Improvement Of Dielectric Strength Research Articles

Condition Assessment of Natural Ester-Mineral Oil Mixture Due to Transformer Retrofilling via Sensing Dielectric Properties.

Mineral oil (MO) is the most popular insulating liquid that is used as an insulating and cooling medium in electrical power transformers. Indeed, for green energy and environmental protection requirements, many researchers introduced other oil types to study the various characteristics of alternative insulating oils using advanced diagnostic tools. In this regard, natural ester oil (NEO) can be considered an attractive substitute for MO. Although NEO has a high viscosity and high dielectric loss, it presents fire safety and environmental advantages over mineral oil. Therefore, the retrofilling of aged MO with fresh NEO is highly recommended for power transformers from an environmental viewpoint. In this study, two accelerated aging processes were applied to MO for 6 and 12 days to simulate MO in service for 6 and 12 years. Moreover, these aged oils were mixed with 80% and 90% fresh NEO. The dielectric strength, relative permittivity, and dissipation factor were sensed using a LCR meter and oil tester devices for all prepared samples to support the condition assessment performance of the oil mixtures. In addition, the electric field distribution was analyzed for a power transformer using the oil mixtures. Furthermore, the dynamic viscosity was measured for all insulating oil samples at different temperatures. From the obtained results, the sample obtained by mixing 90% natural ester oil with 10% mineral oil aged for 6 days is considered superior and achieves an improvement in dielectric strength and relative permittivity by approximately 43% and 48%, respectively, compared to fresh mineral oil. However, the dissipation factor was increased by approximately 20% but was at an acceptable limit. On the other hand, for the same oil sample, due to the higher molecular weight of the NEO, the viscosities of all mixtures were at a higher level than the mineral oil.

Improvement of dielectric strength and properties of cross-linked polyethylene using nano filler

Power cables insulated with cross-linked polyethylene (XLPE) have been utilized worldwide for distribution and transmission networks. There are several advantages for this type of insulation; it has better electrical, thermal, and mechanical properties compared to other types of insulation in medium and high voltage networks. Many studies aimed to improve the XLPE characteristics through introducing nano fillers to the XLPE matrix. Therefore, this paper investigates the AC (HV) breakdown voltage (dielectric strength) of XLPE after adding nano-sized zeolite (Z) fillers with various concentrations of 1 wt%, 3 wt%, 5 wt% and 7 wt%. The dielectric strength is tested in different temperatures of 30 ⁰C and 250 ⁰C. Additionally, it was tested in low and high salty wet conditions. The dielectric strength of the XLPE has been enhanced by inducing the Z nano filler. The results of the tests were used to train the artificial neural network (ANN) to calculate the dielectric strength of XLPE composites with different concentrations of nano Z filler under different weathering conditions. Thermogravimetric analysis, tensile strength, and elongation at break tests were applied to check the thermal and mechanical characteristics of the samples. Experimental findings show that the optimum concentration of nano Z is 3.64 wt% to enhance the electrical, thermal, and mechanical properties.

Optical and dielectric analysis of ZnO nanorods doped polymer dispersed liquid crystal and ethanol gas sensing investigation

ABSTRACT The spectra and dielectric properties of polymer-dispersed liquid crystal films doped with different concentrations of ZnO nanorods were studied. The scanning electron microscopy images show that doping ZnO nanorod concentration can increase the size of liquid crystal droplets to microscale. The absorption spectra of the films also show that the edge of the absorption band of the composite system moves towards high energy with the increase of the concentration of ZnO nanorods. On the other hand, the investigation results show that the dielectric constant and loss increase with the increase of doping concentration from 4 Hz-1 MHz. Further analysis of the Cole–Cole diagram fitted by Havriliak-Negami equation shows that there are three relaxation processes. The improvement of dielectric strength and relaxation frequency of nanocomposites was observed at low frequencies of 100 Hz. A threefold reduce in relaxation time for the doped sample (0.5 wt.%) was obtained compared with that of pure sample without doping. In addition, the studies show that the temperature has a great influence on the activation energy of materials, which have a slight increase (~5°C) in transition temperature of liquid crystals. Practically, in ethanol gas sensing experiment, the results show the value of permittivity increases and impedance decreases.

AC dielectric strength of synthetic ester-based Fe<inf>3</inf>O<inf>4</inf>, Al<inf>2</inf>O<inf>3</inf> and SiO<inf>2</inf> nanofluids — conformity with normal and weibull distributions

This paper is aimed at studying the influence of conductive (Fe 3 O 4 ) and insulating (Al 2 O 3 and SiO 2 ) nanoparticles at various concentrations on the AC dielectric strength of MIDEL 7131 synthetic ester. The Normal and Weibull distribution functions are used to analyze the breakdown voltages with 1, 10, and 50% risk probabilities estimated. It is shown that the breakdown voltage values for most of investigated nanofluids obey both Normal and Weibull distributions. The addition of considered nanoparticles enables to improve the dielectric strength of synthetic ester. However, this improvement depends on the concentration, size and nature of nanoparticles. For a given type and size of nanoparticles, there is an optimal concentration that allows us to reach the highest value of breakdown. On the other hand, the magnetic / conducting nanoparticles give the best improvement of dielectric strength. Indeed, with Fe 3 O 4 (50 nm) the enhancement is of 48% while it is of 35 and 25% with Al 2 O 3 for nanoparticles of 13 and 50 nm, respectively, and it is of 32% with SiO 2 (50 nm).

Dielectric breakdown of alumina thin films produced by pulsed direct current magnetron sputtering

Alumina films (~2 μm thick) were deposited with a mixed Cu/Al interlayer onto copper. Direct current (DC)/Pulsed DC (PDC) magnetron sputtering techniques were independently compared for reactive alumina sputtering. In DC sputtered films, elemental aluminium of 9.2 at.% and nano-crystallites were present within the x-ray amorphous matrix, resulting from target arcing. Defects lead to premature dielectric breakdown/increased current leakage. PDC sputtering improved film quality by removing crystallites, metallic clusters and through thickness cracking. Time dependent dielectric breakdown (TDDB) measurements were carried out using conductive atomic force microscopy identified an improvement in dielectric strength (166 to 310 V μm−1) when switching from DC to PDC deposition power. TDDB suggested that at high applied field the dominant pre-breakdown conduction mechanism was Fowler-Nordheim tunnelling in DC films. Tensile pull-off adhesion ranged from 56 to 72 MPa and was highest following incorporation of an Cu/Al blended interfacial layer. Scratch testing indicated various cracking/buckling failures.

Tailored side‐chain architecture of benzil voltage stabilizers for enhanced dielectric strength of cross‐linked polyethylene

ABSTRACTThe synthesis and physico‐chemical properties of seven benzil‐type voltage stabilizers are reported. The benzil core is substituted with alkyl chains of different length that are linked to the benzil core via an ester, ether, or tertiary amine group. All additives can be melt‐processed with low‐density polyethylene (LDPE). Fourier‐transform infrared spectroscopy confirms that benzil compounds are not affected by the LDPE cross‐linking reaction induced by dicumyl peroxide. Moreover, a combination of gel content measurements, thermal analysis, and small‐angle X‐ray scattering indicates that the presence of benzil voltage stabilizers does not significantly alter the microstructure of cross‐linked polyethylene (XLPE). Electrical tree inhibition experiments under high‐voltage alternating current conditions show that all investigated additives substantially enhance the dielectric strength of the insulating material at a concentration of only 10 mmol kg−1. The highest improvement in dielectric strength, of more than 70% with respect to reference XLPE, is obtained with voltage stabilizers, which carry short (methyl) side chains that are linked to the benzil core via an ester or tertiary amine group. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1047–1054

Optimisation of the dielectric strength of a non-uniform electric field electrode system under positive DC voltage by insertion of multiple barriers

The main objective of this work is the study of the flashover voltage evolution of the air gap of two non-uniform electric field systems as a function of the number of screens inserted in the gap, their position, their isolation and their surface conductivity. The results from this analysis show that in the case of clean and dry atmosphere, Dual-screen isolation systems with rod-rod and rod-plane configurations is respectively 50% and 19% higher than the case of one screen of the same size. Above this value, no further improvement in dielectric strength is found. The isolation of the two barriers by a solid insulating shield, in contact with each other and a conductive grounded base wall, led economically to an important reduction of insertion device size and technically, its optimum withstand voltage remains the same as that obtained in the case of an air isolation layer of the same size. For low conductivity of the polluting solution, uniformly covering both screens of the rod-rod system, the minimum disruptive voltage value is reduced by over 50% compared to that obtained under clean atmosphere, so that these polluted insulating screens barriers had become conductive. So if no cleaning of them is expected in polluted sites, the improvement of isolation provided by adding a second screen to the base system will be completely lost during a low contamination of these barriers and we can add that a single screen system is 24% more rigid than dual-screen system under the same pollution conditions.

Optimisation of the dielectric strength of a non-uniform electric field electrode system under positive DC voltage by insertion of multiple barriers

The main objective of this work is the study of the flashover voltage evolution of the air gap of two non-uniform electric field systems as a function of the number of screens inserted in the gap, their position, their isolation and their surface conductivity. The results from this analysis show that in the case of clean and dry atmosphere, Dual-screen isolation systems with rod-rod and rod-plane configurations is respectively 50% and 19% higher than the case of one screen of the same size. Above this value, no further improvement in dielectric strength is found. The isolation of the two barriers by a solid insulating shield, in contact with each other and a conductive grounded base wall, led economically to an important reduction of insertion device size and technically, its optimum withstand voltage remains the same as that obtained in the case of an air isolation layer of the same size. For low conductivity of the polluting solution, uniformly covering both screens of the rod-rod system, the minimum disruptive voltage value is reduced by over 50% compared to that obtained under clean atmosphere, so that these polluted insulating screens barriers had become conductive. So if no cleaning of them is expected in polluted sites, the improvement of isolation provided by adding a second screen to the base system will be completely lost during a low contamination of these barriers and we can add that a single screen system is 24% more rigid than dual-screen system under the same pollution conditions.

Breakdown conditioning characteristics of long gap electrodes in a vacuum

An improvement in dielectric strength is required in vacuum circuit breakers (VCBs) intended for use in higher voltage systems. In order to develop higher voltage VCBs, it is important to improve the dielectric strength in a vacuum based on consideration of the vacuum breakdown mechanism. Particularly for gaps longer than 10 mm, little is known about the breakdown mechanisms and their quantitative analyses in a vacuum. This paper discusses the breakdown conditioning characteristics of long gap electrodes, under a non-uniform electric field in a vacuum. We treat gap lengths of up to 50 mm in this paper. The conditioning characteristics are investigated under impulse voltage applications. A negative standard lightning impulse voltage was applied to rod-plane electrodes made of Cu-Cr and SUS304 for different tip radii and gap distances until the conditioning effect was completed. We observed illumination spots on electrodes at each breakdown during the conditioning process and calculated the corresponding breakdown field strengths. Experimental results revealed that the tendency of breakdowns associated with long gaps is different from that for the short gaps. As a result, we clarified that the breakdown field strengths are nearly constant at 110-120 kV/mm at the distances longer than 10 mm, and the breakdown field strength is at its maximum when the gap distance is about 5 mm.

界面拡散法によるCVケーブル接続部の絶縁性能向上

界面拡散法によるCVケーブル接続部の絶縁性能向上

Improvement of dielectric strength on transformer winding using new aramid paper

AbstractLarge power transformers using perfluorocarbon liquid have advantages of high dielectric strength, excellent cooling ability, and nonflammability.To make the transformer more compact, it is very important to improve the insulation material.Recently, the authors succeeded in developing a new type of aramid insulation paper which has no fluff on its surface to avoid field concentration.In this study, an outline of the developed paper and the experimental results of the insulation strength on turn‐to‐turn and intercoil models are described.It was found that the new aramid paper increases the insulation strength 1.2 (turn‐to‐case) to 1.7 (intercoil case) times that of the existing insulating paper.

新アラミド絶縁紙による変圧器巻線絶縁の高耐電圧化

新アラミド絶縁紙による変圧器巻線絶縁の高耐電圧化

Electron attachment to perfluorocarbon compounds. II. c-C5F8, c-C6F10, c-C6F12, C7F8, and C8F16: Relevance to gaseous dielectrics

Electron attachment rates αw as a function of the pressure-reduced electric field have been measured for c-C5F8 (perfluorocyclopentene), c-C6F10 (perfluorocyclohexene), c-C6F12 (perfluoro-1,2,-dimethylcyclobutane), C7F8 (perfluorotoluene), and C8F16 (perfluoro-1,3-dimethylcyclohexane). The thermal values (αw)ther of the attachment rate for these compounds are 12.8, 12.9, 5.0, 9.2, and 2.4×109 sec−1 Torr−1, respectively. The electron attachment cross sections σa(ε) as functions of the electron energy ε have been determined using the swarm-unfolding technique; they show distinct structure at ε<1.5 eV. The electron attachment properties for the perfluorocarbons (PFCs) studied in this and the preceding paper depend strongly on the molecular structure. Thus (i) open-chain saturated PFCs do not attach electrons as efficiently as the unsaturated PFCs; (ii) for saturated PFCs an increase in molecular size (chain length) increases αw; (iii) the presence of multiple bonds in the molecule dramatically increases the magnitude of αw for the saturated open-chain PFCs, but it has only a small effect for cyclic PFCs; (iv) the cyclic nature of the PFCs seems to increase greatly σa(ε); and (v) for single-bonded cyclic PFCs, substitution of CF3 groups for F atoms increases (αw)ther more than does an increase in molecular size; in double-bonded cyclic compounds increase in ring size increases σa(ε) at thermal energies. The PFCs studied in this and the preceding paper have higher dielectric strengths than SF6. This improvement in dielectric strength can be attributed to their large σa(ε) over a wider energy range (to ∼1.5 eV). Their use as unitary gases and/or as additives to multicomponent gas mixtures is discussed.

VACUUM TREATMENT OF CLAY SLIPS AND BODIES1

The work contained in this paper was conducted to determine the value of the treatment of clay slips to reduced pressures. Theoretical information obtained in references, however, indicate little possibility of there being much gas adsorbed by the solid particles of a porcelain slip. No improvement of dielectric strength or mechanical strength could be noted as a result of experimental work conducted.

Corona in oil

Experiments were made to determine certain effects of high local voltage stresses in transformer oil. The resistivity of the oil was measured by a special method immediately following a period of voltage stress. The results were 1. A greatly reduced resistivity when the stressing voltage was greater than that producing visible corona. 2. A gradual increase in resistivity following removal of stress. The “rapidly applied” breakdown voltage was found for samples of oil which had been previously subjected to high voltage stresses. The dielectric strength varied in a manner similar to the resistivity, but showed an actual improvement in dielectric strength when the oil was given a rest period of 15 minutes. Definite conclusions are given at the end of the paper.

Corona in Oil

Experiments were made to determine certain effects of high local voltage stresses in transformer oil. The resistivity of the oil was measured by a special method immediately following a period of voltage stress. The results were: 1. A greatly reduced resistivity when the stressing voltage was greater than that producing visible corona. 2. A gradual increase in resistivity following removal of stress. The ``rapidly applied'' breakdown voltage was found for samples of oil which had been previously subjected to high voltage stresses. The dielectric strength varied in a manner similar to the resistivity, but showed an actual improvement in dielectric strength when the oil was given a rest period of 15 minutes. Definite conclusions are given at the end of the paper.