Flashover Performance Of Insulators Research Articles

A Study of Fast Front Transients of an HVDC Mixed Transmission Line Exposed to Bipolar Lightning Strokes

Bipolar lightning strokes are associated with multiple polarity electrical discharge with no current intervals in between, making their behavior quite peculiar. This work presents a fast front analysis of a mixed high voltage direct current (HVDC) transmission link, evaluating the factors that influence the line transients due to shielding failures and backflashovers (BFOs), considering both overvoltage and repeated polarity reversal at the cable sending terminal. The research process includes a detailed modeling of a bipolar lightning stroke, frequency-dependent HVDC overhead, and underground transmission line sections. Noticeable findings include the occurrence of only a positive polarity insulator BFO for the adjacent and subsequent tower, despite the dual polarity of the lightning stroke with relatively small values for the lightning parameters. The influence of traveling waves on the insulator flashover performance of the line with varying parameters (such as the riser section length, the tower grounding impedance, and the location of the lightning stroke) is recorded and explained.

Studying AC Flashover Performance of Suspension Insulators Under Natural Cold Fog and Wet Deposition Conditions

The electrical performance of insulators under comprehensive conditions including low air pressure, pollution and icing, is an important basis for the selection of external insulation. However, the problem of cold wet deposition has not attracted extensive and considerable interest in research. Based on flashover tests of suspension insulators in field test station, the AC flashover performance of insulators was studied. The results show that the duration of existence of the arc throughout the composite insulator surface in case of condensation was significantly lower than that in case of freezing fog flashover because of a change of the sheds surface roughness, which leads the arc melt the frozen fog droplets on the sheds surface and extends its flashover time. Compared with the flashover characteristics of the hydrophilic insulators in frost fog, the composite insulators exhibited clear arc extinguishing and arc starting phenomena. Moreover, the wind velocity has affected the flashover performance of suspension insulators in cold fog. Studying flashover characteristics of suspension insulators can contribute to the design of the external insulation of transmission lines and substations under natural cold fog and wet deposition regions.

Influence of Sheds Damage on the AC Pollution Flashover Performance of Different Voltage Class Composite Insulators

The damage of high-voltage terminal shed of composite insulators can influence the flashover performance of the insulators. The study was carried out experimentally to investigate the flashover characteristics of composite insulators with sheds damage at different voltage levels. The results show that sheds damage would decrease pollution flashover voltage, creepage distance flashover voltage gradient E L and the dry arc distance flashover voltage gradient E H , and such decrease range was proportional to damage degree. Besides, with the increase of composite insulator's dry arc distance, effects of sheds damage on pollution flashover voltage, creepage distance flashover voltage gradient E L and the dry arc distance flashover voltage gradient E H were gradually decreasing. With the ESDD ranging from 0.1 mg/cm to 0.3 mg/cm 2 , AC pollution flashover voltage of 35 kV, 110 kV and 220 kV composite insulators decreased by up to 35.92%, 15.03%, 9.11% respectively; the creepage distance flashover voltage gradient E L decreased by up to 19.15%, 6.05%, 1.34% respectively; the dry arc distance flashover voltage gradient E H decreased by up to 35.92%, 15.03%,9.11% respectively.

Study on Surface Rainwater and Arc Characteristics of High-Voltage Bushing With Booster Sheds Under Heavy Rainfall

The flashover performance of insulators can be improved by BS (booster sheds) in the rain, which is mainly attributed to the reasons that BS break up long cascades of water and block connections of arcs. However, surface rainwater characteristics and arc characteristics of bushing have not been quantitatively studied under heavy rainfall. In this article, the artificial rain tests were conducted on a 500 kV transformer high-voltage bushing equipped with and without BS under the rainfall intensity of 10 mm/min. X (the total length of water column) and L arc (the critical length of arc) on the bushing surface were taken as the feature parameters of surface rainwater characteristics and arc characteristics, respectively. The effects of BS on E h (the rain flashover voltage gradient along the insulation height), X and L arc were investigated, respectively. Furtherly, the relationships were studied among Eh, X and L arc . Results indicate that E h has a sharp rise as the number of BS (NBS) is from one to two, however the rise of E h gradually decreases when NBS exceeds two. X decreases while L arc increases with the rise of NBS, however both the change ranges of them continually fall. Furthermore, L arc presents remarkable negative correlation to X because of the effect of the electric field. E h rises nonlinearly with the decrease of X, which is due to the change of the wetting uniformity on the bushing surface and the potential redistribution along air gaps in the presence of the local arc.

Prediction of flashover voltage using electric field measurement on clean and polluted insulators

The economy of transporting large amount of power over long distance makes high voltage transmission line an important component of the power system. Outdoor insulators provide electrical insulation as well as mechanical support between energized conductor and grounded parts of the tower. Its performance is closely related to the reliability of power delivery. Various parameters affect the insulator flashover performance, but the measurement and analysis of all those parameters would be a challenging and complex task during online inspection. The performance of the insulators is directly dependent on the electric field distribution inside and surrounding them. The aim of this work is to obtain the relationship between flashover voltage and electric field distribution. Accordingly, a novel electric field measurement technology, which was recently commercially available, was utilized in this paper to measure the electric field distribution around six different varieties of composite and ceramic insulators. The relationship between conductivity of the contamination layer and the environmental relative humidity was initially evaluated. Further, the patterns of the electric field distribution for clean, uniform and non-uniform contamination cases were studied and its relationship with the flashover voltages was explored. Electric field calculation with commercially available package was performed to validate the experimental results and extend the conclusion to higher voltage levels.

Influence of pollution chemical components on AC flashover performance of various types of insulators

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− < NO3 − < SO4 2−, K+ < Na+, NH4 + < Ca2+. For a glass insulator, the flashover gradient is 4.9–17.0% higher than that of a porcelain insulator. For a silicone rubber insulator, this gap reaches 14.6–24.4 and 83.3–90% for freely soluble pollution and CaSO4, respectively, this phenomenon can be explained by the combined action of the slightly soluble character of CaSO4 and hydrophobicity transference of silicone rubber.

Study on pollution flashover characteristic of insulators on 500 kV AC transmission lines in high‐altitude area

Pollution flashover performance of insulators is one of the key factors to be considered in the selection of external insulation for transmission lines at high-altitude areas. In pollution laboratories at Tibet high-altitude test base and Beijing ultra high voltage direct current (UHVDC) test base, artificial pollution tests of insulators adopted in AC 500 kV transmission project are performed both in sea-level region and real high-altitude regions, through which flashover voltages at typical equivalent salt deposit density are obtained. The relationship between altitude and flashover voltage is acquired as well.

Study on the dc flashover performance of standard suspension insulator with ring‐shaped non‐uniform pollution

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.

Tailoring charge transport in epoxy based composite under temperature gradient using K2Ti6O13 and asbestine whiskers

The solid-gas interface flashover performance of insulators under dc is influenced to a considerable extent by the residual charge effect. The investigation of the charge origin and charge transport in dielectric materials is a significant topic. In this work, the Al2O3-filled epoxy resin composite was manufactured as the basic dielectric composite. To investigate the charge transport behavior and restrain the charge injection under the temperature gradient, asbestine and K2Ti6O13 whiskers were doped into the basic matrix and post type model insulators were prepared. Experimental results demonstrate that the introduction of K2Ti6O13 whiskers can effectively restrain heat propagation due to its excellent thermal barrier property, which can restrain the charge transport under temperature gradient to some extent. The charge injection level increases for insulator doped with asbestine whisker, which is due to the introduction of large amounts of shallow traps with trap level density ranging from 0.75–0.91 eV. This work is helpful to understand DC charge transport behavior under temperature gradient and provides important significance in design of a suitable insulator inside HVDC GIL.

Flashover Performance Test with Lightning Impulse and Simulation Analysis of Different Insulators in a 110 kV Double-Circuit Transmission Tower

The flashover performance of insulators is the foundation of the calculation of precise lightning protection. The material and structure of an insulator affect its flashover performance but the research on the differences in the flashover performance of different insulators and the analysis of the reason for these differences have not been clear. In this paper, lightning impulse flashover tests of two types of insulators (composite and glass insulators) were carried out in the National Engineering Laboratory (Kunming) for Ultrahigh-voltage Engineering Technology. In these tests, the insulators were hung in a 110 kV double-circuit transmission tower and a standard lightning impulse (1.2/50 μs) was applied to the insulators. The discharge path and the 50% impulse flashover voltage of the insulators were recorded. At the same time, the electric fields in the vicinity of different insulators were calculated using the finite element method with COMSOL Multiphysics. The electric field distribution and the uneven coefficient were analyzed. Combined with the flashover test data and the electric field simulation, the relationship between the flashover performance (discharge path and 50% impulse flashover voltage) and the electric field (electric field distribution and uneven coefficient) is found. The simulation results are in accordance with the test data and the conclusions can provide useful references for the external insulation design of transmission lines and the optimization of insulators.

Study on Influencing Factors of Insulators Flashover Characteristics on the 110 kV True Tower Under the Lightning Impulse

Lightning strike is the main reason for the transmission line tripping. Studies on the insulator flashover characteristics and the accurate flashover criteria under the lightning impulse are the basis for correctly evaluating the lightning resistance performance of the transmission line. In this paper, an experimental study on the flashover characteristics of the insulators with different materials was carried out on the 110-kV double-circuit transmission line tower. The insulators were hung on the umbrella-shaped tower and the drum-shaped tower in the test, and both the standard lightning impulse (1.2/ $50~\mu \text{s}$ ) and the short-tail lightning impulse (1/ $10~\mu \text{s}$ ) were used. The flashover test of a single composite insulator was also carried out for comparatively analyzing the influence of the true tower on the insulator flashover performance. According to the test results on the true tower, the lightning flashover criteria of the insulators with different materials are given. The influencing factors of the insulator flashover characteristics, including the impulse waveform, the insulator material, the tower type, and the insulator suspension position, were further compared and analyzed. Meanwhile, the results of the single insulator test proved that the true tower has great influence on the insulator flashover characteristics and that the flashover test on the true tower can reflect actual situation better and obtain more accurate results. In addition, based on the simulation analysis results of the electric-field distribution around the insulators, the difference of 50% impulse flashover voltage ( $U_{\mathrm {50\%}}$ ) of the insulators in different conditions was partly explained.

Effects of ring-shaped non-uniform pollution on outdoor insulation electrical property

The inner area close to cap and pin of insulator is sometimes heavier polluted than the outer area, causing the contamination pattern on insulator surface look like ring-shape. This kind of non-uniform pollution may affect insulator flashover voltage. In this paper, artificial pollution tests of 4 types of porcelain and glass insulators with ring-shaped non-uniform pollution were carried out. Experimental results indicate that the SDD (salt deposit density), the non-uniform pollution degree K (the SDD ratio of inner part to outer part), the heavily polluted area with radius r, and the insulator profile all have significant effects on insulator flashover performance. The effects of K and r on flashover performance can be reflected by the change of pollution layer conductivity during flashover. Under ring-shaped non-uniform pollution, the relationship between Uso and SDD can still be expressed by negative power function, while coefficient a varies obviously with pollution non-uniformity, but exponent n does not. Finally the correction coefficients of flashover voltage considering non-uniform pollution degree K were also calculated for outdoor insulation design.

Evaluating flashover performance of insulators under fire fighting conditions

EHV and UHV high voltage transmission lines frequently pass through high vegetation areas as well as national forests where there is a significant risk of wildfire by natural or manmade causes. Firefighters use aerial vehicles to douse fires by dumping large amount of fire retardant liquid that is conductive, thus increasing the risk of insulator flashover and power interruptions. In this paper, an ammonium polyphosphate based fire retardant liquid was evaluated to understand its effect on insulator flashover properties. Composite and ceramic insulators, suitable for 115 kV transmission lines, were studied by the solid layer test method. Tests were conducted with and without external wetting by fog. Results indicate that there is a high probability of flashover if the entire insulator is coated with the fire retardant liquid. Statistical methods were applied to extend the validity of testing results to predict flashover voltages at higher voltage levels of 230, 345 and 500 kV. It is shown that standard insulator dimensions used for EHV and UHV lines in fire prone areas could be deficient and additional insulation required to improve the flashover performance to acceptable levels in such locations has been determined.

Study on the flashover performance of various types of insulators polluted by nitrates

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.

Study on the influence rules of soluble contaminants on flashover voltage of disc suspension insulators

The soluble contaminants existing in external insulation are various, which affect insulator flashover performance. This paper conducted experiment to investigate the flashover performance of disc suspension insulator string polluted by eight kinds of soluble common contaminants respectively. Based on the test data, the influence rules of soluble contaminants on flashover voltage of insulators were analyzed, and the flashover voltage prediction formula based on freely soluble contaminants was proposed. Research results indicate that, the flashover performance of disc suspension insulator string varies with the type of soluble contaminants as well as ESDD (equivalent salt deposit density), negative power function can be used to characterize the relationship between flashover voltage and ESDD; under the same ESDD, sulfates correspond to the highest flashover voltage, while NaCl corresponds to the lowest one; the characteristic exponent n of nitrates are within 0.18 – 0.26, which are higher than that of sulfates but lower than the value of NaCl; the flashover voltage prediction formula based on freely soluble contaminants can be expressed as U ƒ = (b−kn) × ESDD−n, the application of which can reduce the workload and cost of laboratory tests in the external insulation design.

AC flashover characteristics of insulators under haze–fog environment

The occurrence of large-scale haze–fog weather in some developing countries brings a new threat to the safe operation of outdoor insulation. In order to study the effect of the haze–fog on the flashover performance of insulators and the flashover mechanism, kieselguhr, calcium sulphate (CaSO­ 4) and ammonium sulphate ((NH4)2SO4) in different particle sizes were mixed with ultrasonic fog to simulate the haze–fog environment and quantitative ac flashover experiments on the standard suspension porcelain insulator were carried out. The test results indicate that the flashover voltage is related with the pollution degree, particle component, particle size and fog-water conductivity, and may decrease by more than 20% under haze–fog environment. The effect of salt aerosols on flashover voltage is determined by the solubility and larger than that of non-soluble mineral aerosols or organic carbon aerosols. The flashover voltage increases with the increase of the particle size, but decreases with the increase of fog-water conductivity. The flashover of insulator under haze–fog condition is a special kind of pollution flashover in essence.

Additional salt deposit density of polluted insulators in salt fog

The surface pollution layer of polluted insulator absorbs fog water and gets damp in salt fog. Meanwhile, the salts in the salt fog deposit on insulator surface, which increases the surface conductivity and degrades the flashover performance of insulators. At present, the effects of fog water conductivity (γ 20) and equivalent salt deposit density (ESDD) are analysed separately, which does not reveal the essence of the flashover in salt fog. In this study, a plenty of experiments on porcelain, glass and composite insulators are carried out in salt fog conditions, and the 50% flashover voltage stress, leakage current and variation of ESDD during the wetting process are studied. The concept of additional salt deposit density (ASDD) is proposed to quantitatively analyse the additional ESDD caused by salt fog. The test results indicate that the ac flashover voltage stress decreases with the increase of both ESDD and fog-water conductivity. Both the ESDD and leakage current become larger in salt fog compared with those in clean fog. The ASDD can be applied to analyse the combined effect of ESDD and fog-water conductivity and be expressed as ASDD = k × γ 20 × ESDD. Then the flashover voltage stress is E = A × (ESDD + ASDD)−a . The values of k for the porcelain, glass and composite insulators are 0.179, 0.191 and 0.230, respectively. The flashover in salt fog environment is a special kind of pollution flashover in essence.

Evaluation of wetting condition and its effects on pollution flashover voltage of aerodynamic insulators

The flashover performance of insulators is significantly affected by the degrees and ways of wetting the pollution layer. However, there have only been a few reports on the wetting condition and its effects on the flashover characteristics of pollution on insulators to date. In this paper, a phase-angle difference-based method is proposed to test the wetting conditions of the pollution layer on insulators. A corresponding measuring device is also developed. The wetting process of the pollution layer on aerodynamic-type glass insulators is studied in an artificial climate chamber using the developed device. The related flashover performance is studied via the even-rising voltage method. Test results show that the phase-angle difference (PAD) curve accurately reflects the entire wetting process of the pollution layer from the initial stage, through the saturated stage to the oversaturated and washing away stages. Over the wetting process, the PAD curve initially drops sharply, and then remains stable for a few minutes before eventually rising slowly to the initial value. The PAD curve has its lowest value when the pollution layer is wetted to saturation, which occurs after approximately 5 min in our experiments. The flashover voltage characteristics of insulators are also significantly affected by the wetting conditions of the pollution layer. The flashover voltage in the saturated stage is shown to be lower than that in the other stages. Therefore, to improve the accuracy of the artificial pollution tests, quantitative methods (including PAD) should be used to obtain the exact time of wetting to saturation.

Source strength impact analysis on polymer insulator flashover under contaminated conditions and a comparison with porcelain

A mathematical model that considers the source-insulator interaction is presented for modeling flashover of polymer and porcelain insulators under contaminated conditions. The model is shown to be capable of handling polymer insulators with varying degrees of surface wettability or hydrophobicity. While a powerful source such as a practical power system causes insulator flashover performance to be lower than what can be expected from standard laboratory test, the relative reduction is more significant for porcelain than for polymer insulators. Line post and suspension insulators up to 500 kV have been considered for this study.

Effects of pollution materials on the AC flashover performance of suspension insulators

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.