Dielectric Properties Of Insulation Research Articles

Synergistic effect of positive temperature coefficient and thermally conductive micro‐nano fillers on electrical and thermal properties of epoxy resin

AbstractThis study investigates the effect of positive temperature coefficient (PTC) fillers, i.e., barium strontium titanate (BST‐60) and nano hexagonal boron nitride (h‐BN) fillers, on thermal and dielectric insulation properties of epoxy (EP). The electrical resistivity, DC breakdown strength, space charge, thermal conductivity, thermogravimetric analysis, and glass transition temperature (Tg) of EP composites were measured. Results indicate that, below curie temperature, the EP composites containing 0.5 and 1 wt% of BST and 5 wt% of h‐BN have superior electrical resistivity, breakdown strength, and thermal conductivity. However, above curie temperature, the EP composites containing 2 and 2.5 wt% of BST and 5 wt% of h‐BN have superior insulation properties than the pure EP and other composites. While the thermal properties of EP improve with an increase in h‐BN loading to 10 wt%, its dielectric properties are slightly compromised due to the presence of tiny defects in the composite material. It is hypothesized that the BST‐60 suppresses the negative temperature coefficient effect (NTC) of EP by regulating the electrical resistivity above the curie point, whereas nano h‐BN enhances the thermal conductivity of EP by constructing a heat dissipation path. The proposed mechanism for charge dynamics along with heat conduction‐dissipation explains the fillers' behavior at different temperatures.Highlights Analysis of epoxy composites with polydopamine‐modified BST and h‐BN fillers. PTC fillers are used to mitigate the negative temperature effect of epoxy. Optimal fillers wt% improves dielectric and thermal properties. This study shows that optimized epoxy composites can be used for packaging.

Effect of Glass Transition Temperature on Enhanced Dielectric Breakdown Strength and Lifetime of Multilayer Polymer Films.

High temperature, high energy density, and low loss dielectric films are promising candidates for miniaturized capacitors in electric vehicles and high-speed trains. However, single-component polymers could not achieve these desired properties simultaneously. Polymer multilayer films (MLFs), which combine a high dielectric constant polymer [e.g., poly(vinylidene fluoride) (PVDF)] and a high breakdown/low loss polymer [e.g., polycarbonate (PC)] in a unique layered structure, have the potential achieve them at the same time. In this work, the effects of PC glass transition temperature (Tg) on the dielectric insulation properties (breakdown strength and lifetime) were investigated at high temperatures of 100-150 °C. Three PC materials had Tg values of 145 (PC1), 165 (PC2), and 185 °C (PC3), respectively. It is observed that MLF-PC3 with the highest Tg of PC exhibited the highest Weibull direct/alternating current (DC/AC) breakdown strength and the longest DC/AC lifetime, whereas MLF-PC1 with the lowest Tg showed the lowest Weibull DC/AC breakdown strength and the shortest DC/AC lifetime. A high-temperature high-volage leakage current study revealed that MLF-PC3 exhibited the lowest bulk conductivity at all temperatures under different electric fields. The knowledge obtained from this study will help us design better MLFs with high performance for next-generation miniaturized capacitors.

Improved DC Dielectric Performance of Cross-Linked Polyethylene Modified by Free Radical-Initiated Grafting BMIE.

To enhance the direct current (DC) dielectric properties of cross-linked polyethylene (XLPE) for high-voltage (HV) cable insulation, the polyethylene molecular chain is modified by grafting bismaleimide ethane (BMIE), which creates carrier deep traps within the polymer material. Compared to the traditional modified molecule maleic anhydride (MAH), BMIE has a significantly higher boiling point than the production temperature of XLPE. Additionally, it does not release bubbles during the production process and, thus, preserves the dielectric properties. It was proved by infrared spectroscopy and a gel content test that BMIE was successfully grafted onto the polyethylene molecular chain and had no effect on the crosslinking degree of the polymer while reducing the amount of crosslinker, thereby reducing the influence of the by-products of the decomposition of dicumene peroxide (DCP) on the electrical resistance of polymers. The analysis of DC breakdown field strength, current density, and space charge distribution at various temperatures demonstrates that grafting BMIE can greatly enhance the dielectric properties of insulation. Polar groups in the BMIE molecule create deep trap energy levels in XLPE-g-BMIE, and these trap energy levels contribute to the formation of a charged layer near the electrode, which is shielded by Coulomb potential. As a result, the charge injection barrier increases. Additionally, the presence of these polar groups reduces the mobility of charge carriers through trap-carrier scattering, effectively suppressing the accumulation of space charge within the material. First-principle calculations also confirm that bound states can be introduced as carrier traps by grafting BMIE onto polyethylene molecules. The agreement between experimental results and simulation calculations indicates that grafting BMIE to enhance the dielectric properties of polyethylene is a new and feasible research direction in the exploitation of materials for HVDC cables.

Influence of dielectric barrier effect and thermally‐conductive network on thermal and electrical properties of epoxy under different frequencies and temperatures

AbstractEpoxy resin (EPR) insulations play a vital role in the insulation of modern power electronic equipment owing to their excellent dielectric properties. However, due to the high‐power density and miniaturization of power equipment which causes high heat fluxes under high voltage and high‐frequency stresses, EPR with good thermal and insulation properties is urgently needed. In this study, the polydopamine functionalized micro‐BN and core‐shell nano TiO2–SiO2 particles are dispersed in EPR to simultaneously improve thermal and dielectric insulation properties. It is revealed that the addition of micro‐nano particles significantly improves the thermal and dielectric performance. Particularly, the high thermal conductivity of micro‐BN and the dielectric barrier effect due to the core‐shell structure of nano TiO2–SiO2 are the main reasons for improved thermal and dielectric insulation performance, respectively. The EPR composite containing 3 wt% of micro‐BN and 1 wt% of nano TiO2–SiO2 exhibits the optimal performance with 0.49 W/mK thermal conductivity and the highest dielectric strength among all the samples, that is, 60.61 kV/mm even at 10 kHz and 90°C. This study found that the crucial factors are the surface encapsulation, weight percent, and homogeneous dispersion of particles in EPR, the dielectric barrier effect, thermal conductivity, and the mismatch between the dielectric constant of EPR and particles. This study proposes the optimal weight percent of suitable micro‐nano particles for EPR to produce suitable composites for high‐frequency and high‐temperature applications.

Synergistic effects of Micro-hBN and core-shell Nano-TiO2@SiO2 on thermal and electrical properties of epoxy at high frequencies and temperatures

Epoxy (EP) resin insulations are widely used in electronic devices and power systems. They are gradually finding their way as winding insulation material for medium frequency transformers (MFT) due to their attractive insulating and adhesive properties along with low cost and easy processing. However, their dielectric insulation properties are compromised at high frequencies and high temperatures due to poor thermal properties. In this context, the surface-treated hexagonal boron nitride (h-BN) micro fillers and core-shell TiO2@SiO2 nanofillers are doped in epoxy (EP) to enhance their thermal and electrical insulation properties under high frequencies and high temperatures. Results reveal that the thermal conductivity of the EP increases continually, whereas the electrical breakdown strength is reduced above the threshold content of micro/nano fillers. Particularly, the micro h-BN addition is beneficial for thermal conductivity enhancement, whereas the core-shell structure of TiO2@SiO2 improves the electrical insulation properties of the EP. The multi-layer core model is used to explain the dynamics of the fillers. The homogeneous dispersion and optimal content of suitable micro-nano fillers can produce efficient EP composites as the electrical insulation for medium frequency transformers.

Reducing Hydrophilic Characteristics of Kraft Paper Insulation by Reinforcing With Surface Modified Rutile-TiO2 Nanoparticles

The hydrophilic nature of cellulose compromises the dielectric properties of kraft paper insulation when exposed to water and moisture. The effort to address this setback is an ongoing research enquiry. In the present work, surface-modified rutile-titanium dioxide nanoparticles (rutile-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> NPs) are reinforced into cellulose pulp to fabricate a nanocomposite kraft paper insulation. Surface modification of the nano-filler particles is intended to reduce the hydrophilicity of the resultant nanocomposite kraft paper insulation. The nanoparticles (rutile-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> NPs) improve other dielectric properties of the insulation. The chemicals used for the surface modification of the rutile-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> NPs were the alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA). The nanofiller retention, r dispersion and hydrophobic properties of the resultant reinforced kraft paper were experimentally analyzed. The results show that more than 50% of the nanofillers were retained within the reinforced kraft paper. Scanning electron microscopy (SEM) images showed evenly distributed nanofillers, with some traces of agglomeration. The moisture absorption property of the kraft paper specimen modified with rutile-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> NPs containing 5 vol/vol% ASA improved by 74% compared to the control (reference) specimen. Water vapor transmission rate of the surface-modified nanoparticles reinforced kraft paper insulation decreased by 30%. Compared to the unfilled paper, the contact angle of water droplets on the surface of the reinforced kraft paper improved by 12%. Water absorption rate improved by being 4 times slower in paper specimens containing rutile-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> NPs 5 vol/vol% ASA. Dielectric dissipation factor measurement results showed that the specimen modified with rutile-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> NPs containing 5 vol/vol% had 40% lower dielectric losses than the reference samples. This study, therefore, has successfully improved the hydrophobic properties of kraft paper by filling it with surface-modified rutile TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> using 5% ASA as the nanoparticle surfactant.

Effect of Rice Husk Filler on the Structural and Dielectric Properties of Palm Oil as an Electrical Insulation Material

This study uses natural fibre, which is moisture absorbent, as an innovative and economical filler for insulating oil. Rice husk (RH) is a natural fibre known to have water-absorbing properties and used as a supplementary cementitious material. This research utilises the water-absorbing properties of RH to improve the physicochemical and dielectric properties of insulating oil. RH was refined into a fine powder at a diameter of less than 63 µm. Palm oil (PO) was synthesised with RH at concentrations of 0 g/L, 0.01 g/L, and 0.1 g/L. The moisture-absorbent properties of RH were analysed by using Fourier-transform infrared (FTIR) spectroscopy. The particle size and distribution of RH in PO were also obtained using a scanning electron microscope (SEM) and a Zeta particle analyser. The breakdown voltage (BDV) strength of PO with RH was measured according to IEC 60156, and the dielectric frequency response was investigated in the range of 102–105 Hz. RH is proven to absorb moisture from PO, as the O–H band intensity at the 3350 cm−1 is increased from the experimental results. Therefore, the mean BDV and dielectric insulation properties of PO dispersed with RH also increased accordingly.

Evaluation of dynamic loading capability for optimal loading strategies of power transformers

Increasing needs for operational flexibility encourage concepts of using thermal operational limits of grid equipment instead of nominal limits, e.g., “dynamic line rating” for transmission lines. Using the available loading flexibility of complex and valuable power transformers requires a comprehensive approach. The thermal state and condition of a power transformer are among the most considerable impact factors for planning overload capability and lifetime management in the long term. This paper investigates the dynamic loading capability of power transformers by using a dynamically calculated initial thermal state, set thermal limits and a dynamic thermal model as proposed in the IEC 60076-7:2017. Different thermal limits can be set depending on the operation state, e.g., long-term emergency or insulation-friendly. The suggested thermal limits also consider ageing and dielectric properties of insulation. Thermal limits of equal ageing velocity for different moisture content in solid insulation can be used to operate the transformer with respect to long-term asset management strategies. Using the proposed reverse calculation of the thermal model’s steady-state loading limits, e.g., nameplate rating, the permissible short-term loading beyond the steady-state limits can be obtained for given timespan. The dependency of the steady-state loading limit from ambient temperature is approximated as a polynomial function with linear, quadratic and cubic components. For a safe loading of the power transformer the steady-state limits of the subsystems have to be compared. The study shows that the hot-spot steady-state limits for normal and long-term emergency loading are lower than the top-oil steady-state limits. Power transformers are usually subject to variations in the loading profile. The current thermal state becomes essential for the estimation of the thermal reserve and loading capability, especially when permissible overloading for a time of up to 60 minutes is scheduled. When determining the possible overload for a longer time span, the validity of the results is decreasing due to growing uncertainties. Moreover, the paper proposes a system to enhance the model by a moisture migration model for mineral-oil-immersed power transformers, decision-making under uncertainties assistance and transformer’s intelligent condition assessment system.

Temperature-dependent DC conductivity and space charge distribution of XLPE/GO nanocomposites for HVDC cable insulation

Nanocomposites have been proven to be an important dielectric material, which provides advanced dielectric properties of insulation to power equipment. For the use of polyethylene (PE)-based nanocomposites in HVDC cable insulation, their temperature dependence needs to be considered, because it could deeply affect the conductivity and space charge behavior. To investigate the temperature dependence of conductivity and space charge properties of the cross-linked polyethylene (XLPE)/graphene oxide (GO) nanocomposites, XLPE samples with GO mass fractions of 0, 0.001, 0.01 and 0.1 wt% were prepared. The conductivity, space charge behavior and breakdown strength of the nanocomposites were tested. Then, the surface potential decay (SPD) was measured to elucidate the mechanism of the GO nanoparticles in the XLPE matrix according to the trap theory. The obtained experimental results showed that the XLPE/GO nanocomposites with the mass fraction of 0.01 wt% exhibited a lower conductivity, lower space charge accumulation, and higher DC breakdown strength than those of the neat XLPE. This observation is related to the nanoparticle-polymer interaction regions and the deep traps introduced by the GO nanoparticles. In addition, the temperature dependence of the electrical properties of nanocomposites decreased when appropriate GO amounts were added. This result indicates that the XLPE/GO nanocomposites are more suitable for use in HVDC cable insulation.

Nacre-like nanocomposite film with excellent dielectric insulation properties and mechanical strength based on montmorillonite nanosheet and aramid nanofiber

Unlike the mechanisms behind their enhanced mechanical properties, application of artificial nacre-like materials has attracted very little attention from scholars. In this study, we designed a unique artificial nacre-like material that integrated outstanding mechanical properties and dielectric insulation properties by assembling MTM nanosheet and aramid nanofiber (ANF) through vacuum-assisted filtration. Compared with other nanocomposite films, the layered ANF/MTM nanocomposite film displayed superior mechanical properties (the tensile strength of ~ 126.5 MPa and elongation at break of ~ 2.56%) and dielectric insulation performance (77.2 kV/mm). In addition, it exhibited good transparency and flexibility. Therefore, this nanocomposite film has great potential for application in flexible electronics and high-voltage electrical insulation.

IMPROVING THERMAL WITHSTANDING CAPACITY OF THREE PHASE INDUCTION MOTOR USING NWCC METHOD

Three phase induction motors are widely used in industries due to their various technical as well as economical advantages. About 60% of industrial electricity is consumed by these motors, but the heat generation inside the motor due to various losses is a critical factor which degrades the dielectric properties of insulation and shortening the lifespan of motor. This paper presents the capillary based water cooling method of three phase squirrel cage induction motor. In this method, a jacket of capillaries made of jute and cotton is used as a outer cover for the motor casing. A small air gap passage is provided in between motor casing and inner surface of jacket. Natural Water Cooling Capillaries (NWCC) method is used to lower down the surrounding temperature of motor which reduces heating limitations of motor. For this purpose we select a 1.1kW three phase, 50Hz squirrel cage induction motor. Heat run test is performed on this motor to determine the total loss of energy dissipated as heat on three phase squirrel cage induction motor with and without adopting capillary based water cooling method. Comparison is done on the basis of experimental result which shows that the temperature withstanding capacity of induction motor will increase by 15.5 % after adopting capillary based water cooling method. This is an economical method and no external energy is required for cooling purpose.

EFFECT OF NANOPARTICLES ON WATER TREEING CHARACTERISTICS IN XLPE INDUSTRIAL I NSULATING MATERIALS

Water treeing is still now one of the main causes of cable deterioration and so, it is widely known that water treeing is the responsible for the reduction of the service life of polymeric cables. This paper has been studied the effects of adding various amounts of nanoparticles as ZnO, and Al2O3 on water treeing characteristics in XLPE industrial materials. This research has focused on studying development of the nano-composite materials performance with water tree growth resistance superior to the unfilled matrix, and has stressed particularly the effect of filler volume fraction on the water tree length and via various frequencies. Finally, this research aims to present a systematic and comparative study about the effect of adding nanoparticles to change dielectric insulation properties, and to understand the role effect of these nanoparticles in the industrial insulation materials which can make significant improvement in water tree resistance, life-time of insulation and decreasing the water tree length in the industrial insulation materials.

Influences of Corrosive Sulfur on Copper Wires and Oil-Paper Insulation in Transformers

Oil-impregnated paper is widely used in power transmission equipment as a reliable insulation. However, copper sulphide deposition on oil-paper insulation can lead to insulation failures in power transformers. This paper presents the influences of copper sulfur corrosion and copper sulphide deposition on copper wires and oil-paper insulation in power transformers. Thermal aging tests of paper-wrapped copper wires and bare copper wires in insulating oil were carried out at 130 °C and 150 °C in laboratory. The corrosive characteristics of paper-wrapped copper wires and bare copper wires were analyzed. Dielectric properties of insulation paper and insulating oil were also analyzed at different stages of the thermal aging tests using a broadband dielectric spectrometer. Experiments and analysis results show that copper sulfide deposition on surfaces of copper wires and insulation paper changes the surface structures of copper wires and insulation paper. Copper sulfur corrosion changes the dielectric properties of oil-paper insulation, and the copper sulfide deposition greatly reduces the electrical breakdown strength of oil-paper insulation. Metal passivator is capable of preventing copper wires from sulfur corrosion. The experimental results are helpful for investigations for fault diagnosis of internal insulation in power transformers.

Behavior of machine insulation systems subjected to accelerated thermal aging test

Accelerated aging tests have been extensively used on motor and generator insulation systems to simulate in a short time period the deterioration mechanisms occurring during normal operation of many years, to identify insulation system designs with longer life time and to support the qualification of a new system. The contribution presents results from extensive investigations on the behavior of two VPI machine test insulation systems (reference and candidate) with thermal class F (155°C), which were subjected to three thermal overstresses in repeated cycles. After each thermal cycle the insulation system is subjected to mechanical stress, moisture, and voltage. In order to check the condition of the insulation systems in a non-destructive manner several properties of insulation like dissipation factor, insulation resistance and partial discharge quantities were measured after each thermal cycle. After that the end of life of the insulation systems were checked with ac voltage and impulse voltage. The results support the qualification by comparative evaluation and they indicate, how the changes in electrical and dielectric properties of insulation take place during the accelerated aging test.

Relationships between Processing Parameters and the Foaming Performances of Polyethylene for Coaxial Cable Insulation

Coaxial cable is cable that transmits electrical energy and signal from the receiver devices to the antenna in the base station. Nowadays, as the telecommunication technology develops, demand for a high signal quality is required in the coaxial cable. In this cable, the electromagnetic signal passes through the insulation, which is usually composed of foamed high-density polyethylene (HDPE)/low-density polyethylene (LDPE) blend. To improve the dielectric properties of insulation and reduce the signal power loss (attenuation), it is necessary to increase the foaming rate of insulation. In this study, two types of foaming experiments were performed: batch type foaming and continuous extrusion foaming. The relationship between the foaming rate and the electrical properties of insulation was investigated in the batch type foaming process. Also, the effects of processing conditions, such as resin composition, die temperature, screw speed, and pressure drop rate of die on the foaming performances were studied in the continuous foaming extrusion for coaxial cable insulation.

Determination of dielectric properties of insulator materials by means of ANFIS: A comparative study

In this study, dielectric properties of polyesters which have abundant use in communication cables and power systems are aimed to be determined by means of adaptive neuro-fuzzy inference system (ANFIS). Measuring the dielectric properties of insulators under various fields of application and operating conditions requires advanced equipments and takes a long time. After measuring in a wide range and different conditions, determining the other required dielectric properties by means of ANFIS will supplement time and cost. To this end, ANFIS models are trained which are built for every measurement value of dielectric permittivity (ɛr) and loss factor (tan δ) with respect to heat-frequency values of polyesters used as insulators. Then, used data in ANFIS and unused data are tested separately. In the models built for determining dielectric permittivity and loss factor, absolute mean percent error-cumulative absolute percent error are found to be 0.0030–0.1092% and 0.0021–0.0766%, respectively. The results of ANFIS are compared with a previous study of multilayer perceptron (MLP) artificial neural networks (ANN) in which the same data sets are used. ANFIS models exhibit good learning precision and generalization according to mean absolute errors (MAEs). Hence, it becomes easy to determine the dielectric permittivity and loss factor without measuring the given temperature–frequency values by ANFIS. The percent effects of dielectric permittivity and loss factor that are determined by ANFIS on total dielectric loss are discussed based on representative analyses.

Non-Contact Scanning Nonlinear Dielectric Microscopy Utilizing Higher Order Nonlinear Dielectric Signal

The contact-type scanning nonlinear dielectric microscopy (SNDM) can measure the distribution of linear and nonlinear dielectric properties of insulators with sub-nanometer resolution. SNDM, however, has tip-abrasion problem and this problem is one of the obstacles for the high quality image. In this paper, in order to improve SNDM, we propose a non-contact scanning nonlinear dielectric microscopy (NC-SNDM) which focused on the nonlinear dielectric signal for controlling the non-contact condition. With NC-SNDM technique, topographic and SNDM images were obtained successfully.

Formal analogy between multiple-trapping and polarization models: a physical picture for the Cole–Cole formula

Dielectric properties of insulators can often be explained with a complex dielectric constant taking the well-known form proposed by Cole and Cole. The latter consists in correcting the simple Debye expression to account for anomalous dispersion in time required to line up dipoles in a dielectric submitted to a static field. Despite its success, the Cole–Cole formula has not received any convincing physical explanation. It introduces anomalous dispersion from a mathematical trick and is not based on a particular description of the polarization at the microscopic scale. The physical reasons for the dispersion are therefore obscure. Carrier transport is another field where anomalous dispersion occurs. In this case, however, numerous published works have contributed to draw some clear physical pictures for it, the most successful models involving hopping and multiple-trapping (MT) in the presence of an exponential distribution of traps. This work shows that the Cole–Cole formalism is formally equivalent to the MT model of highly dispersive transport when written under adequate conditions. This analogy provides a possible physical framework for the Cole–Cole formula, leading for instance to an expression of the characteristic time involved in this formula as a function of the Debye relaxation time.

Non-contact scanning nonlinear dielectric microscopy

Contact-type scanning nonlinear dielectric microscopy (SNDM) can measure thedistribution of linear and nonlinear dielectric properties of insulators with sub-nanometreresolution. SNDM, however, has a tip-abrasion problem, and this problem is one of theobstacles in obtaining a high-quality image.In this paper, in order to improve SNDM, we propose a non-contact scanning nonlineardielectric microscopy (NC-SNDM) which focuses on the nonlinear dielectric signal forcontrolling the non-contact condition. With the NC-SNDM technique, topographic andSNDM images were obtained successfully. Moreover, from the calculation results, it wasconfirmed that NC-SNDM has atomic-order sensitivity to the gap between the tip and thespecimen.

Analysis of electron transfer between electron irradiated metallic ball and insulators in vacuum: A specific alternative to the mirror method

In order to improve the knowledge of dielectric properties of insulators, we have imagined an original method of characterization of the charge buildup. Electrons of an electron beam are implanted through a metallic ball directly in contact with the insulator in a scanning electron microscope. By calculating and modeling the capacitance and the electrostatic force between the ball and the insulator plane, it has been possible to determine the relationship between the injected charges in the metallic ball and its surface potential. The major role of the dielectric thickness has been evidenced when the insulator is placed on a grounded metallic plane. At high potential values, a dielectric breakdown of the medium surrounding the sphere occurs and electrical charges are transferred from the ball to the dielectric sample. This transfer has been evidenced and quantified in the case of sapphire and quartz. Analytical calculations and numerical simulations using the finite-element method have been performed for interpreting these experimental results.