Dielectric Specimens Research Articles

Super Pixel-Level Contamination State Detection Model of Composite Insulator Specimens Under Different Lighting Conditions Based on Hyperspectral Technology and Model Transfer

Super Pixel-Level Contamination State Detection Model of Composite Insulator Specimens Under Different Lighting Conditions Based on Hyperspectral Technology and Model Transfer

Experimental and numerical investigations of multi-layered ship engine room bulkhead insulation thermal performance under fire conditions

Abstract The thermal insulation layer of the ship’s engine room bulkhead is typically constructed from multiple layers of mineral wool. This layer is designed to keep the temperature on the surface of the bulkhead below 140°C in case of a fire. However, measuring the inner and external temperatures of each wall panel bulkhead type during a fire can be difficult. To address this issue, this study was conducted to evaluate the multilayer heat transfer capability of the bulkhead insulation in the event of an engine room fire. The study used heat transfer theory, experimental models, and numerical analysis to assess nine bulkhead insulation specimens. These specimens were fabricated from three original specimens and included a 10–30 mm air layer (W-75, W-50, and W-25). The results showed that all improved specimens had better thermal insulation than the originals. Among them, the improved specimens derived from the W-25 specimen exhibited the most significant temperature reduction compared to those derived from the W-75 and W-50 specimens. The results demonstrated a slight difference between the three methodologies, indicating a high level of reliability in the research findings.

Bonding strength enhancement of a 30-wt.% GF/PET–2024 aluminium alloy joint by synergistic action of multi-scale mechanical interlocking and multiplex H-bonding

In this study, micro-scale cellular and nano-scale labyrinth structures were obtained on the surface of A2024 aluminium alloy adherends through ultrasonic-aided corrosion using an FeCl3 etchant solution to significantly enhance the mechanical interlock of it and a 30-wt.% glass-fibre-enhanced polyethylene terephthalate substrate. The tensile shear strength of the 30-wt.% GF/PET–A2024 joint was increased to 31.84 MPa. The subsequent treatment of an appropriate amount of KH-892/PVA transformed the surface of the aluminium alloy from hydrophobic to hydrophilic to significantly improve its wettability. Furthermore, this treatment generated multiplex hydrogen bonding of O–H⋯O⋯H–N at the interface, further improving the tensile shear strength of the joint up to 50.94 MPa, which is significantly higher than the values obtained using exiting methods. The tensile shear tests of a ring-shaped insulator specimen according to GB 7251-87 verified the improved fracture work equation herein and the bonding strength enhancement of the 30-wt.% GF/PET–A2024 aluminium alloy joint treated by ultrasonic-aided corrosion with an FeCl3 etchant solution and pre-coating of an appropriate amount of KH-892/PVA.

STUDYING SOME OF PHYSICAL PROPERTIES FOR POLYMER REINFORCED WITH CHOPPED CARBON FIBERS (CCF) COMPOSITES

The research includes the study of some physical. Properties (thermal conductive and dielectric strength) for two types of polymer composites; epoxy resin (EP) reinforced by chopped carbon fibers (CCF) and unsaturated polyester resin (UPE) reinforced by chopped carbon fibers at different weight percentage (1%, 3%, 5%, 7%). Hand –lay up. Method was used to prepare sheets of composites (EP, UPE, EP/CCF, and UPE/CCF) on. glass mold which were cut according to the standard dimensions of the thermal conductive and dielectric strength tests specimens. The results showed that the. Sample of UPE/CCF have highest values of thermal conductive at weight. percentage (7%) compared with prepared samples at other weight percentage. Also the results showed that the thermal conductive of. Pure UPE and EP increased with increase in weight percentage of carbon fibers addition. On the other hand, the results showed that the polyester composites have higher values of thermal conductive compared with epoxy composites. Also the results showed that higher value of dielectric strength at weight. Percentage (1%) for both types from prepared composites of carbon fibers.In addition, the results showed that the polyester composites have higher values of dielectric strength compared with epoxy composites.

Study on water absorption and dielectric response of epoxy resin impregnated paper bushing

AbstractEpoxy‐impregnated paper transformer bushings are prone to water absorption due to the presence of water‐attracting moieties in the cellulose chemical structure and epoxy resin, leading to moisture‐induced insulation defects. To understand the mechanism of moisture's impact on epoxy‐impregnated paper, this study developed a testing platform that integrated moisture diffusion, adsorption, and dielectric properties of insulating materials. Insulation specimens made of epoxy resin and epoxy‐impregnated paper were prepared and their water diffusion characteristics were investigated using the Fick diffusion model and the Langmuir model, respectively. The Guggenheim–Anderson–de Boer equation was used to examine the vacuum vapour adsorption characteristics of the materials, providing a theoretical foundation for examining the form of water present within them. The dielectric response of materials with varying moisture levels was then tested to study the effect of water on the dielectric spectrum curve. Using the Extended Derivative Method, the characteristic frequency reflecting the water content of the epoxy‐impregnated paper was extracted. The results demonstrate that 0.01‐Hz characteristic frequency provides superior accuracy for moisture assessment in epoxy resins compared to 50 Hz, and extracting and fitting the relaxation peak characteristic frequency in epoxy‐impregnated paper moisture evaluation yields higher accuracy than using tan δ at the power frequency.

Square Waveguide Cavity for Complex Permittivity and Permeability Measurement by Perturbation Method Without Repositioning

This article presents a novel configuration for the simultaneous complex permittivity and permeability determination with only a single sample placement inside the resonant cavity. The proposed structure incorporates two waveguide orthomode transducers (OMTs) and a square cavity. The square cavity is designed to overcome the limitation of permeability measurement by the rectangular cavity perturbation method. These components allow the product of two orthogonal transverse electric (TE) resonant modes (TE10L and TE01L) with the same cutoff frequencies. Based on Maxwell equations and perturbation theory, new relations are developed to characterize the complex permeability of specimens at TE01L modes. The proposed structure eliminates the main drawbacks of previous studies such as depolarization effect of partial-height samples, radiation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${H}$ </tex-math></inline-formula> -plane sample hole, need to apply the sample holder and precision positioner, multistate positioning, iterative calculations, extra calibration process, and employing complicated mathematical equations. The proposed configuration was designed using CST Microwave Studio (MWS) and ANSYS high-frequency structural simulator (HFSS) and then manufactured and measured. The operating frequency has been selected from 7.5 to 12.5 GHz. Good agreement is achieved between measurement and simulation. The performance of the developed structure was verified by measuring the material characteristics of bar-shaped dielectric and magneto-dielectric specimens and compared with other studies.

Дослідження характеристик резонансної системи на основі прямокутного хвилеводу з двома діафрагмами

Microwave methods for the study and control of the dielectric properties of substances using regular and irregular rectangular metal waveguides have many applications and are of great practical interest. The dielectric properties of materials are assessed based on the determination of the reflection and transmission coefficients in a resonant system. This article considers resonant phenomena of electromagnetic oscillations in a prismatic cavity based on a length of a rectangular waveguide with two diaphragms and with a dielectric specimen in the form of a cylinder centrally positioned between them. The goal of this work is to evaluate the resonant properties of a prismatic cavity based on a length of a rectangular waveguide with two diaphragms as a function of the geometrical dimensions of the rectangular diaphragm windows, the distance between the diaphragms, and the dimensions and dielectric properties of the specimen located at the center of the waveguide length. By computer simulation, the resonant properties of the system were determined as a function of the dielectric constant and diameter of the specimen and the geometrical dimensions of the diaphragm windows. It was found that for each value of the specimen diameter there exists a maximum allowable value of the dielectric constant at which the resonant properties of the system are kept in a selected operating frequency range. The effect of the height of the diaphragm windows on the resonant frequency of a prismatic cavity and the reflection and transmission coefficients was studied, and the behavior of the intrinsic Q factor was assessed qualitatively. A minimum possible diaphragm window height was found such that both the required coupling between the rectangular cavity and the external microwave line is provided and the maximum value of the loaded Q factor in the absence of a dielectric specimen is kept. The results of this work may be used in the design of high-frequency sensors for controlling the dielectric properties of materials in various industries.

Aging State of Stator Bar Insulation Based on the Entropy Weight and Dielectric Response Methods

This article describes a novel research method for evaluating the aging state of the primary insulation material of large generator stator bars based on combining dielectric response methods with the entropy weight method. Vacuum pressure impregnation (VPI) epoxy-mica insulation specimens subjected to different electrical aging time periods were characterized via frequency domain spectroscopy (FDS), polarization and depolarization current (PDC), and thermal stimulated depolarization current (TSDC) tests. The comprehensive characteristic (VPI) parameters obtained by calculating the characteristic dielectric response parameters using the entropy weight method were used to describe the aging state of the VPI epoxy-mica insulation. The results show that during electrical aging, microcosmic structural variations within the insulation directly manifested as changes in the characteristic dielectric response parameters and indirectly affected the comprehensive evaluation parameters of the entropy weight method.

Experimental and Statistical Approach to Detect the Corrosion Rate and Influencing Profiles for Enhancing Corrosion Rate of High-Voltage Insulator Materials.

The influence of temperature, pollutant, and pH on the local corrosion rate of insulators installed in industrial, marine, and rural installation sites is investigated based on experimental and statistical investigations. The tensile load test confirms that corroded insulator specimens collected from industrial sites aged more than 10years represent a minimum fracture load, 19,892 lbs. It was further observed that more than 91.24% and 64.62% corroded insulator specimens suffered from shell break and pin detachment, respectively. The microstructural and XRF analysis reveal that insulator specimens collected from industrial sites (age > 10years), represented the highest wt% of O (19.2) and lowest wt% of Zn (0.34) among industrial, marine, and rural installation sites. The 3D stationery mechanical simulation reveals that insulator specimens aged > 10years experienced maximum stress (600MPa) in the pin-cement interface. Using full two-level factorial designs, temperature, concentration of pollutants, and pH were found significant factors for corrosion rate. The immersion test results further confirm the above-mentioned factors significant for the dissolution behavior of galvanized coating of insulator pin. Following immersion test results, the industrial region shows the highest corrosion rate (5.58-12µm/year) among all installation sites.

Characteristics of electrical tree morphological in glass fibre reinforced epoxy resin under power frequency voltage

To study the influence of the arrangement of glass fibres on the growth morphology of electric dendrites, this paper conducted experiments on the initiation and cultivation of electric dendrites on insulation specimens with different glass fibre arrangements. The experimental results show that when the electric field is parallel to the glass fibres, the electric dendrites show linear growth with multiple branches; when the electric field is perpendicular to the glass fibres, the electric dendrites show aggregated growth. The analysis suggests that when the applied electric field is parallel to the glass fibre, to release the discharge energy in the main channel of the dendrite, the discharge in the track of the electric dendrite will drive the electric dendrite to grow along the interface area between the glass fibre and the epoxy resin. When the externally applied electric field is perpendicular to the glass fibre, the discharge within the electric dendrite discharge channel is high. To release the energy in the electrical tree channel, the blast in the electrical tree track induces the electrical tree branch to grow along the interface area and forward.

Estimation of the moisture absorption rate of glass fiber reinforced plastic using electromagnetic induction testing

We present a novel estimation method for the moisture absorption rate (MAR) of glass fiber reinforced plastics (GFRPs) using electromagnetic induction testing (EIT). The relationship between the permittivity and MAR were analytically calculated using a simple moisture absorption model. Moreover, we experimentally obtained the magnitude of the displacement current for dielectric specimens with known permittivity values. The equations were derived from the magnitude of the displacement current measured during EIT by combining the analytical and experimental results. To verify the validity of the proposed method using the derived equations, we applied EIT to GFRPs with different MARs. It was found that MAR can be qualitatively evaluated using the proposed method.

Insulation Performance and Simulation Analysis of SiO2-Aramid Paper under High-Voltage Bushing.

The long-term safe and stable operation of oil-impregnated paper (OIP) bushings is of great significance to the operation of power systems. With the growth of OIP bushing, its internal insulation will gradually decay. Aramid insulation paper has excellent thermal aging characteristics and its insulation performance can be improved by using nano-modification technology. In this paper, the nano-SiO2 particles were used as the modified additives, and the modified aramid insulation paper was prepared through four steps: ultrasonic stirring, fiber dissociation, paper sample copying and superheated calendering. The microscopic physical morphology and chemical components of the insulation specimens before and after modification were analyzed by atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), and an OIP bushing model based on the modified aramid insulation paper was constructed and its electric field distribution was analyzed. The simulation results show that the use of SiO2-modified aramid insulation paper can improve the electric field distribution of OIP bushings and increase the operating life of power transformers.

Verification and validation of an advanced guardedhot plate for determination of thermal conductivity

The guarded hot plate (GHP) method is a widely used technique to measure thermal conductivity of thermal insulation specimens in steady-state conditions. In this paper, an advance of GHP has been developed in the Laboratory for Thermal Technique and Fire Protection in Institute IMS. The innovative GHP has been applied for measuring thermal conductivity using additional heat flux meters. The design of this GHP is similar to the design of the plate for GHP apparatus, however, it has different design: smaller width of thermal barrier, which is filled with insulation glue. Heaters inside hot and guard plates are built from wire for thermocouples, which is a unique type of heater. Geometry of heater has been optimized inside plates to achieve uniform temperature distribution along the specimen surface. Temperature uniformity of GHP and energy balance were experimentally determined. The verification and validation results of improved GHP have been shown. The test method was validated comparing test results of thermal conductivity with results of the round-robin test. Four national companies participated in the round-robin comparison on thermal conductivity measurement by GHP method. The measurement was performed on the same specimen of thermal insulation material (expanded polystyrene) according to SRPS EN 12667 at temperatures ranging between 10-40?C. The measured thermal conductivity of all participants in the round-robin test was input data for statistical processing according to SRPS ISO 5725-2 and ISO 13528. To evaluate the performance of the participants, the ?z? score has been used. Measurements were conducted successively for all participants. Since 2020, the Accreditation Body of Serbia also approved this test method.

A deterministic breakdown model for dielectric interfaces subjected to tangential electric field

The contact area between solid insulations, namely solid–solid interfaces, affect the dielectric characteristics of an entire insulation system. With the theoretical and experimental studies covered in this paper, we intend to investigate the effects of the elastic modulus, interface contact pressure, and surface smoothness/roughness on the tangential alternating current (AC) breakdown strength of contact surfaces at dielectric interfaces that undergo a tangential electric field. Four distinct solid dielectric specimens with various elastic modulus values were employed. The interfaces between the same materials were subjected to AC breakdown and partial discharge (PD) tests at different contact pressure values. In addition, the interface surfaces were polished using four different sandpapers of various grits to scrutinize the effect of surface roughness. A deterministic model built around the tribology of solid surfaces was proposed to determine the sizes of the interfacial cavities and to simulate the 3D displacement and deformation of the surface protrusions based on the surface roughness, contact pressure, elastic modulus, and hardness of an interface. The estimated sizes of cavities and contact areas were then coupled with a comprehensive breakdown model that addressed the cavity discharge and breakdown of contact areas, separately. Finally, the results by the model were correlated with the results of the AC breakdown and PD experiments to elucidate not only how cavities were linked at solid–solid interfaces but also the effects of roughness, elasticity, contact pressure, and gas pressure inside of the cavities.

A Coupled-Cavity Resonator Technique for Simultaneous Sensing of Dielectric Load and Position of Continuous Material Streams

An electromagnetic coupled-cavity resonator equipped with permanent insertion holes and designed to sense continuous dielectric material streams through the resonator is presented. The resonating structure comprises a pair of pill-box-shaped cylindrical cavities mutually coupled to each other through an aperture-loaded metal sheet of finite thickness. The input and output insertion holes in the top and bottom lids of the cavity structure form a channel for the flow of dielectric samples under test. The arrangement of aperture-coupled cavity sections results in antisymmetric and symmetric field distributions with split resonances. The resonating antisymmetric field is primarily utilized to detect dielectric load variations of the sample stream, similar to the commonly practiced permittivity based measurements in TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">010</sub> cavities. The additional resonance mode with the symmetric field distribution is more sensitive to position offsets of the sample in its concentric flow along the axis. Due to the added resonance, the arrangement provides for 2-D sensing of dielectric specimens under test (dielectric load variation and position offset) and improved accuracy can, thus, be achieved. The working principle of the coupled-cavity technique is analyzed qualitatively by numerical computations. The technique is validated by measuring samples of commercially available cotton fibers with a realized prototype.

Cyclic response of insulated steel angle brackets used for cross-laminated timber connections

In cross-laminated timber (CLT) buildings, in order to reduce the disturbing transmission of sound over the flanking parts, special insulation layers are used between the CLT walls and slabs, together with insulated angle-bracket connections. However, the influence of such CLT connections and insulation layers on the seismic resistance of CLT structures has not yet been studied. In this paper, experimental investigation on CLT panels installed on insulation bedding and fastened to the CLT floor using an innovative, insulated, steel angle bracket, are presented. The novelty of the investigated angle-bracket connection is, in addition to the sound insulation, its resistance to both shear as well as uplift forces as it is intended to be used instead of traditional angle brackets and hold-down connections to simplify the construction. Therefore, monotonic and cyclic tests on the CLT wall-to-floor connections were performed in shear and tensile/compressive load direction. Specimens with and without insulation under the angle bracket and between the CLT panels were studied and compared. Tests of insulated specimens have proved that the insulation has a marginal influence on the load-bearing capacity; however, it significantly influences the stiffness characteristics. In general, the experiments have shown that the connection could also be used for seismic resistant CLT structures, although some minor improvements should be made.

Drilling of different materials through microwave radiations: A review

Microwaves are the electromagnetic radiation with frequency ranging between 300 MHz and 300 GHz, with the wavelengths ranging from about 1 m to 1 mm. Microwave drilling is a machining process in which electromagnetic radiation is concentrated at 2.45 GHz by using a concentrator into a small region. In this review paper, the use of microwave radiations for drilling metals and non-metals is studied in brief and parameters, such as immersion depth, concentrator material, type of dielectric material, susceptor powder and specimen. A characterization of drilling is studied namely Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Micro tensile test, impact test, 3-point bending test, Fractography and porosity.

Electrical Detection of Degradation in Specimens of HVDC Cable Insulation

One of the challenges in laboratory investigation of degradation and ageing of HVDC cable insulation is related to securing, or in other words, imitating the real service environment of the material specimens. So far, the published data refer to experiments conducted in thermo-oxidative conditions, which is not the case during normal cable operation. In service, the cable insulation is protected by a metallic barrier that blocks the transfer of any substances in and out of the construction. By-products from the cross-linking reactions cannot diffuse out and any foreign substances are blocked from entering the insulation. Thus, in order to generate results that are valid, these conditions must be replicated in laboratory experiments. This contribution presents a methodology elaborated for performing ageing experiments in a hermetically sealed environment. Degradation of the material is evaluated through changes in the electrical tree inception voltage and test object capacitance over time. Securing the environmental isolation is accomplished with an isolation system consisting of a glass enclosure with attached metallic electrodes. Indium is used to create a glass-to-metal seal between the glass and the electrodes. The electrode geometry is of needle–plane type and the needle injection process is semi-automated to ensure specimen repeatability.

Investigation on space charge properties of Nomex insulation paper in the mining dry type transformer during hygrothermal ageing

Nomex insulation paper has been widely used in mining dry type transformer insulation because of its excellent performance. However, the injection of space charge can affect the electrical properties of the Nomex paper and accelerate the ageing process of the insulation system, which causes the insulation system failure or even breakdown. Therefore, it is of great practical value to study the space charge characteristics of Nomex insulation paper in different hygrothermal ageing conditions. The accelerated hygrothermal ageing tests of Nomex insulation specimens with different initial moisture contents were conducted. The space charge distribution of specimens impregnated by varnish were measured by the pulsed electro-acoustic method at different ageing stages. Simultaneously, the space charge characteristics during polarising and depolarising phases were deeply analysed. Experimental results show that negative charges accumulate between the varnish-paper interface in specimens near the anode. The number of negative charges gradually decreases with the increase of ageing degree. Under the influence of humidity, Nomex paper can be hydrolysed, which increases the trap density and trap level. Finally, a large amount of positive and negative charges in aged specimens would result in the distortion of the local electric field, which could decrease the breakdown strength of Nomex insulation specimens and be a threat to transformer insulation.

Accumulative effect of partial discharges at impulse voltage wave tail

This study describes a technique for evaluating partial discharges at fast impulse voltages. Fast transient voltages are increasingly present in power equipment and industrial environments due to the ubiquitous application of power electronics switching devices. Special attention should be paid to the dangers caused by the accumulative effect of repetitive impulse voltages on electrical insulation. This study describes a method of analysing partial discharges occurring during impulse voltage stimuli. A repetitive train of pulses is applied to a dielectric specimen and the accumulated PD pattern is obtained. This study focuses on the reverse discharges which occur during the tail part of the impulse voltage waveform acquired in time-resolved mode. Investigations of the partial discharges in the wave tail can potentially lead to greater degradation of the insulation due to the formation of longer, repetitive sequence occurring along the decaying impulse voltage tail. Experiments were performed which investigated the influence of rise time, fall time and peak voltage on discharge pattern reshaping, intensity and time to inception on the tail. In the future, such an approach might be further developed for diagnostic applications and for extended factory tests.