Quasi-uniform Field Research Articles

Formation of directed wide-aperture flows of runaway electrons in air-filled magnetized diodes.

This paper presents the results of research, development, and testing of magnetically insulated air diodes with replaceable graphite and stainless-steel tubular and coaxial cathodes of various configurations capable of generating directed bunches of runaway electrons. At the anode, the bunches have cross sections shaped as circles or rings with an outer diameter of 1-2cm. The durations of the bunches, which carry currents of a few to tens of amperes, range from tens of picoseconds to 100ps, and their charges range from tenths of a nanocoulomb to a few nanocoulombs. The kinetic energy of the bunch electrons at the peak of the current pulse is typically of the order of 150keV. The bunch parameters are set (and varied) by varying the amplitude and duration of the subnanosecond high-voltage pulse driving the diode; they depend on the cathode material and on the strength and profile of the applied external magnetic field. The bunches, retaining their cross-sectional structure, are brought out from the diode, along the field lines, through a thin foil or mesh anode into the open space with a quasi-uniform magnetic field between two Helmholtz coils. In this space, the samples to be irradiated with electrons, similarly to objects exposed to radiation in various experiments and technological applications, can be placed.

Studying the breakdown electric field in uniform and non-uniform air gaps

High voltage is essential in power grids, but it inevitably leads to high electrical stress and the associated risk of electrical discharges. Due to the complexity of the phenomena involved in electrical discharges, there are no analytical formulas for predicting the electric field strength at which they initiate, so experimental data and numerical methods are required for this purpose. According to many sources, electrical discharges can occur in air at normal pressure and temperature when the electric field strength is approximately 3 kV mm−1 or greater. This paper analyzes and discusses this threshold in detail by examining relevant electrode geometries used in high voltage applications from experimental data found in the scientific literature and using 2D finite element analysis simulations. Uniform, quasi-uniform, and non-uniform field gaps are analyzed to help students draw conclusions and gain insight into the nature of gas breakdown and the applicability of the 3 kV mm−1 threshold. The approach proposed in this paper is well suited for a practical session or group project for undergraduate or even graduate courses. Despite the important effects and design implications of electrical discharges on high voltage equipment, apparatus and systems, this topic is often not covered in sufficient detail in regular courses.

Impact of Electrode Distance in a Quasi-Uniform Model Electrode System on Lightning Impulse Breakdown Voltage in Various Insulating Liquids

This study presents findings on the influence of gap length distance on the lightning impulse breakdown voltage of three dielectric liquids of different chemical origins. The liquids were tested in a model electrode system with a quasi-uniform electric field distribution and a pressboard plate placed on the grounded electrode. The experimental studies were supported using calculations and simulations to show the individual relationships between the lightning impulse breakdown voltage and gap distance, which represent the so-called volume effect of the most stressed liquid. The results of the experiment, which involved four considered gap distances of 2, 4, 6, and 8 mm, show that a dynamic increase in lightning impulse breakdown voltage with an increase in gap distance is associated with mineral oil and bio-based hydrocarbons. However, similar trends were not observed for synthetic ester. Calculations that allowed us to assess the impact of gap length distance on lightning impulse breakdown voltage support the observations from the experimental studies. The curves obtained in this field can be considered in the process of designing insulating systems for transformers.

Sample heating above 1400 K in a diamond anvil cell

In high-pressure experimental methods, sample heating in the pressure chamber of a diamond anvil cell is an important topic, and numerous efforts have been made to improve and develop new technologies. In this paper, we propose a new type of internal resistance heating technique, the composite heating gasket, prepared by integrating an annular heater into the sample chamber for direct heating of the sample. As the effective heating area covers the entire pressure chamber wall, a relatively quasi-uniform temperature field is formed within the sample chamber. At the same time, the integration design reduces the risk of diamond oxidation and enables direct measurement of the spectroscopic properties of samples at high temperatures. The preparation of the composite heating gasket is simple and repeatable, and its heating performance is stable at temperatures above 1400 K. When the sample diameter is 210 μm and no thermal insulation is used, the diameter of the temperature zone in which the temperature difference is less than 10 and 20 K exceeds 120 and 170 μm, respectively. The composite heating gasket represents a significant advancement in providing a uniform temperature field for in situ measurements with diamond anvil cells at high pressure and temperature.

Experimental investigation and theoretical analysis of the breakdown time delay and jitter of multi-gap gas switch gaps.

The gas gap of a multi-gap gas switch can be classified as trigger and self-breakdown gaps based on the breakdown condition. A two-gap gas switch consisting of a trigger gap and a self-breakdown gap is developed to independently study the breakdown characteristics of these two types of switch gaps. Trigger experiments for the switch are conducted under various trigger voltage rise rates and different working coefficients. The experimental results indicate that the trigger gap has significantly more jitter than the self-breakdown gap, and the overall performance of the gas switch is determined primarily by the trigger gap. A novel pre-ionization structure with disks is implemented into the two-gap gas switch, considerably decreasing the breakdown delay of the trigger gap and reducing the jitter to a quarter or even less compared to that without pre-ionization. A calculation model of the breakdown time delay for the trigger gap is provided based on the foundational development of the avalanche. The probability distribution of the time required for the initial electron generation is derived in the absence of pre-ionization. The calculated breakdown time delay agrees well with the experimental results in cases with and without pre-ionization under most trigger settings. The method and principle of calculating the breakdown time delay can analyze the collapse of a gas gap with different electrode configurations (quasi-uniform or uniform electrical fields) and various gas media under a nanosecond pulse voltage.

Dynamics of a Plasma Cloud Generated by a Compact Coaxial Gun upon Expansion into Vacuum and Large-Volume Background Plasma in an External Magnetic Field

Results of experiments on injection of dense plasma clouds created by a small-scale coaxial generator into vacuum and large-volume background plasma in an ambient magnetic field are presented. The regime of an “infinite” background medium that allows studying the plasma-cloud dynamics on the scale of about one meter in the directions perpendicular and parallel to a quasi-uniform magnetic field is realized on “Krot” plasma device. The dynamics of the diamagnetic cavity appearing upon magnetic-field expulsion by a plasma blob, the electromagnetic noise appearing in the cavity, along with the evolution of plasma-cloud structure during injection and at the stage of its decay, were studied. It is demonstrated that the key properties of the cloud dynamics that are typical of the active space and high-energy laboratory experiments, including complete expulsion of the magnetic field from the cloud and development of the flute instability at its boundary, are reproduced at low injection speed (below 30 km/s) and low plasma energy (on the order of 0.1 J).

Analysis of the Insulation Characteristics of Hexafluorobutene (C4H2F6) Gas and Mixture with CO2/N2 as an Alternative to SF6 for Medium-Voltage Applications

This paper investigates C4H2F6, a promising environmentally friendly insulating gas that possesses high dielectric strength and a low global warming potential. The study focuses on examining the insulation properties of C4H2F6 when combined with CO2/N2, aiming to assess its suitability as a substitute for SF6 in gas-insulated applications. Finite element analyses are performed to evaluate the field utilization factor and electric field distribution in the proposed mixture. The properties of liquefaction temperature were examined in this study to determine the optimal mixing ratio for applications that require a minimum working temperature. Extensive experimental investigations were carried out to assess the dielectric strength characteristics of the gas mixture in both uniform and quasi-uniform electric fields. It was found that pure HFO-1336mzz (E) exhibits a dielectric strength approximately 1.2–1.6 times higher than SF6. Experimental results have revealed that the insulation performance of a 30% HFO-1336mzz (E)/CO2 mixture closely resembles that of SF6, with a matching efficiency of up to 90% in a weakly uniform electric field. This remarkable performance can be attributed to a positive synergistic effect between HFO-1336mzz (E) and CO2, combined with the gas mixture’s excellent self-recoverability property. These experimental findings are further supported by finite element analysis, which confirms the observed results. The 30% HFO-1336mzz (E)/CO2 gas mixture at 0.15–0.20 MPa pressure and constant 0.6 mm air gap reveal superior insulation tolerance and less sensitivity to the electric field, confirming its promising medium-voltage engineering applications. The associated results of this research provide a critical reference for the engineering application of the alternating (AC) and direct current (DC) insulation characteristics of the HFO-1336mzz (E)/CO2 gas mixture.

Prebreakdown and Breakdown Phenomena in HFO Gas Under Impulse Voltage—Part I: Divergent Field

Pre-breakdown and breakdown phenomena in HFO1234ze(E) gas are studied with a wide range of electrode geometries, characterized by a field enhancement factor η ranging from 1.7 (quasi uniform field) to 90 (very divergent field). As in SF6, breakdown in HFO results from the inception and propagation of leaders. Depending on the field geometry, the measured breakdown voltage represents either the voltage required for leader complete propagation (in divergent field, η > 10), or for inception (in quasi-uniform field, η < 10). This paper mostly concerns results obtained with divergent fields. Leader propagation in HFO is barely influenced by pressure, and the minimum voltage required for propagation to breakdown is slightly lower compared to SF6, explaining that lower breakdown voltages are recorded with HFO in divergent field. In such conditions, breakdown voltage is mainly determined by the gap distance. HFO degradation, evidenced by the formation of black soot after breakdown, has no influence on leader propagation, and hence on breakdown properties in divergent fields.

Electric Field Modulation in the Channel by Lateral Thickness Distribution of High-k Films Formed on GaN HEMTs to Improve Breakdown Voltage

High breakdown voltage owing to a quasi-uniform electric field, upon loading a high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{k}$</tex-math> </inline-formula> film in the gate–drain region, was recently investigated. However, the uniform high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{k}$</tex-math> </inline-formula> film has a limit to modulate the electric field distribution, and the electric field near the gate side is stronger than that near the drain side. This study proposes the modulation of the high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{k}$</tex-math> </inline-formula> film thickness between the drain and gate of a gallium nitride (GaN) high electron mobility transistor (HEMT) in the lateral direction for electric field distribution control in the channel. Compared to a uniform film, the electric field near the gate can also be weakened and possesses a negative gradient compared to a conventional electric field slope. Based on the simulation results, the voltage at which impact ionization occurs can be increased by 20%. This can be explained using the smaller gate leakage current and improved electric field distribution. Furthermore, the maximum electric field increased in the present structure when the slope of the electric field was negative.

Surface Flashover Characteristics of the 252-kV Conical Insulator in C4F7N/CO2 Gas Mixture

This article reports on an investigation into the surface flashover characteristics of the 252-kV conical insulator in a C4F7N/CO2 mixture, which is used as an alternative to sulphur hexafluoride (SF6). This study was done for a quasi-uniform field using a 252-kV experimental platform. The influence of molar fraction and gas pressure is reported and compared to that in SF6. Results show that small quantities of C4F7N mixed in CO2 can increase the surface flashover voltage. Four types of discharge traces are observed in the experiment, and most traces are on the concave side under both ac and lightning impulse (LI) voltage. The polarity effect is observed in C4F7N/CO2 gas mixtures. An empirical formula is fitted for C4F7N/CO2 mixtures by experimental data under LI(–), and 0.7-MPa 9%C4F7N/91%CO2 mixture has great potential to replace 0.5-MPa SF6.

Life on Magnet: Long-Term Exposure of Moderate Static Magnetic Fields on the Lifespan and Healthspan of Mice.

All living organisms on the Earth live and evolve in the presence of the weak geomagnetic field, a quasi-uniform static magnetic field (SMF). In the meantime, although the effects of moderate and high SMFs have been investigated on multiple aspects of a living organism, a long-term SMF exposure of more than 1 year has never been reported. Here, we investigated the influence of a moderate SMF (70-220 mT head-to-toe) long-term continuous exposure (1.7 years) to two different SMF directions on healthy male C57BL/6 mice. We found that not only was the lifespan of the mice prolonged, but their healthspan was also improved. The elevated plus maze test and open field test show that SMFs could significantly improve the exploratory and locomotive activities of the aged mice. The Morris water maze test shows that SMFs could improve their spatial learning ability and spatial memory. Tissue examinations reveal that SMFs have an ameliorative effect on oxidative stress in the brain of aged mice, which was reinforced by the cellular assays, showing that SMFs could protect the PC12 cells from D-gal-induced senescence by increasing superoxide dismutase, catalase, and reducing the malonaldehyde levels. Therefore, our data show that the 1.7-year SMF exposure can improve both the lifespan and healthspan of naturally aged mice due to reduced oxidative stress, which indicates that SMFs have the potential to be used as an adjuvant physical therapy to reduce the ageing-induced health risks to benefit animals, and even humans.

Static Magnetic Fields Protect against Cisplatin-Induced Kidney Toxicity.

Cisplatin is one of the most widely used anti-cancer drugs that can effectively inhibit the growth of multiple types of cancer. However, its clinical application is limited by its severe side effects, especially kidney toxicity, caused by cisplatin-induced oxidative stress, inflammation and kidney cell apoptosis. Here, we found that moderate (a few hundred mT) quasi-uniform static magnetic fields (SMFs) could inhibit cisplatin-induced renal proximal tubular cell death, especially the vertically downward direction SMF. RNA-seq experiments demonstrate that SMFs induced differential gene expressions that are closely associated with oxidative stress, apoptosis, cytokine production, transmembrane transport and DNA repair. In vivo experiments show that SMFs can reduce cisplatin-induced kidney injury in cisplatin-administrated tumor-bearing mice by reducing oxidative stress, inflammation and cell apoptosis. Furthermore, high-dose cisplatin-induced acute nephrotoxicity can be effectively alleviated by SMF treatment of as little as one day, which significantly reduced the reactive oxygen species levels in kidneys and prolonged the mice's survival. Moreover, the concentration of cisplatin in the kidney was significantly attenuated in SMF-treated mice. Therefore, our study demonstrates the effects of moderate SMFs as a novel physical method to reduce oxidative stress, and revealed their future potential to be used against cisplatin-induced kidney toxicity in cancer treatment.

Effects of DC Polarity and Field Uniformity on Breakdown of SF₆ and C₃F₇CN/CO₂ Mixture

To address the issues of greenhouse gases and resultant global warming, there is an urgency to find an electronegative gas to replace SF6 in global power system networks. This article provides an experimental study of various factors on DC breakdown of electronegative gases. Rod-plane geometries with a range of rod diameters (3.5–12.5 mm) and electrode separations (5–55 mm) are used to provide different degrees of field uniformity. SF6 and a 20% C3F7CN / 80% CO2 mixture are tested under pressures ranging between 1 and 5 bar. Breakdown voltages (BDVs) under both polarities rise linearly with gap distance in quasi-uniform fields with the positive being higher. Increasingly nonuniform fields lead to saturation in the case of positive BDV, while in the negative polarity case, the BDV increases linearly. As a result, the two polarities’ BDV magnitudes crossover as field nonuniformity increases. The value at which the negative value exceeds the positive is dependent on the field uniformity, pressure, and gas medium. A simulation based on the streamer criterion model provides a good agreement with experimental results for positive DC breakdown in the range of 2–5 bar. In terms of the insulation characteristics, 20% C3F7CN / 80% CO2 could provide a valuable alternative to SF6 in high-voltage plant for outdoor applications in hot-climate countries and indoor applications in cold-climate countries.

Prediction of Impulse Breakdown Voltage in Dry Air at High Pressure Using Volume-Time Theory

Dry air is promising as an alternative insulation medium for SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas in high-voltage equipment. Since its dielectric strength is about one-third of that of SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas, one of the most effective ways to apply dry air to high-voltage gas-insulated switchgears (GISs) is to further increase gas pressure. In this article, impulse breakdown strength of dry air at high pressure under quasi-uniform electric field is measured. Furthermore, the probability distribution of breakdown voltage is predicted using the volume-time theory in consideration of initial electron generation probability in gas gap and field electron emission from an electrode. The predicted distribution of breakdown voltage by the volume-time theory is in line with the experimental results.

Insulation characteristics of quasi-uniform electric field and effect of metallic particle contaminationin highly compressed dry air

Insulation characteristics of quasi-uniform electric field and effect of metallic particle contaminationin highly compressed dry air

Effect of Copper and Iron Particles on the Dielectric Strength of R410A Gas as an Alternative of SF 6 in GIS

The Gas Insulated Switchgears (GIS) used in high voltage transmission and distribution networks are reliable, compact, and maintenance-free. During the manufacturing process, some metal particles remain inside GIS which affects the breakdown strength of insulating gas and hence deteriorates the performance. The breakdown performance of R410A gas in the presence of free metal particles is presented in this paper. Iron and copper particles of various sizes and shapes are used in the research work. The impact of these free particles on the characteristic of R410A gas at various gas pressures and gap distances is elaborated. Power frequency breakdown voltage test in the presence of metal particles and self-recoverability test are performed in a high voltage lab. The effect of various dimensions of metal particles on the dielectric strength of R410A gas under the quasi-uniform field in alternating current is examined. The impact on the breakdown voltage of R410A is analyzed by increasing the pressure and gap distance. Experimental results are discussed in the paper. As metal particles in between them can disrupt the R410A reaction barrier. According to the results of the experiments conducted, the breakdown voltage was affected by 30% by the addition of iron particles and 46.89% by the addition of copper particles, and it is concluded that iron particles perform better at different gaps distances.

Fracture of Porous Ceramics: Application to the Mechanical Degradation of Solid Oxide Cell During Redox Cycling

Solid Oxide Cell (SOC) is a high-temperature electrochemical device that can be operated in both fuel and electrolysis modes. It is constituted of a dense electrolyte in Yttria Stabilized Zirconia (YSZ) sandwiched between two porous electrodes. The hydrogen electrode is classically made of a cermet composed of Nickel and YSZ (Ni-YSZ). Because of system failures in operation (e.g. a fuel shortage or air re-introduction during the system shutdown), the Nickel is liable to be re-oxidized inducing its volume expansion. The Ni swelling within the cermet leads to a mechanical damage in the YSZ backbone [1] resulting in a degradation of the overall cell performances [2]. Therefore, it is still required to enhance the hydrogen electrode robustness for a higher redox tolerance. For this purpose, it is essential to predict accurately the formation and distribution of micro-cracks in the YSZ network during the re-oxidation. Nevertheless, the modeling of the initiation and propagation of cracks in complex microstructures of porous ceramics is still a subject of investigation. Moreover, there is also a lack of data on fracture properties for the porous YSZ material. In this frame, the present study was focused to propose a relevant model for the initiation and the growth of cracks in porous ceramics in order to simulate the fracture induced by Ni re-oxidation. For this purpose, a coupled experimental and modeling approach has been adopted (Fig.1). For the experimental part, the mechanical characterizations have been performed on thin YSZ membranes of different porosities representative of SOC microstructures. The compression test has been chosen as it generates a quasi-uniform stress field in the tested samples. To this end, micro-pillars of few tens of micro-meters in section have been milled in the YSZ membrane using a xenon plasma focused ion beam. For each investigated porosity, the compressive fracture strength has been measured on several pillars. The tested samples have been carefully characterized to assess the damage in the microstructure. As expected, the compressive strength was found to decrease with increasing the porosity. Moreover, a transition has been observed from a brittle behaviour at low porosity towards a local damage at high porosity [3]. All these experimental data have been used to validate a numerical tool to simulate the fracture in porous microstructure. In this study, a model based on the Phase Field Method (PFM) has been developed. In this approach, the crack is modeled implicitly through a smooth scalar damage variable and its width is controlled by a length scale parameter. Therefore, this method is well adapted to solve the fracture problem in complex and heterogeneous materials since it is mesh independent. The implemented model corresponds to the one proposed by C. Miehe et al. [4] who proposed a staggered scheme for the numerical resolution. A special attention has been paid in this work to evaluate the model capability to accurately predict the crack initiation for porous electrode material. For this purpose, the results of the PFM simulations have been compared to the mixed criterion proposed by D. Leguillon [5]. The study has been conducted on notched samples with different opening angles and for macro-cracks blunted by different cavity sizes. It has been highlighted that the two approaches provide consistent predictions for the crack initiation. Moreover, it has been checked that the PFM model is able to reproduce accurately the expected decrease of the fracture toughness when increasing the material porosity. Once these studies completed, the PFM model has been used to simulate the micro-compression test using a multiscale approach. It has been found that the model is able to reproduce correctly the decrease of the measured compressive fracture strength with the sample porosity. Moreover, the distribution of cracks in the simulations has enabled analysing the transition from a brittle to a local damage detected during the experiments. The validated model has been applied to simulate the fracture induced by the Ni re-oxidation. The repartition and density of micro-cracks in the YSZ backbone has been computed during the Ni reoxidation and the results have been confronted with an available tomographic data set. Finally, the impact on the cermet redox tolerance of the Yttrium content in YSZ has been discussed thanks to the modelling. [1] Faes, A. et al. J. Power Sources 193, 55–64. [2] Laurencin, J. et al. J. Power Sources 192, 344–352. [3] Meille, S. Et al . J. Eur. Ceram. Soc. 32, 3959–3967. [4] Miehe, C. et al. Comput. Methods Appl. Mech. Eng. 199, 2765–2778. [5] Leguillon, D. J. Mech. – A/Solids 21, 61–72. Figure 1

Numerical verification of the two-spike-current behavior in the initial stage of plasma formation in a pulsed surface dielectric barrier discharge

Nanosecond pulsed surface dielectric barrier discharge (SDBD) is a hot topic in many fields, but the mechanisms of discharge evolution and micro-channel propagation are still not clearly understood. In this paper, a plasma fluid model of nanosecond pulsed SDBD is established, and the deductions of the two current spikes are verified and improved by simulation. The two current spikes correspond to the two stages of the discharge: the ionization wave propagation and the repeated re-ignition in the gap between the ionization wave and the dielectric surface. In the first stage, the ionization wave develops rapidly at first and propagates very slowly in the end, which produces the first current spike with a very short rise time and a tailing falling edge. The curve profile of the ionization wave velocity is very similar to that of the first current spike. A certain distance is maintained between the bottom of the ionization wave and the dielectric surface in this stage, which forms the gap for the next stage. In the second stage, the charged particle cloud induced by the quasi-uniform electric field in the middle of the gap and the new micro-channel from the edge of the high-voltage electrode propagate at first, and then merge together to establish a new discharge channel. The reverse electric field in the gap induced by the accumulated charges on the dielectric surface restricts the expansion velocity of the second discharge channel, which results in a longer rise time and falling time of the second current spike.

Impulse Breakdown Characteristics of Transformer Oil with Cellulose Particles in Quasi-Uniform Field

This study presents an experimental comparison of the breakdown characteristics of oil samples with four cellulose particle content levels in a quasi-uniform electric field under lightning impulse voltage. For the experiment to resemble the actual working conditions of transformers and meet the on-site withstand impulse voltage test requirements, both the standard lightning voltage (SLI) and oscillating lightning impulse (OLI) are considered. The experimental results show that an increase in the number of cellulose particles directly weakens the oil insulation strength. At the same particle content level, the oil breakdown voltage under OLI is higher than that under SLI, and the breakdown time lag under OLI is longer than that under SLI. The breakdown time lag is more sensitive to the effect of particle content under SLI. This work is beneficial in deepening the understanding of the impulse breakdown characteristics of transformer oil within cellulose particles and in providing references for the on-site impulse test of oil-immersed power transformers.

Breakdown Voltage of Transformer Oil Containing Cellulose Particle Contamination With and Without Bridge Formation Under Lightning Impulse Stress

This study investigates the effect of cellulose bridge formation on insulating oil. Such effect influences the breakdown voltage and strength of mineral oil (MO) under standard lightning impulse (LI) voltage. A cylindrical test vessel fitted with spherical–spherical electrodes with a 2 mm gap distance between them was used to generate a quasi-uniform field and observe bridge formation between the gaps. A positive LI waveform (1.2/ $50~\mu \text{s}$ ) generated by an impulse generator was applied during the breakdown tests. The rising-voltage method was used to measure the breakdown voltage in accordance with the IEC 60897 standard test method. Weibull cumulative breakdown probability was applied to present the breakdown results statistically, and the outcome was compared with clean oil (CO). Commercial cellulose microcrystalline particles were dispersed into the transformer oil to act as bridge. The concentrations of the samples were 0.004%, 0.008% and 0.012% by weight. The breakdown test results showed that the cellulosic particles reduced the LI breakdown voltage (LIBV) of MO by up to 10%. The influence of cellulose particles on LIBV was also more prominent during bridge formation, with a further reduction of 14%. Evidently, the presence of the cellulose bridge exerted a detrimental effect on the breakdown voltage of MO under LI stress, yielding a reduction of 24% and 27% at 0.004 wt% and 0.012 wt%, respectively. Finite element analysis also indicated that the electrical field strength of cellulosic particle contamination of 0.004 wt% and 0.012 wt% without a bridge skeleton exceeded the critical electric field strength of CO by 20% and 61%, respectively. The presence of cellulosic contamination at any concentration, i.e. either with or without a bridge structure, clearly decreased the breakdown strength of transformer oil. However, the breakdown is not occurred instantaneously. The LI breakdown was initiated and established only at a certain impulse stress level, when the electric field strength along the bridge lines exceeded the critical electric field strength of the oil. Thus, this finding potentially contributes to the formulation of guidelines for condition assessment and contamination monitoring to minimise common issues in power transformer failures that are attributable to the insulation system, and consequently, achieve optimum transformer insulation integrity.