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Particulate matter of 0.3 μm in diameter (PM0.3) poses a serious threat to the environment and human beings. Ultrathin and -light nanofibrous filters with excellent filtration properties can significantly prevent the detrimental effects of these particles. Here, we develop free-standing polyamide PA-66 ultrafine nanofiber papers for PM0.3 filtration using effective and scalable blow and electro-blow spinning techniques. The smallest average fiber diameter is 61.7 nm, which is 2-3 orders of magnitude smaller than that of conventional textiles. Poly(ethylene terephthalate) nonwovens are selected to fabricate free-standing nanofiber papers of various polymers, including polyamide, poly(methyl methacrylate), poly(vinylpyrrolidone), and poly(ethylene oxide) owing to the smooth surfaces of the nonwovens. This underlying principle can be used to create similar free-standing nanofiber papers from other commodity polymers in the future. Mechanisms of capturing particulate matter with different nanofiber morphologies are discussed. Salt and oil particulates are used to characterize the filtration properties. PA-66 papers are promising reusable filters owing to their mechanical particle-capture mechanism. The blow-spun PA-66 papers show filtration performance of 98.75% efficiency and a pressure drop of 125.44 Pa owing to the "slip" effect caused by the ultrasmall diameter. In the electro-blow spinning process, a supplementary voltage supply is conducive to separating nanofiber bundles into random-oriented nanofibers. Electro-blown spun papers possess an ultrahigh efficiency of 99.99% with a reduced areal density of 0.9 g m-2. These PA-66 papers can be used in a variety of applications, such as reusable personal protective equipment, industrial waste gas treatment, and central ventilation purification systems.

In this study, the effect of the pressboard barrier (PB) on streamer pattern in natural ester under the non-uniform electric field is investigated by experiment and simulation. The propagation patterns and characteristics of the streamer in the oil with/without a pressboard barrier are obtained by the shadowgraph technique. The mechanism of the pressboard barrier effect on the streamer pattern and propagation is revealed using a multi-streamer branch simulation method. Results show that the positional relationship between the needle electrode and the pressboard barrier significantly affects the streamer propagation and branching. The barrier effect of the pressboard, the distribution of space electric field affected by the pressboard barrier, and the image force of charge on the pressboard surface are all critical factors for affecting the propagation and branching of the streamer. The greatest threat to the pressboard failure is the z-axis streamer or those branches close to the z-axis that is severely inhibited by other streamer branches. It raises the risk of insulation failure caused by the breakdown of the insulating pressboard.

In this paper, the pollution flashover performance of a superhydrophobic surface with different coating strategies are studied. The results show that a superhydrophobic coating is effective in preventing pollution flashover under high voltage. The superhydrophobic coatings show increased pollution flashover voltage when compared to room temperature vulcanized silicone rubber and to an uncoated glass substrate. A novel suppression mechanism of the superhydrophobic coating is proposed which is driven by the electrostatic force produced by the applied voltage. The coalesced slippery slurry droplets and the expanded dry area on the superhydrophobic surface contributes to increased flashover voltage on the superhydrophobic surface. The superhydrophobic coating promises to be a prospective application in anti-pollution performance of insulators.

In this work, epoxy resin samples incorporated with nano-Al2O3 of various concentrations 0, 1, 3 and 5 wt% were prepared. Surface treatment of nano-Al2O3 was performed by the saline coupling agent of γ-aminopropyltriethoxysilane (KH550) to restrict the aggregation. The thermal gravimetric analysis (TGA) of the epoxy resin/Al2O3 nanocomposites was observed at different filler concentrations. TGA analysis indicated that incorporation of nano-Al2O3 increases the decomposition temperature as compared to pure epoxy resin. Moreover, 3 wt% filler concentration showed increased decomposition temperature because nanoparticles form close connections between molecules due to which thermal stability improved at higher temperatures. Atomic force microscopy was employed for the surface morphology of epoxy resin/Al2O3 nanocomposites which has indicated that surface roughness of epoxy resin/Al2O3 nanocomposites increased slightly with the filler concentration. The distribution of trap energy and trap density obtained by isothermal surface potential decay measurements revealed that 1 wt% filler concentration results in decrease in surface potential decay rate. In addition, significant increase in deep trap energy level and trap density was obvious at 1 wt% filler concentration. AC and DC breakdown strength of the samples was measured to evaluate the effect of filler concentrations on the electrical characteristics of epoxy resin. The results showed that breakdown strength of the epoxy resin/Al2O3 nanocomposites increases with the nano-Al2O3 concentration. Moreover, increase in trap energy results in an increase in the breakdown strength of the nanoparticles.

It is well-known that the high-performance polymeric dielectric films used for high-voltage DC capacitors should have outstanding capabilities in terms of electrical and mechanical properties in order to face harsh operating conditions. Many factors limit the ability of these thin films to face different and growing stresses according to modern electrical requirements. Microstructure properties, additives, impurities, defects formed during manufacturing as well as applied stress types significantly affect the performance of dielectric films and their operational lifetime. This paper presents a comprehensive review of the factors which affect the ageing, degradation and breakdown of metallised polypropylene (PP) capacitors films. The effects of microstructure, surface morphological properties, mechanical properties and defects on the reliability of biaxially oriented polypropylene films (BOPP) are studied. In addition, the phenomena affecting dielectric performance and ageing mechanisms which are induced by electrical, thermal and electrothermal stresses are discussed.

In this paper, we present the synthesis of novel three-dimensional SnO2/MoS2 nanocomposites via electrospinning combined with a hydrothermal method and their application to CH4 gas detection. Morphological characterization proved that the flower-like MoS2 structures were successfully connected to the surface of porous SnO2 nanofibers. The CH4 gas-sensing test results indicated that the gas-sensing properties of SnO2/MoS2 nanocomposites were better than those of pure MoS2; the SnO2/MoS2 nanocomposites exhibited lower optimal operation temperature and higher gas response. Such excellent performance is mainly attributable to the special hierarchical structure and the existence of n–n junctions between SnO2 and MoS2.

The effect of direct fluorination on DC breakdown and flashover characteristics of epoxy/Al 2 O 3 nanocomposites was investigated. Pure epoxy resin and epoxy/Al 2 O 3 nanocomposites with 1, 3 and 5 wt% mass fractions were synthesized, and direct fluorination of the samples was carried out at 55 °C for 30 min using a F 2 /N 2 mixture with 20 vol% F 2 at 0.05 MPa. Fourier transform infrared spectroscopy and atomic force microscopy show that molecular chain scission occur during direct fluorination, which may significantly suppress surface charge accumulation while incorporating Al 2 O 3 nanoparticles into epoxy may restrict surface potential decay of fluorinated epoxy resin. The trap characteristics obtained by isothermal surface potential decay show that 1 wt. % sample has deeper traps and higher trap density. DC breakdown and flashover of samples were measured to evaluate the influence of nano filling and direct fluorination on electrical performance of epoxy resin. The results show that epoxy resin with higher DC breakdown and flashover strength can be obtained by simultaneous application of nano filling and direct fluorination. Compared to un-fluorinated epoxy resin, the DC breakdown and flashover voltage of fluorinated epoxy/Al 2 O 3 nanocomposites with mass fraction of 1 wt% improved 17 and 23%, respectively.

Metallized polypropylene film capacitors (MPPFCs) are widely used in Modular Multilevel Converters (MMC), owing to its high energy density self-healing capability, and high reliability. However, the impact of temperature on the dielectric properties of capacitors is usually neglected for the existing research. It is significant to investigate temperature effect on dielectric properties of MPPFC, and for more accurate assessment of insulation condition. This work is oriented to evaluate the dielectric performance of MPPFCs by using LCR meter, at the frequencies from 20 to 105 Hz and the measuring temperature ranging from 20 to 160 °C. The variations of characteristic parameters of the capacitance and dielectric loss at different temperatures are compared. The results show that, for lower temperatures (up to 140 °C), capacitance as well as dielectric loss acquires a constant value and does not change significantly. While at higher temperatures (>140 °C) as it approaches its melting temperature, increment in temperature results in a sharp decrease in dielectric constant. Compared with room temperature samples, the capacitance of sample is dropped by 12.5% for 150 °C and 31% for160 °C. It could provide a reference for the accelerate aging test of metalized film capacitor in the MMC converters.

RTV silicone rubber coatings have been widely applied to insulators because of their good hydrophobicity and anti-fouling performance. However, challenge is presented as pollution flashovers frequently occur on the insulators operating for years along transmission lines due to the hydrophobicity decline and deterioration of their RTV coatings. In this study, superhydrophobic RTV silicone rubber insulator coatings were fabricated on glass substrate using PDMS as base material and fluorosilane-modified silica nanoparticles as hydrophobic fille via simply high-pressure spraying, with 160° contact angle and rolling angle less than 10°. The prepared coating maintains superhydrophobic stability under a harsh environment involving six low (-15°C, 24 h) high (80°C, 24 h) temperature cycles, and it exhibits excellent acid and alkali corrosion resistance. Rather than laboratory research, this presented technique is facile and scalable, capable of large-area superhydrophobic coating production on the complex surface, thereby boding well for great potentials in practical application.

This article presents the results of the studies on positive streamer behavior in non-uniform field the insulating system, in which the free liquid gap is divided on parts by pressboard barrier. The streamer characteristics were assessed mainly on the basis of registration of light emitted during streamer development and additionally on the basis of propagation velocity evaluated. Three liquids were tested during the studies namely mineral oil, synthetic ester and natural ester. A non-uniform electric field was created by point-to-sphere electrode system, a standard practice in laboratory, although very much unlikely taken place in real transformers. Two liquid gaps 20 mm and 40 mm were considered. The studies included LIBV determination of free liquid gaps, acceleration voltage determination in each case and streamer behavior assessment when 4 mm in thick pressboard barrier was placed 10 mm below HV tip. The results showed the differences in LIBV of the liquids considered in favor of natural ester for 20 mm gap and in favor of mineral oil for 40 mm gap. However when comparing acceleration voltage much better properties indicated mineral oil tested, what was expected based on literature data. When introducing the pressboard barrier the streamers significantly changed their characteristics in relation to the system without barrier. In most cases, at the values of the voltage at which the streamers developed as fast, they have been slowed down. Additionally, the differences in breakdown voltage when barrier introducing were significantly reduced. This fact is a benefit in assessment of ester liquids at lightning impulse stresses and in consideration for their use in power transformers.

Epoxy resin is widely used in DC converter valves, DC bushings, and gas-insulated switches due to its excellent insulation and mechanical properties. Since epoxy resin demonstrates low thermal conductivity and toughness, epoxy resin-impregnated power equipment exhibits poor heat dissipation capability and is easy to crack under mechanical stress. Therefore, research on epoxy resins with high thermal conductivity has received extensive research attention. In this article, a directional freezing method is used to build a thermally conductive network with polyimide and boron nitride nanosheets (BNNS) as the skeleton. The thermally conductive network is immersed in the epoxy resin to improve the thermal conductivity of the epoxy resin matrix. The microstructure of PI/BNNS aerogel and epoxy resin/polyimide interpenetrating networks (IPNs) was observed under the electron microscope. The thermal conductivity and dielectric properties of the composite material were tested. The directional growth of ice crystals contributes to the highly orientated tubular pores in the IPNs aerogel. The BNNS aligned along with the polyimide effectively improves the thermal conductivity of the epoxy resin, reaching a thermal conductivity of 0.745W·m−1·K−1, which is 5 times compared to pure epoxy resin with the BNNS content of 15wt%.

In this study, the pollution flashover performance of the resin-based superhydrophobic coating was investigated. Based on an artificial pollution platform, the static water contact angle of the resin-based superhydrophobic coating was evaluated. It was found that the resin-based superhydrophobic coating could maintain superhydrophobicity under different pollution grades. And the pollution flashover voltage suggested an enhanced pollution flashover performance of the resin-based superhydrophobic coating. Additionally, the correlations between the low water adhesion and the enhanced pollution flashover performance were observed. The water droplets were easy to roll off the surface due to the low water adhesion, thereby enlarged the clean area on the superhydrophobic coating. The enlarged dry area promoted insulation strength, thus increased the pollution flashover voltage of the superhydrophobic coating.

The room temperature vulcanized silicone rubber has been widely used in the electric transmission line. However, the accumulated surface charge leads to the flashover, and degradation limits its development. In this paper, room temperature vulcanized silicone rubbers with a rough surface are prepared via a polymer emulsion template method. The wettability, surface potential decay characteristics, and electrical traps distribution of the samples are investigated. The results show that the rough room temperature vulcanized silicone rubber presents a better hydrophobicity with the increases of the addition of nonsolvent. Besides, the rough room temperature vulcanized silicone rubber with higher addition of nonsolvent performs lower initial surface potential, and the surface potential on it decays faster. Additionally, the addition of the nonsolvent contributes to reducing the trap energy level of the room temperature vulcanized silicone rubber. The rough surface is beneficial to suppress the surface charge accumulation on the room temperature vulcanized silicone rubber.

The life expectancy of a transformer is closely related to the ageing state of its paper insulation. This paper presents novel markers for ageing characterization of paper insulation in oil-filled transformers based on the dispersion staining colors of cellulose fibres. Accelerated thermal ageing experiments were performed for oil-paper specimens at 120 and 130 °C to obtain aged oil-paper samples of different ageing degrees. The average degree of polymerization (DP), crystallinity index, and refractive index of insulation paper were measured; the experimental results show that an increase in refractive index of insulation paper was observed with the increase of ageing degree. Meanwhile, the dispersion staining colors of cellulose fibres in oil were observed using dispersion staining method; the experimental results show that the refractive index of cellulose fibres detached from insulation paper increases with the increment of ageing degree of insulation paper. Finally, two ageing markers—that is, the primary color ratio-based ratio of blue to red and the primary color ratio-based Mahalanobis distance were presented. Their relationship with DP and ageing time of insulation paper were obtained, and were compared with the corresponding relationship with 2-furaldehyde (2-FAL) and carbon oxides. The proposed ageing markers demonstrate an advantage over 2-FAL and carbon oxides and show potential in ageing state evaluation and residual life prediction of paper insulation in transformers.

Diagnosis of aging of the paper insulation in transformers has long been of interest to transformer manufacturers and utilities [1]-[3]. Such insulation is irreversibly aged during long-term operation, losing its mechanical strength and becoming brittle. The loss of mechanical strength is an important factor limiting the life expectancy of the transformer [4], [5]. Consequently diagnosis of aging and evaluation of residual life of paper insulation is important for the safe operation, condition-based maintenance, and replacement of power transformers [6]-[9].

Moisture is a significant factor that affects the insulation performance of outdoor high-voltage insulators in power systems. Accumulation of water droplets on insulators causes severe problems such as flashover of insulators and power outage. In this study, we develop a method to fabricate a micro/nano hierarchical super hydrophobic surface. The as-prepared super hydrophobic surface exhibits a water contact angle (WCA) of 160.4 ± 2°, slide angle (SA) less than 1° and surface free energy (SFE) of 5.99 mJ/m 2 . We investigated the electrohydropdynamic behavior of water droplet on a horizontal super hydrophobic surface compared with hydrophobic RTV silicone rubber surface which was widely used as anti-pollution coating or shed material of composite insulator. Results show that water droplet tended to a self-propelled motion on the super hydrophobic surface while it tended to elongate and break up on the RTV surface. The micro/nano hierarchical surface structure and chemical components with low surface free energy of the super hydrophobic surface jointly contributed to the reduction of skin fraction drag and subsequently made it possible for the motion of water droplet driven by electric field. Furthermore, the self-propelled motion of water droplets could also sweep away contaminations along its moving trace, which provides super hydrophobic surface a promising anti-pollution prospect in power systems.

Outdoor electrical equipment in high humidity area is easy to be polluted by parasitic organisms such as moss, which threatens the safety of power grids a lot, while resin-based superhydrophobic coating is a promising anti-pollution material for insulators. In this paper, we measured the water contact angle and the flashover voltage to study the effect of moss on superhydrophobic coatings, and found that the surface of superhydrophobic coating covered with moss turn to be superhydrophilic, and the wet flashover voltage is almost the same as the glass surface covered with moss. However, the dry flashover voltage of former is higher than the later. Therefore, the resin-based superhydrophobic give a relief to the electrical equipment covered with moss.

In this Letter, we report a simple approach for the preparation of bioinspired nacre-like structured materials with achievable high in-plane or through-plane thermal conductivity via digital light processing 3D printing under optimized printing parameters. Based on the 3D layer-by-layer formation, a vertical force exerted on each printing layer during the 3D printing process makes 2D platelets well-ordered in ultraviolet curable resin (hereafter UV resin), which is proved by the images of the scanning electron microscope and spectra of x-ray diffraction. It is found that a lower printing layer thickness leads to a higher orientation of Al2O3 platelets in the UV resin and greater thermal conductivity of the composites. The thermal conductivity of the structured composites reaches up to 2.622 W m−1 K−1 along the oriented direction at the loading of 30 wt. % of 2D Al2O3 platelets under the designed 3D printing layer thickness of 15 μm, which is about 14 times greater than that of pure UV resin. The surface temperature variations of the composites with time during heating and cooling, observed from the infrared thermograph, indicate the great potential of the 3D-printed structured materials for thermal management applications in electronic devices and electric equipment. It is predicted that fillers with greater intrinsic thermal conductivity and a larger diameter than the 3D printing layer thickness will lead to composites with greater thermal dissipation capability.

Nanofluids, formed by adding nanoscale particles to insulating oil, are stable and homogeneous suspensions that present advanced performance of electrical insulation and heat dissipation. This study shows a comprehensive literature review based on the related results of nano-modified insulating oils (both mineral and vegetable insulating oils) to analyse the preparation methods, stability, dielectric spectroscopy, thermal conductivity, breakdown characteristics under AC and DC voltage, the thermal aging performance of nanofluids and oil-paper interaction. Then theoretical models have been introduced to explain the electrical mechanism of nanofluids. In addition, future research is proposed for nanofluids which would be low cost, highly stable and practically applicable to the power transformers.

Nomex paper has been recognized as an excellent insulation material that was evidenced by its electrical properties and been widely used in high voltage insulation system such as dry-type transformers and oil-immersed transformers. Because of its high surface and volume resistivity, the surface charge accumulation on Nomex paper was difficult to dissipate, which was believed to have great influence on its electrical performance. Surface fluorination has been considered to be an effective approach to modify the surface of paper and to suppress the surface charge. To understand the influence of surface fluorination on surface charge characteristic of dry Nomex paper and oil-impregnated Nomex paper, Nomex T410 paper was fluorinated using a F 2 /N 2 mixture with duration of 15, 30 and 45 min. The oil-impregnated samples were prepared by impregnating the untreated and fluorinated Nomex paper in insulation oil. Attenuated total reflection infrared spectroscopy (ATR-IR, Bruker ALPHA) was used to characterize the surface chemical bond changing caused by fluorination. The surface potential decay characteristics were investigated. The ATR-IR spectra of untreated and fluorinated Nomex paper evidence that the C-Fx bonds were induced to Nomex surface by substitution for hydrogen atoms, accompanying with aromatic chain scission and ring opening reaction. The measured surface potential decay characteristics shows that both positive and negative potential decay were accelerated by surface fluorination. For both untreated and fluorinated Nomex paper, the surface potential decay of dry Nomex paper was faster than oil-impregnated Nomex paper. The surface trap energy and trap distribution of the samples were calculated by isothermal surface potential decay (ISPD) method. The result shows that the fluorination may shallow the traps by modify the chemical chain while the oil-impregnated samples had higher trap energy because some charges were trapped by the insulation oil.

The noise emitted from distribution transformer often disturbs the normal life of people around it, hence it is necessary to research the noise characteristics of the vegetable insulating oil transformer. The characteristic of noise emitted by the 10KV vegetable insulating oil transformer under the condition with no load and different loads were tested in a spacious HV testing hall. As a comparison, the vegetable insulating oil in transformer was replaced by mineral insulating oil, and the noise of the transformer filled with mineral oil has also been measured in the same environment. The experiment results demonstrated that A-weighted sound pressure level of noise emitted from the transformer filled with vegetable insulating oil is about 5~6dB lower than that of noise emitted from the transformer filled with mineral insulating oil. Combined with the characteristics of insulating oil and the classical calculation formula of absorption coefficient, the sound absorption coefficient calculating formula of insulating oil has been derived. Though the calculation, it was found that the sound absorption coefficient of FR3 is about 2 times higher than that of mineral insulating oil. Thus vegetable insulating oil FR3 can reduce the transformer noise better than mineral insulating oil.

Polyhedral Oligomeric Silsesquioxane (POSS), a nanometric hybrid molecule, is being paid great attention in the field of polymeric micro and nanodielectrics, providing strong dispersion and dissolution power in solid and liquid dielectric materials which overcomes the sedimentation and agglomeration issues in nanocomposite insulation materials. This work presents the effect of temperature on electrical properties of Octa-isobutyl POSS (Oib-POSS) mineral based nanofluids prepared with two different concentrations 0.1 and 0.15 wt. % and the results are compared with fresh mineral oil. Dielectric properties that include relative permittivity, dissipation factor and volume resistivity along with the AC breakdown voltage are analyzed within a temperature range of 0-80°C in agreement with the IEC standards. Finally, 2-parameter Weibull analysis is performed over the AC breakdown voltage to validate the consistency of breakdown results. It is found that the POSS nanofluids show improved AC breakdown strength and dielectric properties e.g. relative permittivity, dissipation factor and volume resistivity compared with virgin mineral oil in entire temperature range. The results also show that the increase of POSS concentration not only improve the dielectric properties but also enhance the AC breakdown strength of mineral oil.

Pure epoxy resin and epoxy/Al 2 O 3 nanocomposite sheets were prepared and directly fluorinated. The ATR-IR spectrum proves the existence of C-Fx groups in fluorinated samples. Dielectric permittivity increases because of Al 2 O 3 nanoparticles yet fluorination leads to decreasing of permittivity. Results of surface potential decay and surface morphology measurement indicate that combination of fluorination and nano filling significantly improves corona resistance of epoxy resin. Charge injection to sample matrix tends to be restrained by high surface conductivity of fluorinated layer, while nanoparticles may attenuate the mean kinetic energy of negative charge carriers that arriving sample surface, and increase the probability of charge being captured in matrix. In addition, the high bond energy of C-Fx groups and Al 2 O 3 nanoparticles may contributed to the improved resistance of epoxy resin to corona discharge.

Vegetable insulating oils are gaining importance worldwide to be used in power transformers as an alternative to petroleum based mineral oils because of their biodegradable nature and higher fire points than conventional mineral oils. However, these oils (whether natural or synthetic esters) easily absorb moisture and hence tend to affect the dielectric and physiochemical properties of insulation oils. This paper presents the effect of moisture absorbed by commonly used rapeseed oil on its AC breakdown voltage. A mathematical relationship was developed between breakdown voltage and percentage moisture absorbed to access the values of BDV at any moisture level and this relationship holds good for any moisture content in the oil. Moreover, effect of suspended water molecules on breakdown voltage was simulated using COMSOL Multiphysics. Empirical relationship developed was tested and verified and can be used to find out the BDV of these oils as the percentage moisture increases.

Surface modification is an efficient approach to improve the dispersion of alumina nanoparticles. In addition, it is also useful for improving the electrical properties of polymer nanocomposites. In this work, alumina nanoparticles were treated by the saline coupling agent of γ-Aminopropyltriethoxysilane (KH550) to restrict the aggregation. The chemical bonding was examined by Fourier Transform Infrared Spectroscopy (FTIR). Experiment results confirmed that the chemical bond was formed between alumina and KH550 after modification. The modified epoxy/alumina has more traps which results in larger volume resistivity which is 2.22×1017Ω/cm when the alumina content is 1wt%. With increase of the filler concentration, e.g. 3 and 5 wt%, many over lapping interaction zones occur so that the effect of deep traps is gradually weakened and the effect of shallow traps is increased which resulting the decreased volume resistivity. The modified alumina/epoxy resin nanocomposite shows a higher AC breakdown strength as compare to unmodified nanocomposites.

Ageing assessment of paper insulation is important to safe operation, condition based maintenance, and replacement of power transformers. This paper investigated the effect of oil replacement (OR) upon ageing assessment of paper insulation based on dispersion staining colours of cellulose fibres in oil. Accelerated thermal ageing experiments were conducted for oil-paper specimens for different durations and OR was conducted at different ageing stages. Degree of polymerization (DP) of paper and 2-furaldehyde (2-FAL) in oil were measured, and the dispersion staining colours of cellulose fibres were observed. DP of paper was predicted using traditional ageing marker 2-FAL and proposed ageing marker, i.e., primary colour ratio based Mahalanobis distance (PCR-MD) of (r, b) points. Results show that OR has little effect on DP of paper while it can results in substantial loss of carbon oxides and 2-FAL in oil. The OR has limited effect on Mahalanobis distance of (r, b) points. The prediction errors of DP with ageing marker PCR-MD is smaller than those with 2-FAL, which implies that the proposed ageing marker PCR-MD can be applied to ageing assessment of paper insulation that undergoes OR with accepted error level.

Super-hydrophobic surfaces can be fabricated based on discarded silicone after arc exposure. Hydrophobicity, microstructural development, chemical composition, corrosive liquid resistance, and the microscopic process of contaminant elimination of the surface are presented. High temperature of the arc discharge plays a substantial role in the preservation of surface hydrophobicity.

Insulating oil modified by nanoparticles (often called nanofluids) has recently drawn considerable attention, especially concerning the improvement of electrical breakdown and thermal conductivity of the nanofluids. In this paper, three sized monodisperse Fe3O4nanoparticles were prepared and subsequently dispersed into insulating vegetable oil to achieve nanofluids. The dispersion stability of nanoparticles in nanofluids was examined by natural sedimentation and zeta potential measurement. The electrical breakdown strength, space charge distribution, and several dielectric characteristics, for example, permittivity, dielectric loss, and volume resistivity of these nanofluids, were comparatively investigated. Experimental results show that the monodisperse Fe3O4nanoparticles not only enhance the dielectric strength but also uniform the electric field of the nanofluids. The depth of electrical potential well of insulating vegetable oils and nanofluids were analyzed to clarify the influence of nanoparticles on electron trapping and on insulation improvement of the vegetable oil.

This work prepared super-hydrophobic hybrid thin films that are electric heatable. The hybrid film employed candle soot as template and were mechanically consolidate by sputtered silicon-oxide and fluorocarbon layers. The surface morphology and wettability were analyzed by Field Emission Scanning Electron Microscopy (FESEM) and contact angle measurement. The electric heating performance as stably super-hydrophobicity and thermally-manageable capability were characterized by AC voltages with various amplitudes and frequencies. The monitored nonlinear rise of surface temperature and the steady-state temperature were described by prediction equations. It is found that the silicon-oxide layer exhibits considerable importance of keeping super-hydrophobicity stable. The electric heating performance of hybrid thin film is beneficial to anti-icing, supply heating and heat preservation.

Breakdown strength is an important electrical property of insulating paper. Oil-impregnated paper of various aging states was prepared. Its breakdown voltage was then measured at pulsating direct current (DC) voltage with various ripple factors, and alternating current (AC) and DC voltage for comparison, respectively. The AC breakdown voltage is the smallest, and pulsating DC (r = 1/5) breakdown voltage is the greatest before the paper reaches its end of life. A dielectric model was adopted to investigate the difference in magnitude of breakdown voltage at different voltage waveforms. Meanwhile, it was found that breakdown voltage fluctuated and even increased occasionally during the thermal aging process, and a somewhat opposite changing tendency versus aging time was observed for breakdown voltage at DC voltage and pulsating DC voltage with small ripple factors (r = 1/5 and 1/3), compared with AC voltage. The degree of polymerization (DP) and moisture content of the paper were measured, and the characteristics of the pores and cracks of the paper were obtained to investigate the possible influencing factors of breakdown voltage at different aging states. The results showed that the moisture content, oil absorption ability associated with pores and cracks of paper, and the damage to paper structure all contributed to the variation of the breakdown voltage versus aging time, while the importance of their influence differed as the aging state of paper varied.

This work introduces a super-hydrophobic hybrid thin film which shows the capability of thermal management. The hybrid thin film was obtained by sputtering a fluorocarbon layer on a glass substrate that has been pre-deposited by a layer of soot. Multiple characterization methods, e.g. Energy Dispersive Spectrometer (EDS), Static Contact Angle Measurement and Field Emission Scanning Electron Microscopy (FE-SEM) were employed to detect the samples' surface morphology, weight percentages of elements and super-hydrophobicity. Soot films were introduced as contrast samples to ensure the improved capability of thermal management and super-hydrophobicity for soot/fluorocarbon films. Different thickness of soot/fluorocarbon hybrid thin films were fabricated to investigate the changes of super-hydrophobic characteristics before and after electrical heating. The temperature of soot/fluorocarbon hybrid thin films was manageable to be elevated and maintained at a certain value when they were electrically activated, which shows a prospect of reducing ice/frost accumulation.

The vegetable insulation oil has been widely used in distribution transformers and extended to high voltage power transformers. In order to make the vegetable insulation oil well used in the HVDC applications, it is necessary to study the ion mobility of vegetable insulation oil under different influencing factors. In this paper, we measured the ion mobility of insulation oil using the reversal polarity method, and discussed the influences of voltage amplitude on the measurement results. The properties of ion mobility for vegetable insulation oil were measured at different temperatures and we also measured the ion mobility of the different moisture content in vegetable insulation oil. The results show that the obtained mobility values are independent of the voltage level, when the measurements were performed at low test voltages. The ion mobility of vegetable insulation oil increase with the rising temperature, which satisfies the Arrhenius relationship with the reciprocal of temperature. The moisture content of vegetable insulation oil has some impact on the positive ion mobility.

Vegetable insulating oil is fire-resistant and environmental-friendly liquid dielectrics, which offers excellent electrical and physiochemical properties and has been successfully used in distribution transformers. Vegetable insulating oil differs from mineral insulating oil in their molecular structures, components, physicochemical and electrical properties. This lead to that the two kinds of oil have different acid composition. At the same time, vegetable oil-paper insulation will be affected by many factors such as heat, moisture and oxygen, which makes acidic substance produce more complex. Therefore, this paper explores the change law of acid value in vegetable insulating oil under the condition of hydrolysis, oxidation and aging, studies the influence of different acid value on breakdown performance on positive and negative polarity in small gaps under lighting impulse. The results show that acid value has a significant impact on negative polarity, the breakdown voltage will decrease with the increase of acid value, while it has no obvious influence on positive polarity in small gaps under lighting impulse.

In this Letter, we developed a sprayable superhydrophobic coating with micro-sized disorder indentations to survey the self-ejections of isolated slushes on it during the defrosting process. The microstructures, chemical composition, hydrophobic characteristics, and self-ejection phenomenon of melting slushes on grooved superhydrophobic surfaces are presented. The grooved superhydrophobic surface demonstrates that multiple self-ejections of isolated melting slush off the original locations with no ice bridges or great surface energy release. In addition, the self-ejection of multiple isolated slushes observed generates enough kinetic energy and removes the residual melting slushes in ways of sweeping off. It is also found that the irregular melting slush with a greater deformation energy and surface contact area demonstrates shorter jumping distances compared to that with a spherical shape and low surface contact area. The observed short-distance self-ejection results from the defects of micro-pores on the indentations, leading to great dissipation in vapor pressures and reduced impact from volume fluctuations. Both the volume fluctuation of slush and the evaporation of intermediate liquid generate the pressure gradient in the upward direction and contribute to the self-ejection behavior of isolated melting slush. The results demonstrate the necessity of fabricating grooved superhydrophobic surfaces without micro-pores and conceptual feasibility of employing volume fluctuation of slush for the self-ejection of isolated single melting slush in the case of slushes with no ice bridges, small surface energy, and low inner vapor pressures.

Superhydrophobic surface, inspired by lotus leaf, has aroused intensive interest in research and application on insulators and transmission lines. To improve the superhydrophobicity of composite insulators, they were subjected to surface fluorination in a laboratory vessel using F 2 /N 2 mixture (20% F 2 in vol. %) under 0.1Mpa for 30 min at 25, 40, and 55° C, respectively. A dual nano- and micro-scale hierarchical structure is observed on fluorinated composite insulators which was identified by field emission scanning electron microscope. Surface wettability and surface energy was evaluated by contact angle measurements and surface energy calculation. The fluorination was found to leading the composite insulators' surface with much enhanced superhydrophobicity. For example, the water contact angle elevated from 105 ° to values up to 162 ° after fluorination. The fluorination enables composite insulators removing certain amount of surface contaminants with a water droplet. In addition, the superhydrophobic surface shown good mechanical resistance by no obvious losing of superhydrophobicity after wearing test.

The oil impregnated paper is one of the typical composite structure containing a large number of cavities, which are filled with insulating oil through oil impregnation. In this paper, the oil-impregnated paper structure is simplified and an electrical breakdown model is developed to describe the breakdown progress in oil impregnated paper at pulsating DC voltage. Based on the traditional WZ model, the space discharge and physical time are introduced in the proposed model. The influence of temperature is also investigated. The skeleton types of discharge structure, discharge velocity, and charge movement during the breakdown are simulated. The breakdown simulation results show that the conductivity variation of insulation paper is an important factor which reduces the breakdown voltage under pulsating DC voltage at high temperature.

This paper focuses on the dielectric properties in TiO 2 /XLPE nanocomposites, the TiO 2 nanoparticles were added into XLPE matrix with different mass concentrations (1wt%, 3wt%, and 5wt%). It was found that the volume resistivity of TiO 2 /XLPE nanocomposites was higher than XLPE, which increased with filling concentrations. In addition, the TiO 2 /XLPE nanocomposites had lower permittivity and lower dielectric dissipation, which increased with the filling concentrations and decreased with the measured frequency. The results of pulsed electro-acoustic (PEA) measurements showed that the total space charge was suppressed and heterocharge disappeared in TiO 2 /XLPE nanocomposites, leaving a small amount of negative charge in middle of samples. According to the analysis in this paper, new interface regions introduced many deep traps due to filling TiO 2 nanoparticles, which can shorten the effective distance of “solitary waves” migration and greatly reduce carrier mobility. On the other hand, low carrier mobility enhance the recombination process and weaken the ionization process of impurities, leading to the disappearance of heterocharge in TiO 2 /XLPE. Furthermore, the interface reverse electric field not only suppress the charge injecting from the electrodes, but also strengthen the electric field in the middle of matrix, so that the accumulated space charge can be transferred out of material timely.

The insulating paper in the transformer undergoes degradation during operation. Condition monitoring and assessment for insulating paper is of great significance as the life of insulating paper mainly determines that of transformers. Accelerated thermal aging experiments were performed for insulating papers at 120 °C and 130 °C, respectively. The crystallinity index of insulating paper was calculated with the X-ray diffraction pattern and the refractive index of insulating paper was measured. The results showed that the crystallinity index and refractive index of the insulating paper increase with the decrease of degree of the polymerization in general. The above work is a fundamental attempt in investigating the correlation between paper degradation and its crystalline behavior, which indicates that the crystallinity index and refractive index of the cellulose fibers in insulating oil may be a feasible index to diagnose the paper degradation.

This paper focuses on the space charge characteristics in TiO2/cross-linked polyethylene (XLPE) nanocomposites; the unmodified and modified by dimethyloctylsilane (MDOS) TiO2 nanoparticles were added to XLPE matrix with different mass concentrations (1 wt%, 3 wt%, and 5 wt%). The scanning electron microscope (SEM) showed that the MDOS coupling agent could improve the compatibility between TiO2 nanoparticles and XLPE matrix to some extent and reduce the agglomeration of TiO2 nanoparticles compared with unmodified TiO2 nanoparticles; the volume resistivity testing indicated that the volume resistivity of TiO2/XLPE nanocomposites was higher than Pure-XLPE and increased with the increase of filling concentrations. According to the pulsed electroacoustic (PEA) measurements, it was concluded that the space charge accumulation was suppressed by filling TiO2 nanoparticles and the distribution of electric field in samples was improved greatly. In addition, it was found that the injection of homocharge was more obvious in MDOS-TiO2/XLPE than that in UN-TiO2/XLPE and the homocharge injection decreased with the increase of filling concentration.

The piezoelectricity of a pressure-treated cellular polypropylene (PP) (commercially available, trade name PQ50) film electret was studied by the measurement of direct-and inverse-piezoelectric d 33 coefficient. The sample expanded with optimal parameters has a quasi-static piezoelectric d 33 coefficient of more than 600 pC/N, which is about 40 times as high as that of polyvinylidene fluoride (PVDF). In addition, the hybrid multi-layer system, which properly combines single-layer cellular PP film electrets, shows a quasi-static piezoelectric sensitivity of as high as 2010 pC/N. This is around three times higher than that of well-known lead zirconate titanate (PZT) ceramics. The results are theoretically and technically helpful to promote the application of cellular PP film electrets.

CaCu3Ti4O12 ceramics were synthesized by using the solid-state reaction method, and the roles of the heat treatment on the microstructure and dielectric properties were investigated. It is indicated that no second phase is found after heat treatment. At room temperature, the dielectric constant is decreased slightly and the dielectric loss tangent is effectively suppressed to 0.03–0.04. In the temperature range of 183–273 K, the dielectric loss of CaCu3Ti4O12 ceramics is composed of conductance loss at low frequency and two relaxation loss peaks at high frequency. It is worth noting that the conductance loss at low frequency is reduced apparently by the heat treatment. Furthermore, the results of J-E exhibit that the breakdown field and nonlinear coefficient of CaCu3Ti4O12 ceramics are increased from 226 V/mm and 3.52 to 397 V/mm and 4.51, respectively. The Schottky barrier height of CCTO ceramics is boosted from 0.56 eV to 0.63 eV.

Poly(vinylidene fluoride) (PVDF) has been recently recognized as an attractive candidate for energy-conversion applications that are fabricated on its piezo- and pyroelectric activities. In this work, Commercial poly(vinylidene fluoride) (PVDF) films are uniaxially stretched with varying rates at 110 °C in order to endow PVDF piezo- and pyroelectric mainly via crystalline-phase transition from nonpolar α to polar β that are identified by infrared (IR) spectroscopy and X-ray diffraction (XRD). The fast stretching promotes the α-to-β phase transition and the quite high β-phase fraction in films resultantly. Additionally, fast stretching benefits to producing a slightly increase of γ phase. Sufficient high electric field applied during poling results in α-to-β phase transition according to the XRD observations. The quantified evaluation on XRD data illustrates that the poling accounts for size-increase and better chain-packing of the β-phase crystallites. The energy-conversion performance of the stretched films that were impacted by water drops as well its relation to the piezoelectric-voltage activity was evaluated by measuring the peak-voltage outputs.

In this paper, the electret properties of three types of castings prepared P(VDF-HFP) (poly(vinylidene fluoride-hexafluoropropylene)) copolymer films whose contents of HFP are 4.2, 4.7 and 5.9% are discussed. Experimental results point out that the copolymer film is a polar material, i.e. there exists both oriented dipoles and deposited space charge in the film after charging. Estimating polarizations at different charging fields based on analysis of charging current curves indicate a comparatively high polarization formed under the charging field above 200 MVm-1 for the copolymer film. The mechanically thermal stretching process not only increases the density of the oriented dipoles and deposited space charge but also improves the thermal stability of deposited space charge. However, the increase of HFP content in the copolymer has a negative impact on the density of oriented dipoles and deposited space charge. The measurements by means of laser doppler vibrometer show that the copolymer in which the HFP content is 4.2% and 4.7%, respectively has a notable converse piezoelectric coefficient as high as 140 pmV-1 and 130 pmV -1 . Dynamics of interaction of space charge with oriented dipoles for the copolymer with HFP content of 4.7% are investigated by means of isothermal surface potential decay at different temperatures. At the same time, pyroelectric effect of the copolymer is also discussed

In this paper, the influence of porosity on the stability of charge and piezoelectric d33 coefficient for porous PTFE electrets is studied by using measurements of isothermal surface potential decay, quasi-static direct piezoelectric coefficient, elastic (compression) modulus and a leakage current through porous and non-porous PTFE samples during corona charging, analyses of open-circuit thermally stimulated discharge current spectra, X-ray diffraction spectra and differential scanning calorimetry. The results point out that with the increase of porosity, the stability of charge deposited in the film and the piezoelectricity of the film are improved in a certain range of porosity. The reasons for the influence of porosity on the thermal stability of the charge storage and piezoelectricity are also investigated. Our results also demonstrate that the improvement of the thermal stability of the piezoelectric coefficient at increasing porosity is due to improved thermal charge stability and the improved stability of the mechanical properties of the material. At the same time, the limitation of the improvement of charge storage and piezoelectricity of a PTFE film with very high porosity is discussed.

Uniaxial stretching of P(VDF-HFP) films with optimized stretching parameters does not significantly increase the elastic modulus at room temperature, but induces an enhanced electric polarization. This may explain their outstanding electro-mechanical properties. The increase of polarization stems from the formation of a ferroelectric phase, an additional polarization in the amorphous phase and from melting of secondary crystallites formed in the amorphous phase by annealing or storage.

A process for preparing three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films is introduced. Samples are made from commercial FEP films by means of laser cutting, laser bonding, electrode evaporation, and high-field poling. The observed dielectric-resonance spectra demonstrate the piezoelectricity of the FEP sandwiches. Piezoelectric d 33 coefficients up to a few hundred pC/N are achieved. Charging at elevated temperatures can increase the thermal stability of the piezoelectrets. Isothermal experiments for approximately 15 min demonstrate that samples charged at 140°C keep their piezoelectric activity up to at least 120°C and retain 70% of their initial d 33 even at 130°C. Acoustical measurements show a relatively flat frequency response in the range between 300 Hz and 20 kHz.