these two arrangements in different pollution severities. In addition, the flashover process of these tests was analyzed to study the influence of booster sheds to icing flashover. The experimental results show that, under the condition of 10 mm thick icing, the booster sheds increase the icing-flashover voltage of tension insulator strings by about 7%. The improvement is irrelevant to pollution severities.

Current transformer, the major primary equipment in power system, can transform a large primary current into the standard 5A or 1A current. It provides current for the measure device and the coil of relay protection, meanwhile makes the primary equipments isolate from the secondary equipments. With the development of grid, SF 6 current transformers have been widely used. Once the fault of the internal insulation support in SF 6 current transformer appears, it will threat the safe operation of power systems. In recent years, the fault accidents of current transformer have occurred form time to time. This paper introduces the fault conditions of 500kv current transformer in the area under the jurisdiction of Xiamen EHV Power Transmission and Transformation Bureau. After it be sent back to factory and disassembled, we found that many large area of paint have come off the inner surface of shell of the current transformer, and that there many blister on the inner surface of shell. Meanwhile, send the same batch transformers back to the factory and disassemble, we found that there are many problems in the production process. So this paper suggests that the production process of SF 6 current transformer should be monitored. And after it has been installed, we have to do the aging test and voltage withstand test. To prevent the same fault of 500kv current transformer occurs again, the results in this paper can give some reference and engineering application consult.

In This work, laboratory investigations were carried out on 110 kV composite insulators, 7 units of an IEC standard suspension porcelain insulator string, and 7 units of a XWP-160 antipollution porcelain insulator string with 64 kV voltages in the multifunction artificial climate chamber of Chongqing University. Based on the test results, it puts forward that there are evident differences in ice accretion speed, ice density and ice structure between energized insulators and nonenergized insulators. The electrical field stress has an important influence on ice type and ice structure accrued on the insulators. Both flashover voltage and withstand voltage of the insulators covered with ice at an energized condition are higher than that at a nonenergized condition. Finally, it analyzes the factors that influence the ice type, structure and the flashover performance of an iced insulator.