SF6 Content Research Articles

A Study on the Efficient Degradation of Sulfur Hexafluoride by Pulsed Dielectric Barrier Discharge Synergistic Active Gas

SF6 is a strong greenhouse effect gas, which is widely used in high-voltage electrical equipment such as circuit breakers and high-voltage switchgear because of its excellent insulation performance and arc extinguishing ability. In recent years, the use and emission of SF6 have been rising, and with the proposal of the dual carbon strategic goal, its harmless degradation has become an urgent problem to be solved. In this paper, SF6 was degraded by pulsed DBD plasma technology and O2. Studies have shown that the addition of O2 can effectively promote the degradation of SF6. With the increase in the added O2 content, the DRE and EY of SF6 first increased and then decreased. Under the conditions of the input power of 50 W, SF6 concentration of 2%, and gas flow rate of 50 mL/min, the reaction system obtained the highest DRE and EY of 58.40% and 5.24 g/kWh when the O2 content was 1%, respectively. In the SF6/Ar/O2/H2O system, the addition of H2O could improve the product selectivity of SO2F2, and when the O2 concentration was 1%, the highest selectivity of SO2F2 was 48.96%, and the concentration was 8006.76 ppm. The addition of O2 inhibited the production of SO2, and with the addition of the O2 system, SO2F2 and SOF4 were the main components of degradation products; however, there were also SOF2, SO2, SiF4, SF4, etc. In this paper, the decomposition path of O2 under SF6 was analyzed in detail according to infrared spectroscopy and decomposition products.

Partial Discharge Characteristics of SF6 and SF6/N2 in Nonuniform Electric Field Under Negative DC Voltage

In this study, the partial discharge (PD) characteristics of nonuniform electric fields in SF6 and SF6/N2 gases from PD inception to intermittent breakdown under negative dc voltage were systematically studied. The effects of the gas mixture ratio and electric field distortion on the PD characteristics were analyzed in terms of PD inception voltage (PDIV), PD amplitude, PD repetition frequency, and time-resolved PD (TRPD) patterns. The PDIV of SF6/N2 was higher than that of SF6 within a certain range of gas mixture ratios, which is related to the degree of electric field distortion. The greater the degree of electrical field distortion, the greater the range of gas mixing ratios, in which the PDIV of SF6/N2 was higher than that of SF6. The difference in the value of PDIV also increased with an increase in the degree of electrical field distortion. The PD amplitude and PD repetition frequency in SF6/N2 gradually increased with increasing applied voltage. Moreover, the smaller the SF6 content, the greater the PD amplitude and repetition frequency. In SF6, the PD amplitude significantly increased and the PD repetition frequency significantly decreased when the applied voltage increased to a high value. The shape features of the TRPD patterns in both SF6 and SF6/N2 changed with an increase in the applied voltage, but the change in SF6 was more obvious than that in SF6/N2.

Effect of Working Coefficient on Induced Breakdown Characteristics of Plasma-Triggered-Based Gas Gap Switch in Fast Bypass Conditions

Gas gap switch (GGS) has obvious advantages in fast bypass operation, quick response, simple structure, and strong controllability. The working coefficient (i.e., the ratio of the main gap working voltage to the steady-state breakdown voltage) of GGS is mainly affected by the working voltage, the main gap distance, the type of trigger gas, and the pressure. However, the research mostly focuses on the influence of the above single factors on the trigger performance of GGS. There is a lack of studies on the induced breakdown characteristics of GGS considering the variation of the working coefficient under different factors. In this article, a research platform about the induced breakdown of GGS is established. It is shown that the working process of the GGS trigger cavity is divided into the high voltage pulse prebreakdown stage in the first cavity, the discharge ablation stage in the first cavity, and the discharge ablation stage in the second cavity. With the increase of the working voltage, the critical trigger capacitor voltage is lower, and the plasma jet characteristic parameters are decreased gradually during the critical conduction. The conduction time delay and time delay jitter are decreased rapidly, the triggering stability is significantly improved. Changing the main gap distance, the discharge channel shape is a significant difference at the moment of GGS-induced breakdown. And the electric characteristics of plasma jets are varied obviously. The high trigger gas pressure hinders the development of the plasma jet and causes a greater trigger jitter. With the increase of SF6 content in the trigger environment, the characteristic parameters of GGS decrease obviously. The strong insulation and electronegative gas seriously constrain the plasma jet capability. The research results can provide theoretical guidance for the improvement of trigger stability and the selection of trigger condition parameters of plasma-triggered-based gas switches used in fast bypass conditions.

Research on Micro-Water Treatment Technology of High-Voltage SF6 Equipment without Power Failure

With the frequent occurrence of high-voltage SF6 equipment-related problems in recent years, the exceedances of micro-water content and decomposition products of SF6 and other problems are getting increasingly worse. The routine solution involves multiple processes, including powering off the equipment, recovering the SF6 gas in the equipment, flushing, injecting new SF6 gas, and conducting the voltage withstand test, which requires a huge number of manpower and material resources. Besides, the operating process is risky, and power cutting and re-supplying increase the operational risk and impair the reliability of the grid in the power supply. This paper mainly explores an efficient, safe, and intelligent online SF6 purification technology, which can achieve the online detection and purification of SF6 micro-water and impurity gases when the high-voltage SF6 equipment is powered on and make the purification units’ renewable. Applying this technology can prevent the power cut of high-voltage power supply equipment, thereby making sure that the grids are safe. It is also environmentally friendly as it can reduce the usage of SF6 gas.

Numerical simulation of breakdown properties and streamer development processes in SF6/CO2 mixed gas

In this paper, the dielectric breakdown properties in SF6/CO2 mixed gas, the development of the streamer in SF6/CO2 mixed gas, and the distribution of each component with time were studied. First, the electron transport parameters (mean energy, longitudinal diffusion coefficients, Townsend coefficient, critical reduced electric field coefficients, and electron energy distribution function) in SF6/CO2 mixtures with different ratios in the E/N range of over 50–1000 Td were obtained by two-term Boltzmann equation analysis. Then, coupled with the Boltzmann drift–diffusion equation and Poisson equation, the hydrodynamic model of discharge of SF6/CO2 mixtures in a strongly non-uniform electric field was established. Many different influence factors are considered, such as the gas mixture ratio, applied voltage, space temperature, space pressure, and electrode structures. The results indicate that the increase in SF6 content in the mixed gas will reduce the ionization rate of the total mixed gas, and therefore, it takes a longer time for breakdown. The higher the pressure, the more concentrated the form of the streamer. As the temperature increases, the shape of the streamer head becomes more scattered, and it loses its contoured shape at about 3000 K; in addition, the existence of the maximum electron number density value appears at the tip of the rod electrode rather than at the streamer head. The simulation also revealed that the dielectric strength of SF6/CO2 mixtures is stronger than that of SF6/N2 mixtures and reached a turning point at an SF6 ratio of 60% under extremely non-uniform electric fields, which agreed well with experiments.

Decomposition characteristics of SF6/N2 gas mixture with local overheating

In recent years, Chinese power enterprises have adopted SF6/N2 to replace SF6 gas in transmission electrical equipment. In this paper, an experimental platform was set up to simulate the local overheating fault of gas insulated electrical equipment. The relationship between the decomposition products of SF6/N2 gas and temperature, experimental time, gas pressure and SF6 content was studied. The results showed that the main products of SF6/N2 gas decomposition were SO2, SO2F2, N2O, C2F6, COS, CS2 and CF4. The content of SO2 increased nonlinearly with the increase of temperature, experimental time, gas pressure and SF6 content. However, the contents of SO2F2, N2O, C2F6, COS, CS2 and CF4 decreased first and remained unchanged with the increase of gas pressure, and increased with the increase of temperature, experimental time and SF6 content. And C element in metal also affected the type and quantity of gas products.

Benchmarking the calculation of electrically insulating properties of complex gas mixtures using a multi-term Boltzmann equation model

The accurate calculation of DC breakdown voltage thresholds solely from elementary electron-neutral interactions in complex gas mixtures using a multi-term Boltzmann equation (BE) kinetic model is demonstrated. SF6:N2 mixtures in the 100 Td < E/N < 400 Td field regime are explored to benchmark the model’s effectiveness. A ten-term BE model is found to yield DC breakdown voltages which, on average, agree within 3% of experimental measurements. A two-term BE model is also applied in order to characterize the error introduced in all calculations by the two-term approximation. These discrepancies are largest in pure N2 where error is greater than 10% for diffusion coefficients, within 6% for particular vibrational rate coefficients, and within 5% for breakdown voltages. However, this error falls to within 1% for most parameters and breakdown voltages in mixtures with large SF6 content.

A multi-tracer approach for understanding the functioning of heterogeneous phreatic coastal aquifers in humid tropical zones

ABSTRACT In many areas around the world, phreatic coastal aquifers are vulnerable to contamination and saltwater intrusion. This study aims to explore the hydrodynamic functioning of the coastal phreatic aquifer of the Urabá region, Colombia, based on the identification of hydrogeochemical processes and groundwater residence and transit times. Hydrodynamic data showed a fast response of the piezometric levels to precipitation with a maximum time lag of 48 h. Mean annual recharge was estimated at 19% with respect to precipitation. Another recharge mode is linked to infiltration from surface water, according to the hydrodynamic results. The main geochemical process identified is hydrolysis due to dissolved carbon dioxide, inducing silicate weathering and resulting in predominant HCO3-Ca-Mg facies. Monitoring of δ18O and δ2H compositions combined with 3H, 14C, chlorofluorocarbons (CFCs) and SF6 contents showed a fast recharge in the aquifer. Groundwater dating indicates recent waters mixed with older (>40 years) waters.

Effect of trace SF6 on negative corona characteristics in SF6/N2 gas mixtures under DC voltages

Corona discharge will affect the performance of insulating gas, threatening the safe and reliable operation of gas-insulated power equipment. In this paper, we investigate the negative corona characteristics in SF6/N2 gas mixtures with trace SF6 [φ(SF6) ≤ 1%] under DC voltages. With decreasing SF6 content, two types of the discharge–stage transition process as a function of mean current are observed. One process is from the Townsend stage, through the pulse stage, to the breakdown stage; the other one presents a normal-glow stage between the pulse stage and the breakdown stage. The threshold of SF6 content which can change the discharge–stage transition process in SF6/N2 gas mixtures is between 0.1% and 0.15%. The lower limit of positive-ion flux in the normal-glow stage is 6–7 orders of magnitude higher than the upper limit of positive-ion flux in the Townsend stage. The variation in pulse characteristic parameters, including pulse–time parameters and pulse amplitude, with increasing SF6 content is investigated. The pulse–time parameters show a trend of decreasing first and then saturation with increasing SF6 content, while the pulse amplitude shows a slightly increasing trend. Moreover, the “return stroke” phenomenon of negative corona in SF6/N2 gas mixtures is observed through discharge light emission images. The “return stroke” phenomenon does not necessarily occur in the normal-glow stage, resulting from competition between the position of the negative-ion cloud and the negative-ion flux.

Numerical simulation of streamer discharge development processes with multi-component SF6 mixed gas

In recent years, more stringent environmental protection requirements have led to increasingly higher voltage power equipment that adopt SF6 mixed gas as an insulation medium. In this paper, the streamer development in SF6 mixed gas and the distribution of each gas component were studied over time. Thus, a discharge model of the needle plate electrode with SF6 mixed gas was established. The hydrodynamic model for SF6 mixed gas discharge in uneven electric fields was created by coupling the Boltzmann drift-diffusion equation and Poisson's equation. The influence of different factors, such as the applied voltage, gas mixture ratio, and electrode spacing, on the streamer discharge of SF6/N2 mixed gas, and the influence of the type of mixed gas (air and CO2) combined with SF6 on the streamer development process were studied. The simulation results showed that the higher SF6 content in the mixed gas made breakdown more difficult, and a higher pressure led to a more concentrated streamer. Comparing and analyzing the discharge processes under different SF6 mixed gases (SF6/CO2, SF6/air, and SF6/N2) showed that the streamer for the SF6/CO2 mixture took longer to develop toward the plate electrode with a more difficult breakdown. The presented simulation results provide a microscopic interpretation for the needle-plate discharge defects that appear in power equipment that utilize SF6 mixed gas.

Theoretical investigation of neutral decomposition by-products of SF6 + CO2 mixture in the existence of H2O

Investigation of the decomposition products of SF6–CO2 gas mixture, which is a promising binary gas mixture with dilute SF6 content to be used in gas-insulated systems in power electrical systems, is very important in terms of the sustainability of operating conditions, environmental and human health. This study determines the decomposition products of the SF6–CO2 gas mixture in the presence of H2O, depending on the reaction paths by the B3LYP method at 6-311 + G(d,p) level set employing the density functional theory method. The decomposition products obtained by this method have been confirmed by the findings of experimental studies in the literature. Decomposition pathways of SF6 + CO2 mixture in the absence and presence of H2O are constructed in order to analyze relative energy changes of the decomposed by-products to select the most probable ones.

Performance of high rate MRPC with different gas mixtures

A Time Of Flight system (TOF) based on MRPC technology is widely used and plays an important role in modern high energy and nuclear physics experiments. With the increase of the energy and luminosity of the accelerators, many experiments have higher demand for a better resolution TOF detectors working in an high data rate environment, which undoubtedly represents a big challenge for this technology. FAIR-CBM is a typical high rate experiment and the particle rate of its TOF can reach 30 kHz/cm2. A new type of MRPC, built with low resistive glass with bulk resistivity of about 1010 Ohm cm, has been developed. The rate capability of this high rate MRPC can reach 70 kHz/cm2. The standard gas mixture for the high rate MRPC used for the FAIR-CBM-TOF is 90% C2H2F4, 5% i-C4H10 and 5% SF6. For the high rate MRPC used for the STAR-eTOF the gas mixture contains only two components, namely 95% C2H2F4 and 5% i-C4H10. In order to ensure the long time stability of the high rate MRPC and its use with different gas mixtures, we have studied the performance with different gas mixtures. The first measurements were performed with the standard gas mixture and different gas mixtures were tested by reducing the content of SF6. Three high rate MRPCs were tested at the same time for a cross-check, using a cosmic ray test-stand. The result of the test shows that the working point of the high rate MRPC moves toward a lower value of the high voltage by reducing the SF6 content in the gas mixture. The working point for a chamber efficiency of 95% without SF6 is about 800 V lower than the one with the standard gas mixture. Other properties such as time resolution, noise rate, dark current and cluster size have also been measured with different gas mixtures.

Thermal Decomposition Properties of Epoxy Resin in SF₆/N₂ Mixture.

As a promising alternative for pure SF6, the mixture of SF6/N2 appears to be more economic and environment-friendly on the premise of maintaining similar dielectric properties with pure SF6. But less attention has been paid to the thermal properties of an SF6/N2 mixture, especially with insulation materials overheating happening simultaneously. In this paper, thermal decomposition properties of epoxy resin in SF6/N2 mixture with different SF6 volume rates were studied, and the concentrations of characteristic decomposition components were detected based on concentrations change of some characteristic gas components such as CO2, SO2, H2S, SOF2, and CF4. The results showed that thermal properties of 20% SF6/N2 (volume fraction of SF6 is 20%) mixture has faster degradation than 40% SF6/N2 mixture. As ratio of SF6 content decreases, thermal stability of the system decreases, and the decomposition process of SF6 is exacerbated. Moreover, a mathematical model was established to determine happening of partial overheating faults on the epoxy resin surface in SF6/N2 mixture. Also thermal decomposition process of epoxy resin was simulated by the ReaxFF force field to reveal basic chemical reactions in terms of bond-breaking order, which further verified that CO2 and H2O produced during thermal decomposition of epoxy resin can intensify degradation of SF6 dielectric property.

Suppression of Electron Avalanches in Ultra-Dilute SF6-N2 Mixtures Subjected to Time-Invariant Crossed Fields

Ultra-dilute SF6 + N2 mixtures are subjected to crossed fields with the applied magnetic field normal to the electric field in order to investigate critical magnetic field values for suppression of electron avalanche growth resulting in increased dielectric strength. Electron avalanche growth inhibition together with the related electron mean energy variation and electron energy distribution functions are reported in the binary mixtures subjected to the combined fields. Even if a very small amount of greenhouse gas (SF6) is used, high dielectric strength can be achieved in the binary mixtures with 0.25%, 0.50%, 0.75%, 3%, and 7% SF6 contents subjected to the evaluated combined fields. The magnitude of the crossed magnetic field, which results in avalanche growth inhibition, decreases as the electronegativity of the mixture is increased.

Molecular Mechanism of Loading Sulfur Hexafluoride in γ-Cyclodextrin Metal-Organic Framework.

γ-Cyclodextrin metal-organic framework (γ-CD-MOF) is a new type of highly porous carrier for potential loading of therapeutic or diagnostic gas like sulfur hexafluoride (SF6). Here, loading of SF6 into γ-CD-MOF was investigated for its mechanism by molecular simulation and quantitative determination of SF6 using quantitative nuclear magnetic resonance (qNMR). For the SF6 loading, γ-CD-MOF was first degassed to remove the air without thermal decomposition or loss of framework crystallinity, then placed in the copper tube, and sealed to adsorb SF6 under 1.2 MPa and 25 °C for 12 h. The qNMR was employed for the determination of SF6 loaded in γ-CD-MOF using Span 80 as suspending agent and trifluoroacetic acid as internal standard. Then, the thermodynamic parameters had been estimated. Finally, molecular modeling combining with 19F NMR spectra was conducted to reveal the status of SF6 molecules in γ-CD-MOF. The results demonstrated that the content of SF6 loaded in γ-CD-MOF was 2.67 ± 0.46 wt %. After exposing to the environment of free SF6 at 0.1 MPa for 10 days, the relative content was 74.7%. It was confirmed that SF6 preferred to stay in the cavity of γ-CD-MOF cubes rather than in the γ-CD molecular pairs, which was a nonchemical adsorptive process. In conclusion, this research has established qNMR method and molecular simulation to demonstrate SF6 molecules in γ-CD-MOF and its loading mechanism.

Study of the arc interruption performance of SF6-CO2 mixtures as a substitute for SF6

CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> attracts a lot of attention of researchers as a possible substitute for SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> . The arc interruption capabilities of SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> -CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mixtures for different mixed ratios are investigated in this paper by carrying out tests with a 126 kV puffer gas circuit breaker prototype and calculations. Based on the measured arc voltage and current around current zero, the post-arc current characteristics and the thermal interruption properties are investigated for SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> -CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mixtures under different rise rate of the recovery voltage RRRV and di/dt before current zero. It is found that the interruption performance of SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> -CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> increases greatly with increasing SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> concentration, and the critical RRRV and di/dt before current zero of SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> -CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mixtures for SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> content of 0, 20 and 50% are about 39, 45 and 70% of that of SF6 respectively, at the short-circuit current of 10 kA. The pressure increment at current zero is found to have a great effect on the interruption of the short-circuit current. In addition, the pressure buildup in CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is much lower than that in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> for both no-load and loaded operations.

Luminescence characteristics of the Kr2F* excimer molecules in (Kr-SF6) gas mixtures with a Low SF6 content upon pumping by a pulsed beam of fast electrons

Emission spectra of dense Kr + SF6 gas mixtures with different composition excited by a pulsed beam of fast electrons are measured. The electron energy is 150 keV, and the amplitude and duration of a beam current pulse are 5 A and 5 ns. The composition of gas mixtures was Kr (190–1900) Torr and SF6 (0.1–1.0) Torr. The efficiency of formation of excimer Kr2F* molecules was found to be high in dense (∼1 atm) Kr-SF6 mixtures with a low SF6 donor content. The luminescence times and quenching constants of broadband emission of the Kr2F* molecule (λmax = 404.7 nm) by components of a gas mixture are measured.

AC creeping discharges propagating over solid–gas interfaces

This study is aimed at the characterisation of surface discharges propagating over different solid materials namely glass, bakelite and epoxy resin immersed in three gases and their mixtures (namely SF6, N2 and CO2, SF6/N2 and SF6/CO2) under AC voltage (50 Hz) in a point-plane electrode system. It is shown that the stopping length of discharges (Lf ) increases linearly with the applied voltage and decreases when the gas pressure is increased. Lf is longer in CO2 and N2 than in SF6 [Lf (N2) > Lf (CO2) > Lf (SF6)]; and the increase of SF6 content in the investigated mixtures leads to a significant decrease of Lf . Also, for a given gas, Lf is slightly longer with glass and bakelite than that with epoxy resin indicating that the higher the dielectric constant, the longer the discharge is. Thus, the density of the discharge branches and Lf reduce when the thickness of insulators increases and/or their dielectric constant decreases. Such observations evidence the implication of capacitive charge effect and electric field in the development of creeping discharges.

Electron Transport Coefficients and Effective Ionization Coefficients in SF6-O2 and SF6-Air Mixtures Using Boltzmann Analysis

The electron drift velocity, electron energy distribution function (EEDF), density-normalized effective ionization coefficient and density-normalized longitudinal diffusion velocity are calculated in SF6-O2 and SF6-Air mixtures. The experimental results from a pulsed Townsend discharge are plotted for comparison with the numerical results. The reduced field strength varies from 40 Td to 500 Td (1 Townsend=10−17 V·cm2) and the SF6 concentration ranges from 10% to 100%. A Boltzmann equation associated with the two-term spherical harmonic expansion approximation is utilized to gain the swarm parameters in steady-state Townsend. Results show that the accuracy of the Boltzmann solution with a two-term expansion in calculating the electron drift velocity, electron energy distribution function, and density-normalized effective ionization coefficient is acceptable. The effective ionization coefficient presents a distinct relationship with the SF6 content in the mixtures. Moreover, the E/Ncr values in SF6-Air mixtures are higher than those in SF6-O2 mixtures and the calculated value E/Ncr in SF6-O2 and SF6-Air mixtures is lower than the measured value in SF6-N2. Parametric studies conducted on these parameters using the Boltzmann analysis offer substantial insight into the plasma physics, as well as a basis to explore the ozone generation process.

DC creeping discharges over insulating surfaces in different gases and mixtures

This paper deals with the propagation of discharges over insulators of Bakelite and glass, in gases and their mixtures (namely N2, SF6, CO2, SF6 - N2 and SF6 - CO2) under DC voltage in a point - plane electrode arrangement. The shape and stopping length, Lf, of these discharges are investigated versus the type of insulator material and its thickness, the amplitude of applied voltage, the type of gas/mixture and pressure. The obtained results show that the discharges do not always propagate radially; this depends on the type of solid/gas interface. However, as we reported elsewhere for lightning impulse voltage, Lf increases quasi-linearly with the voltage; it decreases when the gas/mixture pressure increases. Lf is shorter in SF6 than in CO2 or N2; and the increase of SF6 content in the investigated mixtures leads to a significant decrease of Lf. Also, for a given gas, Lf is slightly longer with glass than with Bakelite indicating that the higher the dielectric constant, the longer the discharge is; the dielectric constant of glass and Bakelite being εG = 5.0 and εB = 4.8, respectively. Thus the density of the discharge branches and Lf reduce when the thickness of insulators increases and/or their dielectric constant decreases indicating that the capacitive charge effect and electric field influence the development of creeping discharges.