Solid Insulation Research Articles

Enhanced flexoelectric properties in sodium bismuth titanate-based lead-free relaxor ferroelectrics by inducing polar nanoregions

The presence of polar nanoregions (PNRs) is widely recognized as playing a crucial role in the functionalities of numerous solid dielectrics. However, the contribution of PNRs to the specific coupling between strain gradient and electric polarization (flexoelectricity) has not yet been thoroughly understood. Herein, we investigated the impact of varying PNR concentrations on the dielectric and flexoelectric properties of lead-free relaxor ferroelectric (Bi0.5Na0.5)TiO3 ceramics, achieved through stoichiometric BaTiO3 doping. The temperature-dependent dielectric properties indicated the presence of PNRs in doubly doped ceramics as compared to pure BNT. Complex impedance analysis and piezoresponse force microscopy measurements demonstrated significant differences in PNR concentrations among the different components. The reorientation of PNRs notably enhanced the flexoelectric response of the doubly doped BNT-xBT ceramics, with this enhancement being positively correlated with the concentration of PNR. Specifically, the flexoelectric coefficient was found to be approximately 10 times higher in the morphotropic phase boundary (MPB) region (x = 0.08) compared to pure BNT. These findings illustrate that adjusting the concentration of PNRs in lead-free relaxation ferroelectrics is an effective strategy for improving flexoelectric properties.

Low Thermal Conductivity and Diffusivity at High Temperatures Using Stable High-Entropy Spinel Oxide Nanoparticles.

The realization of low thermal conductivity at high temperatures (0.11W m-1 K-1 800 °C) in ambient air in a porous solid thermal insulation material, using stable packed nanoparticles of high-entropy spinel oxide with 8 cations (HESO-8 NPs) with a relatively high packing density of ≈50%, is reported. The high-density HESO-8 NP pellets possess around 1000-fold lower thermal diffusivity than that of air, resulting in much slower heat propagation when subjected to a transient heat flux. The low thermal conductivity and diffusivity are realized by suppressing all three modes of heat transfer, namely solid conduction, gas conduction, and thermal radiation, via stable nanoconstriction and infrared-absorbing nature of the HESO-8 NPs, which are enabled by remarkable microstructural stability against coarsening at high temperatures due to the high entropy. This work can elucidate the design of the next-generation high-temperature thermal insulation materials using high-entropy ceramic nanostructures.

A novel fuzzy approach for winding paper insulation health monitoring of oil‐immersed power transformer using non‐destructive failure parameters

AbstractPower transformers, a significant and critical component of power system, require regular condition monitoring by the utilities for uninterrupted power supply. The kraft paper made of cellulose is used as the solid insulation of the transformer which undergoes thermal, electrical and environmental stresses causing its progressive ageing. It thereby reduces the physical strength of the paper and ultimately affects the transformer's remaining useful life (RUL). The current state (physical) of the cellulosic (solid) insulating paper is predicted in this article using the fuzzy inference system (FIS) based RUL assessment technique. Three non‐destructive parameters, that is, 2‐Furfuraldehyde (2‐FAL), Carbon Monoxide (CO) and Carbon dioxide (CO2) are used to determine the value of the Degree of Polymerisation (i.e. DP) which directly reflects the current physical state of the insulating material. Four states (Healthy, Poor, Very Poor, and Worst) of the insulating paper are identified based on the values of DP. Five FIS models have been developed separately with different membership functions and a number of fuzzy if‐then rules are formulated to estimate the value of DP. Every model is tested with 30 test samples to validate the system accuracy. The physical condition of the insulation estimated using FIS built with trapezoidal membership function has been found to be considerably precise with an average absolute deviation of 0.33%. Hence, a comprehensive assessment of transformer's health is carried out with the prominent non‐destructive parameters.

An Electrical Method to Detect Both Crack Creation and Propagation in Solid Electrical Insulators.

Fracto-emission is the ejection of electrons and positive ions from matter undergoing a mechanical fracture. The creation and propagation of fractures in insulating material can generate an electrical signal that can be detected using a sufficiently fast signal recorder. The theoretical equations related to crack creation/propagation that induce an externally electric signal are detailed for two conditions: with and without an external applied electric voltage. Results from an experiment with no externally applied voltage are presented for fibreglass-reinforced epoxy laminate samples, in which current signals ranging from 50 mA to 100 mA are measured in a time frame of 200 ns. The signal-to-noise ratio is high enough to consider that the signal that was recorded is not a measurement artifact. This method may help to identify and track a crack propagating inside dielectric materials.

Assessment of Dielectric Strength for 3D Printed Solid Materials in Terms of Insulation Coordination

Insulating materials can be classified into solid, liquid, and gaseous forms. Solid insulation materials are divided into different types such as organic, inorganic, and polymer types. In electrical circuits, solid insulation materials are generally used as components that provide insulation and mechanical support. In recent years, as a result of developing technologies, the production of participation insulation materials with 3D printing technology has become widespread. Three-dimensional printing technology enables the rapid creation of objects by combining materials based on digital model data. It is important to evaluate the materials produced with 3D printing in terms of insulation coordination. Studies have shown that the electrical breakdown strength of solid dielectrics varies depending on factors such as sample type, thickness, the magnitude of applied voltage, and the temperature of the physical environment. According to IEC-60243 standards, there are various methods to measure the breakdown strength of solid insulators applied to different voltage types. In this study, the behavior of PLA, ABS, ASA, PETG, and PC/ABS materials produced with 3D printing and having the potential to be used as insulation materials when exposed to high voltage within the scope of insulation coordination was investigated. The breakdown strengths of solid insulation materials produced with 3D printing were measured in the high-voltage laboratory within the scope of IEC-60243. Breakdown strength was statistically evaluated with the Weibull distribution. Damage analysis of the breakdowns in the test specimens was examined in detail with ImageJ software. With the comparative analysis, the behaviors of PLA, ABS, ASA, PETG, and PC/ABS solid insulation materials were revealed and their superiority over each other was determined.

Iridescent structural color by using ultra-low refractive index aerogel as optical cavity dielectric

Iridescent color-shift pigments have been used in some industrial applications, e.g., for cosmetics and packaging. To achieve environmental-friendly and lasting color, thin-film interference is used to generate structural color. By maximizing the refractive index (RI) difference between the thin films (i.e., using an ultralow RI film), super-iridescent structural color can be produced. While the lowest refractive index of a naturally occurring solid dielectric is close to 1.37 (i.e., MgF2), we synthesized highly porous dielectric SiO2 aerogel to achieve ultralow-RI (n ~ 1.06) and demonstrated a high-refractive index/low-refractive index/absorber (HLA) trilayer structural color. The achieved structural color is highly iridescent and capable of tracing a near-closed loop in CIE color space. By tuning the refractive index, thickness, and geometry of the aerogel layer, we control the reflection dip’s shape, therefore producing a wide range of vivid and iridescent colors.

Contact-electro-luminescence triggered by triboelectric charge

Luminol, a prominent chemiluminescent reagent, is extensively employed in biochemical and environmental analyses for its ability to produce rapid luminescence through oxidation in aqueous solutions. However, the lack of comprehensive studies on the observation and regulation of intermediate species during this luminescent reaction may limit its effectiveness in detection applications. Understanding these intermediates is crucial for enhancing the accuracy and reliability of luminol-based detection methods. To solve the above problems, this work proposes a straightforward and effective approach to directly oxidize luminol via reactive oxygen species (ROS) generated from contact electrification (CE) between inert solid dielectrics and deionized (DI) water, termed contact-electro-luminescence (CEL). Metal-free, cost-effective solid dielectrics present a promising alternative to expensive metal-based catalysts, aligning well with the principles of green chemistry and sustainable development. In such reaction system, ROS can be regulated by various solid dielectrics and additives (such as xylitol and p-benzoquinone), thereby tuning the luminol reaction. More importantly, two distinct luminescent emission peaks are directly observed and the intensity of the emission peaks of the two intermediate states can be regulated. The observation of intermediate is beneficial to reveal the intrinsic of luminescence reactions, exploiting the effective and tunable luminescence system. Moreover, CEL provides a new mechanical-photonic conversion paradigm to study luminol luminescence via triboelectric charge from the CE effect. CEL can offer distinct characteristic signals, offering potential applications in high-efficiency and specific detection for environmental monitoring, biomedical diagnosis and food safety testing etc.

Solid dielectric electrochemical polishing of 3D-printed parts: Performance and mechanisms

Surface post-processing of metal additive manufacturing components is challenging due to their typically complex geometries (e.g., curved surfaces) coupled with high initial surface roughness. Herein, we propose an efficient solid dielectric electrochemical polishing (SDECP) method employing ion exchange resin particles with a porous structure that absorbs and stores electrolytes as a conductive medium. This method enhances the surface quality of additively manufactured components with Bézier curved surfaces to a mirror finish, achieving improvements in Sa, Sq, and Sz of 91.5%, 91.7%, and 86.9%, respectively. Planetary motion strategies are implemented to optimize mass transfer on the anode surface in the discontinuous solid dielectric. Results indicate that bidirectional planetary motion (BPR) in SDECP effectively improves the uniformity of surface roughness and material removal across different regions of the part. Furthermore, we quantitatively describe the relationship between material removal rate (MRR) and average current in SDECP. The intermittent material removal mechanism of SDECP is elucidated utilizing discrete element method (DEM) simulations. Our work offers innovative insights into the material removal mechanisms of SDECP, presenting an efficient approach for overall surface post-processing of metal additive manufacturing components.

Surface flashover in 50 years: II. Material modification, structure optimisation, and characteristics enhancement

AbstractSurface flashover is a gas–solid interface insulation failure that significantly jeopardises the secure operation of advanced electronic, electrical, and spacecraft applications. Despite the widespread application of numerous material modification and structure optimisation technologies aimed at enhancing surface flashover performance, the influence mechanisms of the present technologies have yet to be systematically discussed and summarised. This review aims to introduce various material modification technologies while demonstrating their influence mechanisms on flashover performances by establishing relationships among ‘microscopic structure‐mesoscopic charge transport‐macroscopic insulation failure’. Moreover, it elucidates the effects of chemical structure on surface trap parameters and surface charge transport concerning flashover performance. The review categorises and presents structure optimisation technologies that govern electric field distribution. All identified technologies highlight that achieving a uniform tangential electric field and reducing the normal electric field can effectively enhance flashover performance. Finally, this review proposes recommendations encompassing mathematical, chemical, evaluation, and manufacturing technologies. This systematic summary of current technologies, their influence mechanisms, and associated advantages and disadvantages in improving surface insulation performance is anticipated to be a pivotal component in flashover and future dielectric theory.

Diagnostics of porcelain insulators by partial discharges characteristics

RELEVANCE of the research is in the development of a non-destructive method for diagnosing porcelain insulators of high-voltage electrical equipment based on analysis of the characteristics of partial discharges (PDs). The problem of the final stage of breakdown of both the discharge gap and porcelain insulators recognizing is currently has not solved. THE PURPOSE. Recognition of PDs in solid insulation, study of PDs characteristics in a pre- breakdown situation, recognition of defective insulators based on PDs characteristics analysing. METHODS. The study of PDs characteristics for a defective and functional porcelain insulator were carried out. To study the characteristics of various types of PDs, including in the pre-breakdown situation, a system of surface-needle electrode system was used. RESULTS. The article describes compares the PDs characteristics obtained using standard R-400 device and a digital storage oscilloscope (DSO) using a telescopic antenna. A technique for recognizing the development of the pre-breakdown situation of the surface-needle discharge gap and the porcelain insulator was developed. The technique for recognizing a defective porcelain insulator from the amplitude-phase diagrams (APD) of PDs was developed. The method for assessing the breakdown voltage of porcelain insulators based on the characteristics of partial discharges was developed. CONCLUSION. At the moment of transition to the pre-breakdown stage were observed a sharp increase in the counter-movement of charges of opposite polarity. In the stage immediately before the breakdown, near zero values of the applied voltage of negative polarity, ordered PDs of the same polarity as the applied voltage were recorded in large numbers. These discharges led to an increase in leakage current and were interpreted as initial corona discharges. These discharges were recorded both by a telescopic antenna in a surface-needle system and in porcelain insulators.

Band gaps of elastic waves in 1-D dielectric phononic crystal with the flexoelectric and strain gradient effects consideration

The propagation of Bloch waves in one dimensional phononic crystal consisting of dielectric elastic solids is studied with consideration of the strain gradient, inertial gradient and flexoelectric effects. The transfer matrixes for single layer and one-cell of phononic structure are derived based on the constitutive equations and the governing equations of dielectric elastic solids. The dispersion equation for Bloch waves is obtained by the application of periodic conditions for the generalized displacements and tractions considered within strain gradient theory of electro-elasticity. Based on the numerical solution of the derived dispersion equation, the influences of the micro-stiffness length scale, micro-inertial length scale and flexoelectric coefficients on the dispersion and the bandgap are discussed.

Examining the relationship between dwelling energy efficiency and fuel poverty for retrofit strategies: A case study of the United Kingdom

In the midst of a global energy crisis, fuel poverty has become increasingly prevalent. This study examines the effectiveness of energy efficiency retrofits (EERs) in alleviating fuel poverty in the United Kingdom. Using the England fuel poverty dataset with XGBoost and interpretive analyses like Individual Conditional Expectation (ICE) and SHapley Additive exPlanations (SHAP), the research investigates the impact of EERs on fuel poverty. Findings reveal that while energy efficiency enhancements hold potential for improving conditions for vulnerable households, their effectiveness diminishes beyond a certain threshold. The study identifies solid wall and cavity wall insulation, condensing boilers, well-insulated lofts, and central heating systems as the most effective retrofit measures. However, the optimal selection of retrofit methods varies depending on household and dwelling conditions due to interactive effects. Additionally, comprehensive cost-efficiency analyses show that installing condensing combination boilers in homes without existing boilers and adding cavity wall insulation to houses with uninsulated cavity walls are more cost-efficient options. These insights shed light on the complex relationship between energy efficiency retrofits and fuel poverty, informing the development of targeted strategies to address this pressing issue in the UK and globally.

Impact of sol-gel synthesized α-aluminium oxide nanoparticles for the enhancement of transformer oil insulation properties

Transformers require advanced insulating materials that outperform mineral oil in terms of cooling, insulation effectiveness, eco-friendliness, and operational efficiency in extreme operational and weather conditions. As a result, emphasis has shifted to renewable and ecologically friendly materials. This change in emphasis is motivated by worries about the environment, fuel shortages, and the disposal of waste oil. Transformers have historically employed solid insulators like paper and pressboard as well as mineral or synthetic oil. However, the development of ecologically friendly insulating materials has been spurred by new environmental regulations as well as economic benefits. This research investigates the use of sol-gel synthesized aluminium oxide (Al2O3) nanoparticles to improve the insulation properties of host transformer oil. The synthesized aluminium nanoparticles were characterized with X-ray diffractometer (XRD) and found rhombohedral structure, whose chemical composition was studied by energy dispersive X-ray spectrum (EDAX). Investigation using Fourier transform infrared spectroscopy (FTIR) also revealed the production of AlOOH and hydroxyl vibrational bands at wavenumbers of 690 and 2976 cm−1, respectively. Studies using field emission scanning electron microscopy (FESEM) showed that the nanoparticles are spherical in form and have a diameter of around 38 nm. Additionally, the incorporation of α-aluminium oxide nanoparticles into the host oil showed an improvement in the breakdown voltage and suggested a possible use for better transformer oil insulation.

A fabrication of universal multifunctional coating with excellent adhesion on the surface of solid rocket motor insulation materials through a surface activation strategy based on multiple interaction forces

In this work, a novel universal multifunctional coating for insulation materials was prepared by using polyethyleneimine as the coating substrate, low-temperature molten glass powders and MXene as additives. Meanwhile, the universal coating was tightly adhered to the surface of the insulation materials through a surface activation strategy based on the synergistic effects of hydrogen bonding, electrostatic adsorption, and chemical bonding. The shear strength data showed that the adhesion between the universal coating and the three types of insulation material were higher than that of most commercial adhesives. In the case of EPDM, for example, the adhesion of the coating is 4.17 (epoxy glue), 1.04 (chemlok) and 1.67 (Neoprene) times that of commercial adhesives, respectively. In addition, the universal coating showed good heat resistance and thermal insulation, and formed a complete and dense char on the surface after exposure to flame. The maximum mass loss temperatures of the three coated insulation materials were 2.8, 6.5, and 9.3 °C higher than the original samples, respectively. Thermal insulation tests showed that the coated insulation materials reduced the average top surface temperature by 29.68 %, 40.84 % and 29.06 %, respectively, compared to the original samples. The results of anti-migration tests showed that the prepared universal coatings displayed generally superior anti-migration properties to the insulation materials, with equilibrium migration concentrations reduced by up to more than 80 % compared to the original samples, which is better than the previous products of the same type. In addition, the application potential of the advanced multifunctional insulation materials obtained from the preparation was evaluated, resulting in an increase of more than 35 % in peel strength and more than 20 % in shear strength for all insulation materials with propellants. This work provides a simple and environmentally friendly generalized strategy to develop high performance insulation materials for aerospace fields.

Image De-noising Based on WMF Technique for Electrical Trees Structure in High Voltage Cable Insulation

Electrical treeing is a common problem during the pre-breakdown phenomenon in solid insulations due to the damage caused by Partial Discharge (PD) that progresses through stressed insulation via chemical degradation, which resembles the shape of a tree root. This resulted in a decrease in performance through degrading the insulation, which became a serious problem while dealing with electrical equipment. Hence, a deep understanding of electrical tree structure is vital to improving the quality of solid insulations. Ergo, optical microscopy is primarily used to examine tree structures, shapes, and fractal dimensions to reconstruct electrical tree structures for morphological study. However, optical microscopy images are frequently degraded by noise from readout procedures or image data acquisition systems, noise caused by occlusion, illumination, non-uniform intensity, destroying potential tree pixels, and a critical loss of information about the electrical tree structures. Therefore, this research proposed the Wiener Median Fusion (WMF) filter for electrical tree study. The performance of the WMF de-noising technique improves the image quality for the precise portrayal of the electrical tree structure based on thresholding segmentation algorithm analysis in terms of accuracy, sensitivity, and false positive rate. Based on the analysis of the thresholding segmentation algorithm, Otsu's thresholding exhibits the highest result compared to Niblack. The Otsu's overall percentage in terms of accuracy is 80.2934%, the sensitivity is 99.1513%, and the false positive rate is 82.6265%.

Dielectric metamaterials with effective self-duality and full-polarization omnidirectional brewster effect

Conventional dielectric solid materials, both natural and artificial, lack electromagnetic self-duality and thus require additional coatings to achieve impedance matching with free space. Here, we present a class of dielectric metamaterials that are effectively self-dual and vacuum-like, thereby exhibiting full-polarization omnidirectional impedance matching as an unusual Brewster effect extended across all incident angles and polarizations. With both birefringence and reflection eliminated regardless of wavefront and polarization, such anisotropic metamaterials could establish the electromagnetic equivalence with “stretched free space” in transformation optics, as substantiated through full-wave simulations and microwave experiments. Our findings open a practical pathway for realizing unprecedented polarization-independence and omnidirectional impedance-matching characteristics in pure dielectric solids.

The Catalytic Effect of Low Molecular Weight Acids on the Physicochemical and Dielectric Properties of Oil-Paper Insulation Systems.

In most industrialized countries, power transformers built several decades ago are approaching the end of their operational lifespan. The ongoing energy transition, focused on developing 100% renewable energy sources and accelerating global transportation electrification, further exacerbates these assets. Combined with rising electricity demand, there is an increasing risk of critical transformers' degradation acceleration. In this context, understanding the aging mechanisms of the insulation system inside these essential assets, which form the core of every energy network, becomes paramount for today's managers and engineers responsible for their operations. The acids generated through oil oxidation can be classified into two categories: low molecular weight acids (LMAs), which are inherently more hydrophilic and consequently have a greater impact on the degradation rate of solid insulation through hydrolysis, and high molecular weight acids (HMAs), which do not significantly contribute to the degradation of paper insulation. This study specifically addresses the impact of acids generated through oil oxidation-focusing on LMAs. New oil samples were infused with different ratios of LMAs before impregnation. The impregnated paper samples underwent thermal aging at 115 °C. Different physicochemical and dielectric properties were investigated. The investigations revealed that oils blended with formic acid exhibited more adverse effects on the insulation system compared to other LMAs. This information is essential for industry professionals seeking to mitigate the risks associated with transformer degradation and extend the lifespan of these critical assets during the energy transition.

Locating Insulation Defects in HV Substations Using HFCT Sensors and AI Diagnostic Tools.

In general, a high voltage (HV) substation can be made up of multiple insulation subsystems: an air insulation subsystem (AIS), gas insulation subsystem (GIS), liquid insulation subsystem (power transformers), and solid insulation subsystem (power cables), all of them with their grounding structures interconnected and linked to the substation earth. Partial discharge (PD) pulses, which are generated in a HV apparatus belonging to a subsystem, travel through the grounding structures of the others. PD analyzers using high-frequency current transformer (HFCT) sensors, which are installed at the connections between the grounding structures, are sensitive to these traveling pulses. In a substation made up of an AIS, several non-critical PD sources can be detected, such as possible corona, air surface, or floating discharges. To perform the correct diagnosis, non-critical PD sources must be separated from critical PD sources related to insulation defects, such as a cavity in a solid dielectric material, mobile particles in SF6, or surface discharges in oil. Powerful diagnostic tools using PD clustering and phase-resolved PD (PRPD) pattern recognition have been developed to check the insulation condition of HV substations. However, a common issue is how to determine the subsystem in which a critical PD source is located when there are several PD sources, and a critical one is near the boundary between two HV subsystems, e.g., a cavity defect located between a cable end and a GIS. The traveling direction of the detected PD is valuable information to determine the subsystem in which the insulation defect is located. However, incorrect diagnostics are usually due to the constraints of PD measuring systems and inadequate PD diagnostic procedures. This paper presents a diagnostic procedure using an appropriate PD analyzer with multiple HFCT sensors to carry out efficient insulation condition diagnoses. This PD procedure has been developed on the basis of laboratory tests, transient signal modeling, and validation tests. The validation tests were carried out in a special test bench developed for the characterization of PD analyzers. To demonstrate the effectiveness of the procedure, a real case is also presented, where satisfactory results are shown.

Appropriate analysis on properties of various compositions on fluids with and without additives for liquid insulation in power system transformer applications

Transformer is a well-known power system apparatus utilized in conjunction with solid insulations such as paper and press board, as well as liquid insulations like mineral oil, a petroleum-based fluid. Despite the notable drawbacks associated with mineral oil, such as limited resources for future generations and its non-eco-friendly nature, its usage remains ubiquitous. There is a growing imperative to explore alternative fluids that surpass mineral oil in terms of environmental impact and performance. Amidst the global shift towards green energy, this study focuses on vegetable seed oils such as corn oil, soybean oil, mustard oil, and rice bran oil as potential substitutes. The research evaluates these oils based on key transformer properties including breakdown voltage, water content, interfacial tension, viscosity, acidity, flash point, and fire point. Interestingly, rice bran oil and soybean oil exhibit promising characteristics that suggest they could effectively replace petroleum-based fluids in transformers. Furthermore, the study extends to blending mineral oil with vegetable seed oils in various compositions, incorporating natural and synthetic antioxidant additives ranging from 0 to 1%. Comparative analyses between samples with and without additives reveal that the inclusion of 1% propyl gallate yields outstanding performance improvements. For instance, a blend comprising 25 ml of mineral oil and 25 ml of soybean oil, supplemented with 1% propyl gallate, demonstrates 90% higher effectiveness compared to other blends and additives tested. Moreover, the research employs statistical regression analysis to establish relationships between different parameter variables, providing deeper insights into the performance and compatibility of these blended oils in transformer applications. This comprehensive investigation underscores the potential of vegetable seed oils as viable alternatives to mineral oil, contributing to the advancement of eco-friendly solutions in power systems.

Distribution of the Magnetic Field Generated by a 20-kV Power Conductor with Solid Polymer Insulation

Distribution of the Magnetic Field Generated by a 20-kV Power Conductor with Solid Polymer Insulation