Solid Insulation Research Articles

Simulation of laser-induced ionization in wide bandgap solid dielectrics with a particle-in-cell code.

Modeling the laser-plasma interaction within solids is crucial in controlling ultrafast laser processing of dielectrics, where the pulse propagation and plasma formation dynamics are highly intricate. This is especially important when dealing with nano-scale plasmas where specific phenomena of plasma physics, such as resonance absorption, can significantly impact the energy deposition process. In this article, we report on adapting of a Particle-In-Cell code, EPOCH, to model the laser-plasma interaction within solids. This is performed by implementing a background permittivity and by developing and validating adapted field ionization and impact ionization modules. They are based on the Keldysh ionization theory and enable the modeling of ionization processes within solids. The implementation of these modules was validated through comparisons with a hydrodynamic code and existing literature. We investigate the necessary number of super-particles per cell to model realistic ionization dynamics. Finally, we apply the code to explore the dynamics of plasma formation in the regime of of quantized structuring of transparent films. Our study elucidates how a stack of nano-plasma layers can be formed by the interference of a pulse with its reflection on the exit surface of a high refractive index material.

Development of transformers with natural ester and cellulose or aramid insulation

This study describes selected research studies performed for developing design rules for power transformers usingnatural esters. The presented simulation results verified adequacy of design rules and allowed for adjustments needed forimplementing the new insulation system vs. the one based on mineral oil and cellulose-based solid insulation. They were alsoused as a base for the next transformer design combining natural ester with aramid insulation.

A multi-mobility model for polymer insulation: Role of high-mobility space charge on breakdown with high dv/dt voltages

The failure of polymeric solid insulation under transient voltages is closely associated with space charge. In this study, DC and AC field dependent conduction characteristics of epoxy resin were investigated using the space charge limited current and high voltage broadband dielectric spectrum methods. The breakdown voltages (BDVs) were examined over a wide range of DC voltage ramp rates. The results reveal a threshold effect of AC conduction loss below 600 Hz with respect to the electric field. By analyzing charge migration laws, a mobility distribution was deduced, with an upper limit of 10−9m2V−1s−1, which is much higher than the measured DC mobility of 10−14m2V−1s−1. A multi-mobility bipolar charge transport model was proposed by introducing the obtained mobility distribution. Compared to the single-mobility model, high-mobility carriers induce larger unblocked space charge currents for high dv/dt voltages, leading to lower values of BDV, which is consistent with experiment results. As the voltage ramp rate decreases, both simulation and experiment demonstrate higher BDVs due to the field shielding effect of homocharges.

Study of partial discharge and breakdown phenomena at triple junctions under various conditions of pressure and temperature

The presence of a triple junction constitutes a potential source of failure in many industrial applications. The reinforcement of the local electric field due to differences in relative permittivity facilitates the formation of partial discharges (PD) which weaken the insulation system and eventually lead to complete breakdown. Additionally, severe environmental conditions (temperature, pressure) associated with certain applications (e. g. nuclear or aeronautical sector) should be considered because of their influence on discharge phenomena occurring at triple junctions. In this study, the behaviour of partial discharge inception voltage (PDIV) and of breakdown (flash-over) voltage (BIV) is compared in the case of a triple junction with an aluminium oxide disc as a solid insulator when the system is heated up to 400 °C. While the PDIV decreases strongly for temperatures above 300 °C, this effect disappears for the BIV. It is proposed that this behaviour is due to the fact that relative permittivity of the aluminium oxide disc increases with temperature at low frequencies, but remains relatively stable at high frequencies, regardless of the temperature.

Usable Analytical Expressions for Temperature Distribution Induced by Ultrafast Laser Pulses in Dielectric Solids.

This paper focuses on the critical role of temperature in ultrafast direct laser writing processes, where temperature changes can trigger or exclusively drive certain transformations, such as phase transitions. It is important to consider both the temporal dynamics and spatial temperature distribution for the effective control of material modifications. We present analytical expressions for temperature variations induced by multi-pulse absorption, applicable to pulse durations significantly shorter than nanoseconds within a spherical energy source. The objective is to provide easy-to-use expressions to facilitate engineering tasks. Specifically, the expressions are shown to depend on just two parameters: the initial temperature at the center denoted as T00 and a factor Rτ representing the ratio of the pulse period τp to the diffusion time τd. We show that temperature, oscillating between Tmax and Tmin, reaches a steady state and we calculate the least number of pulses required to reach the steady state. The paper defines the occurrence of heat accumulation precisely and elucidates that a temperature increase does not accompany systematically heat accumulation but depends on a set of laser parameters. It also highlights the temporal differences in temperature at the focus compared to areas outside the focus. Furthermore, the study suggests circumstances under which averaging the temperature over the pulse period can provide an even simpler approach. This work is instrumental in comprehending the diverse temperature effects observed in various experiments and in preparing for experimental setup. It also aids in determining whether temperature plays a role in the processes of direct laser writing. Toward the end of the paper, several application examples are provided.

Substrate integrated waveguide-based frequency-tunable self-octaplexing antenna

This communication presents the first-ever substrate integrated waveguide (SIW)-based frequency-tunable self-octaplexing antenna (SOA) for wireless communication. The structure is arranged by implementing eight distinct patches with planar 50-ohm feedlines at the top of the SIW cavity, which realize eight distinct resonant frequencies. Independent tuning of each resonant frequency is achieved by incorporating appropriately allocated pockets in the antenna substrate, which can be filled with solid dielectrics of different permittivity. The obtained tuning range is broad, from 4.76 GHz to 9.82 GHz, with the capability of tuning each port individually. Furthermore, the port isolation (for all ports) that is better than 20.5 dB. The proposed antenna has a realized gain of 3.91, 3.2, 4.05, 4.14, 3.8, 3.37, 3.55 and 3.28 dBi when using Rogers RT-6010 for pocket filling. Furthermore, the antenna provides stable radiation characteristics with precise tuning. The proposed device is suitable for various applications, including multi-standard wireless communications, due to its small size, wide tuning range, consistent radiation patterns, and comparable isolation levels.

Quantifying Uncertainty in Dielectric Solids’ Mechanical Properties Using Isogeometric Analysis and Conditional Generative Adversarial Networks

Quantifying Uncertainty in Dielectric Solids’ Mechanical Properties Using Isogeometric Analysis and Conditional Generative Adversarial Networks

Multi-factor Aging Characteristics of GIS Solid Insulation: Effect of Electrical, Thermal and Vibration Stresses

Multi-factor Aging Characteristics of GIS Solid Insulation: Effect of Electrical, Thermal and Vibration Stresses

An Evaluation and Correlation Study of Oil and Solid Insulation in Power Transformers using Composite Index Considering Operating Age

An Evaluation and Correlation Study of Oil and Solid Insulation in Power Transformers using Composite Index Considering Operating Age

Numerical modeling and optimization of thermal insulation for liquid hydrogen storage tanks

Liquid hydrogen storage is one of the effective hydrogen storage methods due to its high density of 70.8 kg/m3 compared to gaseous hydrogen of 0.0838 kg/m3 at atmospheric pressure. Liquid hydrogen requires cryogenic storage technology, which minimizes heat flux by stacking multiple insulation layers in a high vacuum (10−1–10−5 Pa). However, large-scale tanks use a medium vacuum (100–10−1 Pa) to reduce maintenance expenses. Solid insulation is applied to prevent liquefaction of residual gas up to 150 K, followed by the stacking of multilayer insulation (MLI) with a vapor−cooling shield (VCS) to minimize the insulation thickness. In this study, a numerical model was developed to calculate the heat flux of a storage tank based on the physical tank shape. The insulation thickness was also optimized for two insulation systems (solid insulation + MLI and solid insulation + MLI + VCS). Optimal VCS placement on solid insulation, determined through sensitivity analysis, reduces 64.2 % of MLI layers under 5 Pa vacuum pressure. Total insulation thickness reduction of 51.4 % at 1 Pa vacuum pressure was obtained based on a hydrogen storage tank volume of 4200 m3. The effects of insulation thickness reduction are remained even when the vacuum pressure is increased to 5 Pa.

Shielded HMSIW-based frequency-tunable self-quadruplexing antenna using different solid/liquid dielectrics

This article proposes a frequency-tunable self-quadruplexing antenna based on a shielded half-mode substrate integrated waveguide (S-HMSIW). In order to reduce the size of the HMSIW cavity resonator and to obtain quad-band characteristics, a modified E-shaped slot is engraved on the top of the metal. The experimental validation is carried out after analyzing the data using a circuit model. Flexibility of each resonant frequency is enabled by integrating solid and liquid dielectrics of different permittivities within the designated etched region of the proposed antenna. With a broad tuning range of 3-8.5 GHz, the operating frequencies can be tuned individually according to the application-driven demands. Furthermore, the port isolation is better than 23.5 dB in all four bands. The proposed antenna, with distilled water-filled pockets, exhibits realized gain of 3.82, 4.1, 4.3, and 5.6 dBi at 5.17, 5.76, 3.55, and 7.03 GHz, respectively. The antenna features stable radiation characteristics in all bands and throughout the entire tuning ranges. Owing to its compact size, broad tuning capability, consistent radiation patterns, and competitive isolation levels, the presented design can be considered a suitable solution for compact tunable devices required by the existing and emerging application areas, especially multi-standard wireless communications.

Environmentally Friendly Wood Adhesives Based on Dextrin/Arabic Gum Blends

Wood adhesives are widely used consisting of urea-formaldehyde resins. Most of the studies aim to find alternative natural materials to replace the carcinogenic chemical adhesives. A mixture of natural materials that are available in abundance, cheap in price and are resistant to water and heat, the failure of the interface between two solid dielectrics is a major source of insulation system failure, hence it is crucial to understand the principles regulating this breakdown occurrence. It is generally agreed that the tangential AC breakdown strength of solid-solid surfaces is primarily determined by the elastic modulus (elasticity), radial/tangential pressure, surface smoothness/roughness, and dielectric strength of the ambient environment. For this purpose, we made use of dextrin and Arabic gum. Physical parameters (Lap shear strength, pull off strength, hardness, roughness, electrical and thermal insulations) of the combination formed with varying amounts of each ingredient were investigated. The adhesive characteristics of D were enhanced by the incorporation of AG, with improved pull off adhesion and lap shear strength at increasing AG levels at 80%. D /AG 20/80 compositions had pull off adhesion values 189 times greater than pure D, and lap shear strength values 820 times higher. It is evident because of the dispersion of AG molecules in decreasing the surface roughness of D/AG films and increasing their hardness on the shore scale. There is a positive correlation between the amount of AG added and the blending matrix of D, therefore boosting A with AG increases A. The dielectric strength, and thermal insulation increases with the increase in the weight ratio of Arabic gum. The blend prepared as an adhesive for wood is a good electrical and thermal insulator at 8% AG/20% D.

Investigation of Novel Solid Dielectric Material for Transformer Windings.

Improvement techniques aimed at enhancing the dielectric strength and minimizing the dielectric loss of insulation materials have piqued the interest of many researchers. It is worth noting that the electrical breakdown traits of insulation material are determined by their electrochemical and mechanical performance. Possible good mechanical, electrical, and chemical properties of new materials are considered during the generation process. Thermoplastic polyurethane (TPU) is often used as a high-voltage insulator due to its favorable mechanical properties, high insulation resistance, lightweight qualities, recovery, large actuation strain, and cost-effectiveness. The elastomer structure of thermoplastic polyurethane (TPU) enables its application in a broad range of high-voltage (HV) insulation systems. This study aims to evaluate the feasibility of using TPU on transformer windings as a solid insulator instead of pressboards. The investigation conducted through experiments sheds light on the potential of TPU in expanding the range of insulating materials for HV transformers. Transformers play a crucial role in HV systems, hence the selection of suitable materials like cellulose and polyurethane is of utmost importance. This study involved the preparation of an experimental setup in the laboratory. Breakdown tests were conducted by generating a non-uniform electric field using a needle-plane electrode configuration in a test chamber filled with mineral oil. Various voltages ranging from 14.4 kV to 25.2 kV were applied to induce electric field stress with a step rise of 3.6 kV. The partial discharges and peak numbers were measured based on the predetermined threshold values. The study investigated and compared the behaviors of two solid insulating materials under differing non-electric field stress conditions. Harmonic component analysis was utilized to observe the differences between the two materials. Notably, at 21.6 kV and 25.2 kV, polyurethane demonstrated superior performance compared to pressboard with regards to the threshold value of leakage current.

High-Sensitivity Detection of Solid and Liquid Dielectrics Using a Branch Line Coupler Sensor

This article presents a sensor based on branch line couplers for detection of dielectric samples. The two <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50~\Omega$</tex-math> </inline-formula> branches of the coupler are used as sensing elements, where one of them is considered as a reference and the other one is the test branch. Loading the material under test (MUT) on the test branch modifies its electrical length and characteristic impedance. This breaks the symmetry in the coupler, which, in turn, affects the isolation level. Using this concept, the MUT can be simply identified just by measuring the isolation level at the center frequency of the coupler. The sensor response is analyzed using an analytical circuit model. In addition, the sensing performance has been verified through the fabrication and measurement of two sensor prototypes for measurement of solid and liquid samples.

Surface Flashover Over a Microprofiled Cylinder in Air

Increasing the lightning impulse (LI) flashover voltage of insulators can require significant changes to the geometry and material properties of the insulator or electrodes. In some cases, such changes can be difficult to implement as they may require more volume or may compromise e.g. mechanical properties of the insulating parts. In this work, a new micro-scale surface profile which inhibits streamer discharge propagation without introducing major changes to the insulation system geometry or materials is demonstrated. Polyoxymethylene (POM) cylinders with 25 % glass fibre in air were machined with 0.5x0.4 mm rectangular grooves spaced 0.6 mm apart and stressed with LIs. A non-profiled cylinder and a cylinder with a larger, semi-circular grooves were tested for comparison. The cylinders were tested in a 48.5 mm atmospheric air gap with an exposed triple junction as the positive electrode. The rectangular surface profiles increased the LI flashover voltage (U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> %) from 70.0 to 96.5 kV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">peak</sub> . High-speed imaging showed positive surface streamer discharges stopping on the surface profiles. Discharge inception calculations were used to explain the influence of the surface profile on the flashover process. The results are relevant for technological applications with electrically stressed solid insulators in gases.

Numerical investigation of subcooled two-phase flow and heat transfer characteristics in a liquefied natural gas cargo containment system

The increase demand in liquefied natural gas (LNG) leads to an increase in the import and export of LNG cargo using LNG carriers with composite insulation and double-hull structures for membrane-type storage tanks. Therefore, the transient characteristics of cryogenic heat transfer in LNG cargo containment systems have become important for understanding the physics when LNG vaporizes into the gas phase to minimize the cargo loss of boil-off gas (BOG). However, public papers on fundamental research on long-term computational fluid dynamics (CFD) simulations are still lacking. In the present study, the transient flow behaviors of LNG fluids and the heat transfer characteristics were numerically studied by applying the simplified single-layer insulation in an LNG cargo containment system, based on an initial temperature of TLNG,0 = 110.5 K and saturation temperature of TLNG,sat = 111.5 K in the boundary condition of IGC code. The proposed analysis procedure can serve as a reference for relative comparison of predicted BOG difference in LNG cargo holds.Consequently, the heat transfer mode between the two-phase methane and the solid insulation could be typically classified into two different regimes: (1) a subcooled regime before reaching the saturation temperature from the initial temperature, and (2) a boiling regime after reaching the saturation temperature. In the subcooled regime, the subcooled liquid exhibited bulk flow motion induced by natural convection. The loss rate of LNG mass per time interval was extracted from the best-fit correlations as −0.0448 kg/s at 1 d (105 s) to cool down the insulation system and −0.0072 kg/s at 5 d (5 × 105 s) to reach the LNG saturation temperature. When the predicted value of the loss rate of LNG mass by the CFD simulation was regarded as the generation rate of boil-off gas, it was smaller than the designed value of the boil-off gas of 0.464 kg/s corresponded to boil-off rate (BOR) about 0.2 %/day. In the boiling regime and under sufficiently cooled conditions, the loss rate of LNG mass per time interval did not consistently exceed the BOR value. The liquid flow exhibited a turbulent-like motion despite the relatively low velocity at 10 d (106 s). Especially, the increase in the effective thermal conductivity was a good indicator for distinguishing between the boiling and subcooled regimes.

Environmentally friendly super-insulating material with three-dimensional nanoporous structure: A promising candidate for future gas insulated transmission lines and carbon sequestration

Gas-insulated transmission lines (GILs) play a pivotal role in facilitating the future long-distance transmission of power generated by renewable energy. However, the extensive employment of gaseous SF6 presents formidable obstacles to scaled application of GILs, due to its relatively low breakdown strength and substantial greenhouse effect. In this work, a fantastic phenomenon was discovered that the breakdown field strength of insulating gases can be significantly enhanced under the confinement of nanostructures, based on it, the starch/polyorganosiloxane biocomposite insulating dielectric (S/PBID) with three-dimensional nanoporous structure was designed and fabricated. Specifically, insulating gases (CO2, N2, SF6) composite with S/PBID, the breakdown field strength is significantly improved by 744.44 %, 520.74 %, and 310.95 % compared to the original gases, respectively. The relative permittivity of S/PBID (<2) is considerably lower than that of most existing insulating materials. Theoretical analysis suggests that the nanopores within S/PBID restricts electron multiplication and transport processes at the lateral scale, leading to a substantial enhancement in gas breakdown field strength. The nanoporous material is expected to be composited with atmospheric CO2 to overturn the conventional gas–solid composite insulation form of the GILs, thereby providing a novel insulation form of high-performance, environmental-friendly, and carbon sequestration.

Effect of Potassium Feldspar Sodium-feldspar Composite Flux on The Sintering Performance of Fly Ash-coal Gangue-phosphogypsum Based Wall Insulation Materials

In this paper, fly ash and coal gangue as the main raw materials, potassium feldspar-albite as the composite flux, phosphogypsum as the bonding agent, the semi-melting method was used to prepare fly ash-coal gangue-phosphogypsum based wall insulation material, the effect of composite flux ratio and the amount of coal gangue added on the melting temperature of wall insulation material was studied. Through the visual of high-temperature deformation analyzer to observe the volume contraction, expansion, passivation, and globalization of different formulations of specimens at high temperatures, and record the corresponding temperatures of the occurrence of various situations, to study the composite flux ratio and gangue addition on the fly ash-gangue-phosphogypsum based wall insulation material melting temperature of the impact, to determine the appropriate ratio of the flux and the gangue addition for the solid waste-based will provide a theoretical basis for the preparation of solid waste-based wall insulation materials. The research results indicate that with the increase of coal gangue content, the initial melting temperature, spheroidization temperature, and passivation temperature of the sample all sharply decrease. With the increase of potassium feldspar: albite, the melting temperature range will significantly increase. When the additional amount of fly ash is 50%, the additional amount of coal gangue is 12%, the additional amount of phosphogypsum is 5%, and the ratio of potassium feldspar to albite is 1:1, the sintering temperature of the fly ash- gangue -phosphogypsum based wall insulation material is the lowest.

Robust mixed FE for analyses of higher-order electromechanical coupling in piezoelectric solids

AbstractDirect flexoelectricity is a size-dependent phenomenon, very prominent at smaller scales, that connects the strain gradients and the electric field. The very existence of strain gradients enhances noncentrosymmetry and heightens the interaction between piezoelectricity and flexoelectricity, demanding fully coupled higher-order electromechanical formulations. The numerical instability of the existing finite elements used to model flexoelectricity alone is revealed due to their reliance on the stabilization parameter. Thus, two new finite elements () and () are proposed for mixed FEM that are numerically robust without any need of such stabilization parameters. Additionally, the existing finite element [ in (Deng et al. in J Appl Mech 84:081004, 2017)], is implemented from scratch to replicate known results and benchmark the performance of newly proposed finite elements. To verify the robustness of these elements, various benchmark problems for flexoelectricity in dielectric solids, such as a thick cylinder and truncated pyramid are simulated. The great agreement of the numerical results with the existing ones reflects the foundational nature of the proposed elements. Furthermore, the proposed mixed finite elements were used to successfully analyze cantilever beam and truncated pyramid problems where piezoelectric effects were taken into account for the first time. Current results are intrumental in simulating flexoelectricity and piezoelectricity together to highlight their interactions using newly proposed numerically robust finite elements.

Life Expectancy of Transformer Paper Insulation Retrofilled with Natural Ester in the Laboratory.

The use of alternative insulating liquids instead of mineral oil in transformers is spreading around the world due to their superior fire resistance. Furthermore, researchers have demonstrated that these oils increase the lifespan of the solid insulation of the transformers and, thus, the life expectancy of the equipment. Retrofilling of transformers with natural and synthetic esters allows companies to benefit from the properties of using these liquids without making an investment into new machinery. This paper investigated the ageing process of Kraft paper that was retrofilled with a natural ester in the laboratory. The Kraft paper samples were subjected to accelerated thermal ageing in an oven at 130 °C, and markers such as the degree of polymerisation and tensile strength were measured. The ageing tests comprised a first period, where the samples were immersed in mineral oil, followed by a replacement of the oil with a natural ester. As moisture is a determinant factor for paper ageing, two sets of samples with different moisture contents were tested. The results showed that the retrofilling of the transformers may slow down the degradation rate of the solid insulation despite the presence of remaining mineral oil adsorbed in the paper.