Flashover Performance Research Articles

Improved flashover performance and tracking resistance of silicone rubber by direct fluorination

ABSTRACTSilicone rubber (SIR) samples are fluorinated using a F2/N2 mixture at different temperatures. Physicochemical characteristics are analyzed by attenuated total reflection infrared spectroscopy, X‐ray photoelectron spectroscopy, and scanning electron microscopy. Basic properties of the fluorinated surface layers are investigated, and the intrinsic alternating current (AC) flashover performance and tracking resistance of the surface fluorinated samples are evaluated. The flashover test results indicate that the fluorination leads to an improvement in AC flashover performance of the SIR material, as a result of the competition between the favorable effect of the increase in surface conduction and the adverse effect of a likely increase in surface layer permittivity. The increase in AC flashover voltage is associated with the voltage‐increasing mode of the flashover test. The flashover test results also show an improved tracking resistance of the SIR material by the fluorination, and the improvement is especially significant for the fluorinated surface layer that has less cleavage of SiC bonds or a good compactness as well as a large thickness. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48556.

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.

Surface trap effects on flashover voltages of epoxy/Al2O3 nanocomposites for high voltage insulation

In this paper, surface trap distribution and the affected surface charge transport properties in epoxy/Al2O3 nanocomposites were studied to provide theoretical and experimental basis for improving surface flashover performance in vacuum. Four different contents of nano-Al2O3 particles were doped into epoxy and different trap distribution of samples was calculated from the surface potential decay method. Surface charge injection was identified by a direct current integrated charge [DCIC-Q(t)] method. Secondary electron emission (SEE) coefficient were obtained combing testing and fitting method. Results showed improved charge injection resistance, reduced charge mobility and SEE coefficient at a slight concentration of 0.5 wt% and 1 wt% by introducing deep traps. Continuing to increase nanoparticles in matrix was proved to reduce trap depth and have adverse effects. The surface flashover voltages reached the highest at 1 wt% nano-doping, which was in accordance with the percolate threshold loading content for trap depth, charge injection field and SEE coefficient. Surface charge injection caused the local field enhancement at the counter electrode and increased the possibility of surface flashover. Deep traps over 1 eV contributed to carrier capture, reduced charge injection and had a positive effect on increasing surface flashover voltage.

Effects of metal particle material on surface flashover performance of alumina-filled epoxy resin spacers in SF6/N2 mixtures under DC voltage

When metal particle is attached on spacer surface, it will distort surface electric field and may reduce surface insulation level. Therefore, it is important to research the relationship between metal particle attachment and surface flashover property. In this study, the effect of metal particle materials on surface flashover performance of alumina-filled epoxy resin spacers in SF6/N2 mixtures is investigated. Three kinds of metal particle materials including copper, iron and aluminum are considered. The results reveal that, at low gas pressure or low surface electric field, the material of metal particle has no obvious effect on flashover voltage. At high gas pressure and high surface electric field, the flashover voltage is almost the same when the iron or aluminum particle is attached, which is lower than that with copper particle attached. It is revealed that at high pressure and high electric field, metal powders and vapor of micron size can be generated from the attached metal particle surface by electric field forces and Joule Heating Effect, which may be responsible for the difference in flashover voltage for different metal particle materials.

The regulation mechanism of SiC/epoxy coatings on surface charge behavior and flashover performance of epoxy/alumina spacers

Surface charge accumulation on insulators becomes an important bottleneck to restrain the development of a DC energy transmission system. This study investigates the effects of SiC/epoxy coating on surface charge behaviors and flashover performance of alumina/epoxy spacers. A nonlinear conductive coating consisting of different contents of SiC fillers and epoxy matrix was fabricated and sprayed on only half surface of the spacers. The dependence of surface charges on the contents of SiC fillers is characterized by three stages. Surface charge accumulation is aggravated at deterioration stage, then almost unaltered at balance stage and inhibited at suppression stage. These three stages are determined by the competition between the increased surface trap density from interface region and the improved surface conductivity from SiC fillers. The variation trend of flashover in air and SF6/N2 mixtures is distinguished by three stages as well. Flashover voltage decreases at descent stage where deficient contents of SiC fillers are appended and then goes up at improved stage with moderate supply of SiC fillers. Finally, it ends up with drastically collapse at degenerated stage when affluent SiC fillers are brought in. The theoretical analysis is proposed based on surface trap distribution and conductivity measurement results to reveal the regulation mechanism of SiC/epoxy coating on surface charge behavior and flashover performance.

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.

Fixed least squares support vector machines for flashover modelling of outdoor insulators

Many theoretical and experimental studies have been performed to optimize the maintenance times of outdoor insulators. But most of the proposed methods are both time consuming and difficult to apply in practice and also they rely mainly on pollution charts and they are not reliable to weather variations. Artificial intelligence has been extensively applied to solve a large number of electrical and high voltage engineering modeling and optimization problems. In this paper, a methodology based on modified LS-SVM strategy using a fixed set of support vectors is proposed to evaluate the flashover performance of outdoor insulators under contaminated conditions, where the candidate support vectors are selected from the training set according to a quadratic Renyi criterion. The obtained results are promising and ensure that the presented technique can help high voltage engineers to assess insulator performance including such important factors as the flashover behavior, the insulator geometrical parameters, aging, and contamination accumulation. Further comparative analysis of the estimated results with the measured data collected from the site measurement demonstrate the effectiveness of the use of fixed-size LS-SVMs models for flashover prediction.

Synergic effect of adsorbed gas and charging on surface flashover

Flashover is a crucial issue in both high-voltage engineering and surface physics. It not only challenges the existing theories about its dynamic evolution, but also inhibits the clean energy revolution by limiting the accessible voltage rating of power equipment. It is of significance to elucidate the microscopic process along the interface to improve the flashover performance. In the present study, the synergic effect of adsorbed gas and surface charging is investigated, which reveals a long ignored factor for determining the flashover voltage. Depending on the relative amount of adsorbed gas, the flashover voltage varies, which exhibit different behavior from the bulk breakdown of the same gas. The amount of N2 gas adsorbed on epoxy resin (EP) surface is much larger than that on Al2O3 ceramic surface, corresponding to the observed higher flashover voltage on EP. It is proposed that the adsorbed gas molecules not only modify the local surface charging state via their interaction with the trapped charges, but also capture free electrons due to the distortion of their electronic distribution. Both effects suppress the free path length of electrons in the gas-solid interface. This work explores another possibility to improve the surface flashover performance.

Wet Snow Flashover Characteristics of 500-kV AC Insulator Strings with Different Arrangements

In order to study the wet snow flashover characteristics of 500-kV AC insulator strings under different arrangements, wet snow flashover tests were carried out in the large climate chamber of China Electric Power Research Institute (CEPRI). The wet snow flashover voltages were obtained by the even-rising method and the flashovers were filmed by a camera. The test results showed that the installation of an anti-icing shed of large diameter could increase the wet snow flashover voltage. The distance between the two insulators was a key parameter that influenced the discharge process and the flashover voltage. Under Λ-string arrangement, for common insulators, the flashover performance of iced insulators increased with the connection angle; for anti-icing insulators, the flashover performance increased at first and then decreased with the connection angle. In wet snow conditions, when the connection angle was at the commonly adopted angle of 60°, the flashover performance of the common insulators under the V-string and Λ-string arrangements was almost the same. For anti-icing insulators, however, the V-string arrangement was recommended according to the tests. The results obtained in this study can provide a reference for external insulation design in wet snow conditions.

Surface charge accumulation behavior and its influence on surface flashover performance of Al2O3-filled epoxy resin insulators under DC voltages

Surface charge accumulation on insulator surface can have great influence on surface flashover performance. An experimental system is established to investigate surface charge accumulation and decay characteristics of Al2O3-filled epoxy resin insulators in 0.1 MPa SF6 under DC voltages. Surface potential is recorded by a Kelvin vibrating probe connected to an electrostatic voltmeter. By pre-depositing charges on insulator surface, the influence of surface charges on surface flashover performance is studied. The results reveal that surface charge distribution appearance is the combined effect of electrode injection, back discharge and gas ionization. Surface charge distribution has obvious polarity effect. It is concentrated near the HV electrode under positive voltages and dispersed under negative voltages. The difference in positive and negative surface flashover voltage is attributed to the difference in surface charge distribution under DC voltages of different polarities. Surface charge decay contains two stages, which satisfies the law of double exponential function. At first stage, surface charge decays fast, which corresponds to charges escaping from shallower traps. While it decays slowly at the second stage, which corresponds to charge escaping from deeper traps. Surface charge decay process is dominated by surface conductivity mechanism. The pre-deposited charges on insulation surface have great influence on surface flashover performance. The deposited positive charges can increase positive flashover voltage but decrease negative flashover voltage.

Charge transport and accumulation around the cone‐type insulator in SF 6 ‐filled 550 kV DC GIL

The gas-insulated transmission line (GIL) has the advantages of large transmission capacity, high operation reliability and environmental friendliness. It is suitable for the high voltage direct current (HVDC) power transmission of hydropower stations or nuclear power stations. However, due to the serious surface charge accumulation problem of the cone-type insulator under DC, its flashover performance is reduced, which restricts the engineering application of HVDC GIL. In this study, a charge transport model of SF6 gas and epoxy is established for a 550 kV cone-type insulator and the charge accumulation characteristics of the concave and convex surfaces of the cone-type insulator are obtained. Computational results show that the gas conductivity around the cone-type insulator is not uniform due to the charged particles drifting under a constant electric field. Compared with the AC, the difference of the electric field distribution caused by charge accumulation under DC stress is analysed. Based on the established model, the influence of the gas ion-pair generation rate and epoxy volume conductivity on the charge accumulation on the surface of 550 kV cone-type insulator was studied. This study is helpful to further optimise the geometry and material properties of the cone-type insulator and reduce the surface charge accumulation and critical area electric field to improve HVDC GIL insulation design.

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.

Influence of trap characteristics on DC surface flashover performance of low density polyethylene in vacuum

Surface flashover is the primary limitation to the development of power system and the increase of voltage level. Previous work has proved that the trap can greatly influence flashover performances, but the relationship between trap parameters and surface flashover voltage is not clear. In the paper, we study the effects of crystallization behavior, microstructure, and trap parameters on DC surface flashover performance of semi-crystallinity polymer through adding phenolphthalein which is regarded as nucleating agent in low density polyethylene (LDPE). Micro-IR spectroscopy result proves that phenolphthalein exactly exists in LDPE/phenolphthalein composite. Differential scanning calorimeter (DSC) and scanning electron microscope (SEM) are used to investigate the effect of nucleating agent (phenolphthalein) on crystallinity behavior and microstructure of LDPE, and their results indicate that the phenolphthalein modification greatly changes the crystallization behavior of LDPE. The SEM results show that the spherulite size of LDPE decreases and is distributed more uniformly with the increase of phenolphthalein concentration. The DSC results show that the crystallinity and lamella thickness increase. Thermally stimulated depolarization current (TSDC) is used to characterize the trap parameters of LDPE/phenolphthalein composites. The TSDC results indicate that the shallow trap level (&lt;i&gt;γ&lt;/i&gt; peak) increases from 0.19 eV to 0.65 eV and the deep trap (&lt;i&gt;α&lt;/i&gt; peak) increases from 0.81 eV to 0.99 eV with the increase of phenolphthalein concentration. Relationship between microstructure and trap parameters shows that the smaller spherulite size indicates the deeper trap level (for LDPE, the trap level increases from 0.81 eV to 0.99 eV when the spherulite size decreases from 23.2 μm to 14.9 μm), and larger crystallinity means smaller trap density (for LDPE, the trap density decreases from 1404 pC to 612 pC when the crystallinity increases from 34.51% to 43.25%). The DC surface flashover performance increases with the increase of phenolphthalein concentration, and reaches a highest value: when the concentration is 1 wt%, the highest value is increased by 48.42%. Finally, it is concluded that the microstructure of semi-crystallinity polymerinfluences the trap parameters, which affects the surface flashover performance through affecting the carrier transport process in the development process of surface flashover. The trap level and trap density play complementary, cooperation and mutual transformation roles in improving the surface flashover performances as indicated by the analysis of the " U-shaped” relationship between trap parameters and flashover voltage.

Comparison of Surface Pollution Flashover Characteristics of RTV (Room Temperature Vulcanizing) Coated Insulators Under Different Coating Damage Modes

In recent years, room temperature vulcanizing (RTV) coated insulators have been increasingly applied in transmission lines. A large number of RTV coating damages on insulators are currently found. In this paper, three typical damage modes of the RTV coating were simulated, and then, the pollution flashover characteristics were studied. Results indicated that there is a big difference between the ac flashover performances of RTV coated insulators under different damage modes. The flashover voltage, as well as the critical leakage current, is largely influenced by salt deposit density (SDD), damage modes, and damage area. The relationship between SDD and U 50 still meets negative power function when RTV coating was damaged. Insulators with fan-shaped damage have lowest flashover voltage and greatest critical leakage current. Both 2D and 3D FEM models of the RTV coating damaged insulator pollution layer were developed to calculate leakage current density on the damaged surface; it is discovered that the starting position of the arc usually occurs where the current density is maximum. Different damage modes lead to the change of the surface pollution layer conductivity and the uneven distribution of the current density, which is the main reason that pollution flashover characteristics are directly related to the damage modes.

Computational analysis of the electrostatic environment and flashover performance of a suspension linkstick for live-line work on high voltage power lines

What are the root causes of recent live-line tool flashovers at electric field gradients of less than 100 kV/m when tools are supposed to withstand three times that stress? Computer simulations of electrical discharges from the head of the tool connected to the energized conductor suggest a role of highspeed surface partial discharges in initiating the flashover process.

Analysis of Transient Behavior of Mixed High Voltage DC Transmission Line Under Lightning Strikes

Growing urbanization coupled with increased power demands have led to increasing use of mixed power transmission lines with sections of overhead lines (OHL) and underground cables. Due to differences in surge impedance of cables and OHL, voltage surges experience reflections and refractions at their boundaries which make the transient behavior of mixed high voltage direct current (HVDC) line quite peculiar. Lightning strikes on overhead sections of lines induce voltage surges that travel along OHL and enter the cable section. Lightning overvoltage can cause OHL insulators to flashover and stress cable insulation or cause its permanent breakdown. In this paper, we simulated a fast front model of a mixed HVDC transmission line using an electromagnetic transient simulation program (PSCAD) to analyze its transient behavior under a lightning strike. The leader progression model has been used to predict the dielectric performance of OHL insulators. It has been shown that transition towers adjacent to the cable section are much less vulnerable to flashover than subsequent towers. The length of a riser section (connecting OHL and cable) and tower footing impedance have shown to significantly influence the flashover performance of OHL insulators. In addition, the length of cable segments and sheath grounding impedance has been found to influence cable overvoltage. This paper can be used to evaluate the insulation coordination and overvoltage protection requirements for a mixed HVDC transmission line.

Unraveling the “U-Shaped” Dependence of Surface Flashover Performance on the Surface Trap Level

The effects of surface traps on surface flashover remain controversial. To clarify the relation between surface flashover and surface trap level, in this work, the surface trap level of epoxy composites was modified by nanoparticles incorporation, electron beam irradiation, and ozone treatment. Surface trap characteristics were analyzed by surface potential decay. Surface flashover voltages were measured in a vacuum for dc voltage and in SF 6 for ac voltage. The “U-shaped” curve is founded to describe the relation between surface flashover voltage and surface deep trap level, surface flashover voltage first decreases and then increases with surface deep trap level. Enhancement of surface flashover voltage is attributed to reduced surface charge density, which was calculated by a double-trap flashover model. The simulation results indicate that the surface charge density on left side of “U-shaped” curve is controlled by surface shallow traps, whereas that on the right side is determined by surface deep traps. The effects of surface shallow and deep traps on surface charge accumulation and dissipation are used to demonstrate the reduced surface charges and improved surface flashover voltage for the “U-shaped” curve. The proposed “U-shaped” curve offers a promising way to improve surface flashover performance for high-voltage applications by tailoring surface trap characteristics with surface modifications.

Internal Temperature Detections of Contaminated Silicone Rubber Under Discharge Conditions Based on Fiber Bragg Gratings

Composite insulators are widely used in power grids at home and abroad. The anti-pollution flashover performance, hydrophobicity and hydrophobic migration, and pollution flashover mechanism of composite insulators have been the focus of international attention. Temperature is one of the important parameters to describe the flashover of composite insulators. In this paper, the internal temperatures of a composite insulation sample were detected based on FBGs (fiber Bragg gratings). The sample was layered and made of one layer of silicone rubber 3 mm in thickness and one layer of epoxy-GFRP (Glass Fiber Reinforced Plastics) similar to 220 kV composite insulators. Three gratings were embedded in the interface of the two layers between long and short electrodes on the surface of the sample. Temperature calibration tests were carried out on the gratings in the sample. Then, discharges tests were done on the sample with embedded FBGs. Internal temperatures as well as leakage currents were measured, and the arcs process of the discharges on the sample was photographed. The test conditions are as follows: the output voltage was increased by approximately 0.5 kV/s; the equivalent salt deposit density (ESDD) was about 0.5 mg/cm2 and the non-soluble deposit density (NSDD) was about 1.2 mg/cm2.The results show that the internal FBGs could sense the flashover on the surface of the sample. An interface temperature rise caused by the discharges is within 11 °Cin this paper. There is a time delay of 1.4 s from the leakage current arise to the temperature rise during the discharges. The arcs always develops from the small electrode to the large electrode on the surface of the sample, the rise process of the internal temperature at the interface under the middle surface position of the two electrodes is significantly different from that near the small electrode, the arcs starting electrode, while similar to that near the large electrode, the flashover electrode.

AC Flashover Performance of FRP Hot Stick for Live Working in High Altitude Areas

Fiberglass reinforced plastic (FRP) hot sticks are the most common tool used in live working on 10-kV overhead power lines. However, the flashover voltage of hot sticks significantly decreases with increase in altitude. Until now, test data for live working in areas where the altitude exceeds 3000 m have not been available. In this paper, an experimental study on the flashover performance of the FRP hot sticks was carried out in different high-altitude areas. Power frequency discharge tests were performed in Yambajan (4300 m), Lhasa (3600 m), Nyingchi (2900 m), and Wuhan (28 m), respectively. The flashover characteristics in different areas were obtained and analyzed, and a voltage correction method was proposed. Based on these test results, the minimum approach distance of hot stick in high-altitude conditions was determined, and the safety margin was verified. The test results show that for altitudes between 2500 and 4500 m, the hot stick flashover voltage decreases by 7.725% with every 1000 m of altitude increase, and the minimum approach distance of hot stick is 0.6 m (without end fitting) and 0.9 m (with end fitting), and the safety margin is 3.

Distribution of polymer surface charge under DC voltag and its influence on surface flashover characteristics

This study investigates the characteristics of polymer surface charge and their influence on DC surface flashover, which is significant to the study of surface discharge theories and the insulation design of high-voltage direct current equipment. Silicone rubber (SiR), epoxy resin (EP), and polymethyl methacrylate (PMMA), were selected as experimental materials in this paper. Using the active capacitance probe method, the charge accumulation, and dissipation characteristics of the polymers were studied under needle-plane electrode corona discharge. In cases of no surface charge accumulation, DC flashover tests were conducted in multiple electric fields. Furthermore, the influence of surface charge on flashover performance was quantitatively evaluated by means of optical and electrical tests as well as mathematical analysis. The results indicate that the surface potential of EP and PMMA increases more prominently as the applied voltage increases, but SiR is the least to do so. On the other hand, the surface potential of SiR and PMMA decays faster, with EP as the slowest of all. After the polymer accumulates surface charges, the flashover voltage, the luminance of the flashover arc channel, and the fractal dimension of the arc path all decrease. Besides, the flashover voltage of PMMA is extremely sensitive to surface charge, and the flashover voltage under extremely non-uniform electric field is less sensitive than that under the slightly non-uniform electric field. The analysis suggests that the surface charge exerts an influence on the flashover characteristics by affecting the surface field intensity and the surface discharge development process, and these factors ultimately determine the flashover characteristics.