DC Flashover Research Articles

New experimental study on the DC flashover voltage of polymer insulators: combined effect of surface charges and air humidity

This research provides conclusions that can partially cover the lack of knowledge related to the effects of ambient atmospheric conditions on DC electric power transportation. Two polymer insulating materials are used for completing this research study. Inside a climate chamber, the relative humidity is controlled and adjusted from 20% to 80% RH. Then, the samples are charged with positive or negative charges by applying ± 20 kV to the corona-generating ring of needles. The surface potential is measured using an electrostatic voltmeter and is converted into surface charge density later by applying the probe response matrix method. The pre-charged samples are then stressed with high-voltage negative or positive DC values inside the climate chamber over a range of controlled values of the surrounding air humidity until flashover takes place. The space charges, which can drift in the air gap to reach the solid surface, are highly affected by the level of the relative humidity of the surrounding air. Also, increasing humidity results in a reduction of the DC flashover inception voltage and a shorter time to flashover regardless of the voltage polarity. Moreover, the positive or negative flashover inception voltage of both materials increases with pre-deposited negative charges while it decreases with pre-deposited positive charges.

Constellation of Condensation and Raindrops and Its Effect on the DC Flashover Voltage of Inclined Silicone Insulation

The purpose of this paper is to characterize the arrangement of raindrops and natural condensation and its effect on the direct current voltage’s flashover of a hydrophobic silicone insulating surface under variable inclination. Two devices have been realized, one for on-site exposure of insulation samples under rain and condensation, and the other for laboratory use in order to allow the variation of the angle of their inclination. The temporal evolution of the constellation of raindrops and natural condensation as well as the electrical discharge produced in the laboratory was visualized. The results emanated from the on-site investigation allowed the identification of the most reproducible type of arrangement of rainwater and natural condensation drops as well as the estimation of their respective volume. The results from the laboratory clearly show the existence of an interval of the insulation’s inclination angle, which allows the best approximation of its electrical performance to what was obtained in the absence of these drops. Moreover, these same results also inform us about some very critical situations to avoid regarding the insulation by a judicious choice of its inclination angle.

DC Flashover Dynamic Model of Post Insulator under Non-Uniform Pollution between Windward and Leeward Sides

Experience shows that under unidirectional wind or certain terrain, the surface of post insulators is non-uniformly polluted between windward and leeward sides, which affects the flashover characteristics. In this paper, a formulation of residual pollution layer resistance was proposed under this non-uniformity and a typical post insulator was taken as an example to analyze and calculate its residual resistance. The theoretical resistance was verified by numerical simulations using COMSOL Multiphysics. The proposed resistance formulation was then implemented in a DC flashover dynamic model to determine the flashover voltage (Ucal), which was validated by artificial flashover tests. Then the factors affecting DC flashover voltage were analyzed. Research results indicate that: the residual resistance formulation agrees well with simulation results, especially when the arc length exceeds 70% of the leakage distance. The good concordance between theoretical and experimental flashover voltages with most relative error within ±10%, validates the flashover model and its residual resistance formulation. Ucal gets impaired under this non-uniformity. The degree of reduction is related to salt deposit density ratio (m) of windward to leeward side and leeward side area proportion (k).

DC breakdown and flashover characteristics of direct fluorinated epoxy/Al2O3 nanocomposites

The effect of direct fluorination on DC breakdown and flashover characteristics of epoxy/Al 2 O 3 nanocomposites was investigated. Pure epoxy resin and epoxy/Al 2 O 3 nanocomposites with 1, 3 and 5 wt% mass fractions were synthesized, and direct fluorination of the samples was carried out at 55 °C for 30 min using a F 2 /N 2 mixture with 20 vol% F 2 at 0.05 MPa. Fourier transform infrared spectroscopy and atomic force microscopy show that molecular chain scission occur during direct fluorination, which may significantly suppress surface charge accumulation while incorporating Al 2 O 3 nanoparticles into epoxy may restrict surface potential decay of fluorinated epoxy resin. The trap characteristics obtained by isothermal surface potential decay show that 1 wt. % sample has deeper traps and higher trap density. DC breakdown and flashover of samples were measured to evaluate the influence of nano filling and direct fluorination on electrical performance of epoxy resin. The results show that epoxy resin with higher DC breakdown and flashover strength can be obtained by simultaneous application of nano filling and direct fluorination. Compared to un-fluorinated epoxy resin, the DC breakdown and flashover voltage of fluorinated epoxy/Al 2 O 3 nanocomposites with mass fraction of 1 wt% improved 17 and 23%, respectively.

Interfacial E-field self-regulating insulator considered for DC GIL application

The non-uniform electric field (E-field) distribution is typically considered a key factor in triggering surface flashover inside gas insulated lines (GILs). To improve the E-field distribution along GIL insulators, this study proposes the concept of an interfacial E-field self-regulating (IER) insulator. The capability of the IER insulator with a resistivity-nonlinear (ρ-nonlinear) skin layer to regulate the E-field distribution is validated via theoretical analysis. In this paper, cone-type insulators are coated with epoxy (EP)/SiC composites to fabricate EP/SiC coated IER insulators. Next, an electrical simulation model is built to calculate the E-field distributions and energy losses of the insulators. As the coating thickness or SiC content increases, the E-field regulation function of the EP/SiC coated insulator becomes increasingly significant with increasing leakage current and energy loss. Under DC voltage, nearly 24 hours is required for the E-field distribution along the conventional insulator to reach the steady state. When the SiC content in the coating layer is increased, the transient time of the EP/SiC coated insulators decreases because of the reduced volume resistivity. Furthermore, coating thickness measurements and DC flashover tests are conducted. The coating thickness of the EP/SiC coated insulator is found to increase with increases in the SiC content because of the higher viscosity of the liquid-state coating. The DC flashover voltage of the EP/SiC coated insulator is found to be higher than that of the conventional insulator, with the flashover voltage increasing with the SiC content in the coating layer.

DC breakdown and flashover characteristics of direct fluorinated epoxy/Al2O3 nanocomposites

The effect of direct fluorination on DC breakdown and flashover characteristics of epoxy/Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> nanocomposites was investigated. Pure epoxy resin and epoxy/Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> nanocomposites with 1, 3 and 5 wt% mass fractions were synthesized, and direct fluorination of the samples was carried out at 55 °C for 30 min using a F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> /N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> mixture with 20 vol% F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> at 0.05 MPa. Fourier transform infrared spectroscopy and atomic force microscopy show that molecular chain scission occur during direct fluorination, which may significantly suppress surface charge accumulation while incorporating Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> nanoparticles into epoxy may restrict surface potential decay of fluorinated epoxy resin. The trap characteristics obtained by isothermal surface potential decay show that 1 wt. % sample has deeper traps and higher trap density. DC breakdown and flashover of samples were measured to evaluate the influence of nano filling and direct fluorination on electrical performance of epoxy resin. The results show that epoxy resin with higher DC breakdown and flashover strength can be obtained by simultaneous application of nano filling and direct fluorination. Compared to un-fluorinated epoxy resin, the DC breakdown and flashover voltage of fluorinated epoxy/Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> nanocomposites with mass fraction of 1 wt% improved 17 and 23%, respectively.

Novel insulator with interfacial σ-FGM for DC compact gaseous insulated pipeline

Electric field optimization plays a significant role in the insulation design of gaseous insulated pipeline (GIL) for the DC power system. Functionally graded material (FGM) with spatial distribution of conductivity σ (σ-FGM) provides a potential method to uniform the electric field and thus reduces flashovers to some extent. However, researches at present are nearly at a standstill for the complicated fabrication techniques and huge bulk conduction losses. In this paper, a novel cone-type insulator with surface conductivity gradient was proposed based on direct fluorination treatment. Theoretical analysis shows that the electric field distortion near the high voltage electrode can be effectively suppressed with increasing the thickness and conductivity of the fluorinated layer. Epoxy cone-type insulators with homogeneous and graded surface conductivity were then prepared by different fluorination treatments to verify the effectiveness of σ-FGM design. As for the homogeneously fluorinated cone-type insulators, the DC flashover characteristics were improved with the rise of fluorination time. Especially, the flashover voltage of the insulators with σ-FGM layers can be raised by up to 36.3 % compared with that of the traditional ones, owing to a more uniform electric field distribution. Theoretical and experimental results prove that the insulator with surface σ-FGM is a promising choice for DC compact GIL.

Surface coating affecting charge distribution and flashover voltage of cone-type insulator under DC stress

In order to scale down the dimensions of gas insulated line (GIL) insulators and to improve their reliability at ultra high voltage (UHV), considerable research has been conducted to raise the flashover voltage of the insulators. Usually, a non-uniform electric field distribution is an important factor triggering flashovers. Under AC and impulse voltages, the application of functionally graded materials (FGM) with spatial distribution of dielectric permittivity (e-FGM) has been shown to be an effective solution. But at DC, the electric field distribution depends not only on the conductivity of the dielectrics but also the charge distribution on the surface. Based on the cone-type insulator, this paper reports on the surface charge accumulation and DC flashover of epoxy (EP)/graphene (GR) coated insulators in air. The EP/GR coated insulators with different filler amounts were prepared to test their surface charge accumulation and flashover characteristics. The results show that the uniformly distributed homopolar surface charge is helpful in reducing the peak electric field thereby raising the flashover voltage. The 0.1% EP/GR composite has the slowest surface potential decay process. Accordingly, the 0.1% coated insulator has the highest flashover voltage among the other EP/GR coated insulators and uncoated insulator. The bipolar surface charge regions caused by wire-type metal particles distort the electric field distribution. Flashovers of the 0.1% coated insulator occurs at lower voltage than the uncoated insulator when a wire-type particle attaches to the surface.

Effect of temperature on polyimide dc flashover characteristics in different vacuum degrees

The properties of electrical insulation in vacuum and at cryogenic temperatures are important for designing the insulation of large, low-temperature superconducting devices. Particularly, gas pressure and temperature turbulence could easily lead to insulation failure. In this paper, the results of measurements that reveal the dc flashover characteristics of polyimide film from 300 to 20 K at helium pressures of 10−3–103 Pa were reported. The results show that, in high vacuum, the flashover voltage is generally independent of pressure. As pressure increases, the flashover voltage first decreases and then increases gradually. Electron multiplication in helium or on the surface of PI is the main factor affecting flashover. For high to medium vacuum, the N (gas molecular density) at which electrons begin to collide with helium and reduce flashover voltage at higher temperature is less than at cryogenic temperatures. For medium to low vacuum, by comparing the flashover curve with a curve fit based on Townsend discharge theory, a result was found that when a certain specific N is exceeded, the electron impact ionization on the solid surface will no longer be the main influencing factor of flashover. This N increases as temperature decreases except 200 K, and reaches the minimum at 200 K. After analysis, it may be the larger mean free path of electrons increases the likelihood of electron impact ionization on the PI surface. Therefore, in order to maintain a high flashover voltage in engineering, it is necessary to maintain a higher degree of vacuum at higher temperatures; and when the pressure rises to low vacuum, the flashover is mainly determined by the gas discharge.

Flashover characteristics of G-11 at different cryogenic temperatures

The feeder busbar joint system is an important component in the International Thermonuclear Experimental Reactor (ITER) project. The insulation materials for the feeder busbar joint play a crucial role in both mechanical support and high voltage insulation. The G-11 block, which is a kind of industrial high-pressure, glass-reinforced epoxy laminates, has been chosen for the feeder busbar joint as insulation and mechanical support. The electrical characteristics of the G-11 depend significantly on temperature. As the feeder system works at the temperature of 77K, to ensure safe operation, the DC flashover voltage is required to be studied in a cryogenic environment. In this work, DC flashover characteristics of the G-11 at both room and cryogenic temperatures have been investigated using a self-developed test system. The results show that the surface flashover voltage rises as the flashover times increase. In addition, results indicate that the flashover voltage of the G-11 depends on orientations and it increases as the temperature decreases.

DC surface flashover characteristics of G-11CR in vacuum from room temperature to cryogenic temperatures

A helium-free cryostat for DC flashover voltage measurement was constructed. The G-11CR film specimens were tested in vacuum at the temperatures of 293 K, 77 K and 9 K and Weibull distribution was obtained. The results show that the surface flashover voltage of G-11CR film increases as the temperature decreased. The surface flashover voltages did not decrease after 80 times successful flashover and slightly increased as the flashover time increased. Further analysis was performed to investigate the change of G-11CR surface after flashover was characterized by atomic force microscopy (AFM). G-11CR film presented different surface morphologies after 80 times successful flashover at different temperatures.

Prediction of DC flashover voltage of cap‐and‐pin polluted insulator

The aim of this work is to estimate the flashover voltage of cap-and-pin insulator using a novel improvement of the pollution resistance evaluation. In cap-and-pin suspension insulators, local arcing starts around the pin where the dry bands first appear due to the evaporation of the conductive layer. In appropriate conditions, a unique dominant arc starting from the pin propagates toward the cap resulting in a complete flashover. Since the highest current density is concentrated around the arc root during propagation, the pollution layer residual resistance variation, with respect to the arc position, is non-linear. Thus an original three-dimensional model, which depicts a real insulator using COMSOL Multiphysics TM to evaluate the residual resistance, is developed. This model takes into consideration the actual geometry of the insulator and the current restriction at the arc root for a specific arc radius. A novel mathematical model for calculating the residual resistance of this polluted insulator was elaborated for all possible arc radii. The mathematical model and the 3D simulation results are in satisfactory agreement. To evaluate the flashover voltage, a new approach based on Saddle Point Finding Method is proposed. The new approach and model have been validated using experimental results of previous studies.

Study on DC flashover characteristics of plastic sheds in small scale

The statistics of tripping data of transmission lines show that external damage has become the main cause of transmission lines tripping. The foreign object cutting-out, which accounted for 61.8% of the total, was the most common external damage in transmission line. This paper focuses on the DC flashover characteristics of small-scale plastic sheds of greenhouses with distances of 1m and 2m. The results of the study showed that the surface of plastic sheds was less dirty and the average surface conductivity was lower. In the dry condition, it can be regarded as an insulator, and in the case of moisture, however, its insulation resistance is reduced to a megohm level. Under the electric field of 70 kV/m, no flashover occurred in both the wrinkled and the tiled plastic sheds. The flashover voltage per unit length of the wrinkled is about 15%-21% lower than that of the tiled, and wrinkled plastic sheds flash over more easily. As the pole pitch increases, the flashover voltage per unit length decreases, and the flashover voltage per unit length of both the wrinkled and the tiled is reduced by about 30%.

Surface charging and its effects on DC flashover strengt of insulating materials

Surface charging is a common occurrence during service for all HV insulators as well as live-line tools during live-line maintenance work. The main sources of non-uniform charging are corona discharges from nearby and/or direct contact with energized objects and cloth wiping by tribo-charging effect. This paper focuses on studying the DC charging characteristics of three different insulating materials and the flashover performance of such pre-charged insulating materials. The samples used are cylinders of 130 mm length with different diameters. Surface charging by cloth wiping is found to be much more than that of corona discharges from energized objects. Surface charges take more than four hours to decay by 80%. Moreover, surface charges can increase the positive and negative flashover voltages of pre-charged samples by up to 13%. Mitigation methods to improve the flashover performance of insulating materials are identified. The research on surface charging forms hypotheses and recommendations which may improve the live-line maintenance procedures in ways that protect live-line workers from accidents.

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.

Contribution of surface charges on high‐voltage DC silicon rubber insulators to DC flashover performance

In this study, the results of measurement for DC flashover characteristics of silicon rubber insulators in the presence of surface charges and in terms of geometric characteristics have been examined and analysed. In the carried out experimental tests, the surface of different insulators was charged by an external corona source and both the metal end fittings were kept grounded while the needles have been connected to a certain applied negative and positive DC voltages and then surface electric charge measured. A series of flashover tests have been carried out on the charged insulators under positive DC voltages to investigate the effects of surface charges. The experimental results revealed that positive electric charges reduced flashover performance while negative charges increased the flashover voltage level. Also, the experimental modelling of DC flashover of charged insulators regarding the ratio of shed spacing to shed depth and specific leakage distance in terms of surface electric charges, revealed that flashover voltage gradient of the charged insulator is most affected by specific leakage.

High voltage DC partial discharge and flashover characteristics with surface charging on solid insulators in air

Electrical insulation for high voltage DC (HVDC) applications exhibits many features that differ from those of conventional AC insulation. It is therefore necessary to concentrate research activities on aspects that will facilitate a better understanding of its performance, e.g., DC-partial discharge activity, charging characteristics on solid insulator surfaces, and electric field distributions. All of these are complex phenomena and are not yet well understood [1]–[7].

Optical Sensing Techniques for Condition Monitoring of Transformer Insulation Material

Non contact optical sensing techniques—probing of surface flashover and probing of laser induced plasma, spectrally and temporally, were used to study the thermal ageing induced degradation of oil impregnated pressboard (OIP) insulation material of transformer. Optical probing of surface flashover discharge, gave parameter values comparable with, flashover discharge current measurements. Optical emission persistence time was increasing with ageing level of the insulation. Optical emission spectroscopy of impulse surface flashover gave atomic emissions from the copper sulphide contaminant, diffused in the OIP material due to ageing. Failed insulation samples showed larger Cu emission intensity. Optical emission intensity during AC, + DC and − DC flashover was used to indicate ageing and to identify initiation and arcing stages of surface flashover. Alternatively, laser induced breakdown spectroscopy (LIBS) indicated neutral Cu emission in the aged OIP material, indicating diffusion of Cu contaminants. Quantitative analysis was performed on OIP samples with surface deposited Cu and thermally aged specimen. A minimum detection limit of 4.5 µg/cm2 was achieved. LIBS plasma persistence time was identified as an alternative tool for understanding ageing level of OIP material of transformer.

Effects of arc ablation on surface electrical performance of epoxy resin insulation

Operation of a high-voltage, direct current circuit breakers can cause irreversible damage to the surface electrical performance of insulation due to arc ablation. To analyze the failure mechanism, the epoxy resin (EP) insulation was ablated under experimental arcs of different energy values. The rated current of the experimental arc is 5 A, and the ablation energy values were set as 0 J, 31.3 J, 78.2 J, 109.6 J and 156.5 J respectively by changing the arc duration. The destruction of the EP surface molecule structure was investigated by using x-ray photoelectron spectroscopy and gas chromatography. Furthermore, the surface potential decay and distribution characteristics of surface traps were studied by using the isothermal surface potential decay (ISPD) method. The test results indicate that when arc energy reaches 156.5 J, the DC flashover voltage of EP decreases by 64.7% and the surface conductivity increases by 3.22 times, compared with that of the virgin sample. Reductive groups in EP molecules are oxidized when suffering arc ablation, thereby leading to the breakage of molecular chains and severe carbonization, forming amorphous carbides with high conductivity on EP surface. ISPD results show that the density of deep traps and the center energy level of traps on the EP surface increase when the arc energy increases. These traps can form multiple charge centers and distort the electric field distribution on the EP surface. Thus, the partial discharge is promoted and flashover finally occurs, thereby decreasing the surface electrical strength of EP.

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.