235 research outputs found
Temperature gradient effect on the space charge behaviour in multilayers of oil and pressboard
No full textThe presence of space charge within oil and pressboard (PB) can lead to distortion of electric field within converter transformers. Electric field distortion can cause materials to be overstressed which accelerates degradation, and can result in breakdown. Therefore, it is important to analyse the factors that affect space charge formation and dissipation such as temperature, moisture, ageing, multilayers and electric fields. This paper focuses on the temperature gradient and multilayers of oil and PB on space charge. Considering the inhomogeneous acoustic wave velocity caused by a temperature gradient and different dielectric properties of multilayers of materials, an improved space charge recovery algorithm has been developed, in which the distortion of the acoustic wave caused by the attenuation, dispersion, different dielectric properties and temperature effects have been considered. It has been found that for two layers consisting of PB and oil with the bottom electrode fixed at 20 ᵒC, the electric field in the oil increases under the temperature gradient while decreases at ambient temperature. For a three layered sample consisting of PB, oil, and PB with the bottom electrode at ambient temperature, the electric field gradient exists under the temperature gradient and the maximum electric field occurs near the cathode while occurs near the anode at ambient temperature
Comparison of two models on simulating electric field in HVDC cable insulation
No full textSpace charge accumulation in cable insulation is one of the major technical problems in the further development of HVDC cables. A conductivity model and a bipolar charge transport model are developed to respectively calculate the space charge and electric field distribution in polymeric insulation. In this paper, both models are employed to simulate the field distribution in a medium voltage polymeric cable. Comparisons are made between theoretical and simulation results. The limitations of the conductivity model which is widely used in HVDC cable design are presented, and the results of the bipolar charge transport model are more consistent with the experimental observations. Moreover, transient current in the cable is simulated to anticipate the field distribution within the insulation when subjected to a thermal transient. The results suggest that the thermal transient can affect the space charge and electric field distribution significantly. A field inversion can only take place with higher temperature and larger temperature gradient, and this can be maintained even with temperature decreasing
Study of DC breakdown in multilayered insulation systems
No full textMultilayer insulation systems have been widely used in both high voltage alternating current (HVAC) and high voltage direct current (HVDC) power systems. A well designed multilayer insulation structure could benefit the performances and reliability of the overall insulation system and as well as costs reduction. In this present work, DC breakdown strengths of various combinations of multilayer polyethylene terephthalate (PET) films (from one to four layers) with a same total thickness were investigated. It has been found that multilayer symmetrical structures can enhance the breakdown strength of the insulation system. Space charge characteristics of these various configurations have been initially studied, in order to understand the results of the DC breakdown tests. The space charge results indicate that large amount of fast moving charges presences immediately after the voltage application, which can be partially blocked and trapped by the interfaces between film layers. The trapped charges could further distort the electric field distributions in the insulation system, and resulting field enhancement in thin layers, particularly in unsymmetrical configurations. However, more evidences and investigations are required for better understanding the mechanisms of breakdown in multilayer insulation system.</p
Modelling space charge in HVDC cable insulation
No full textThe design of high-voltage direct-current extruded cable is one of the most challenging issues in the cable industry, as the electric field distribution across the insulation can be strongly affected by the presence of space charge, which can subsequently affect its long-term reliability and life expectancy. In this study, the bipolar charge transport model was utilized to calculate space charge and field distribution in a polymeric cable insulation, and the result was compared with the one obtained by the conductivity model which is commonly used in the cable industry. It is shown that the simulation results of the bipolar charge transport model are more comparable with the previous experimental work, and the shortcomings of the conductivity model are presented. At last, the feasibility and potential issues of the new method are discussed for further development
Advanced High-Temperature Alloys and Oxides: Design, Performance, and Optimization of Complex Concentrated Alloys and Thermally Grown Oxides
No full textThe increasing demands for high-temperature (HT) structural materials in energy, aerospace, and waste-to-energy industries have driven research into advanced alloys capable of withstanding extreme environments. This study investigates the oxidation, hot corrosion resistance, and thermodynamic stability of commercial alloys, complex concentrated alloys (CCAs), and thermally grown complex concentrated oxides (TG-CCOs) for HT applications.
The research begins by evaluating the hot corrosion performance of Fe- and Ni-based commercial alloys in simulated combustion environments, identifying their limitations at 850°C under Na₂SO₄-NaCl salt mixtures. The study then shifts focus to Fe-Cr-Ni multi-principal element alloys (MPEAs), which exhibit improved corrosion resistance at 700–900°C. Element selection plays a pivotal role in optimizing hot corrosion resistance in MPEAs. It is important to avoid using elements with high corrosion activity, such as Mn, and to reduce the relative contents of Fe and Ni. This is essential to control the formation of unprotective corrosion layers and prevent the decomposition of the alloy.
Phase stability and diffusion mechanisms during oxidation of FeCrNi medium entropy alloys (MEAs) are analyzed through experimental observations and density functional theory (DFT) calculations. Key findings include the importance of temperature-dependent transformation from FCC to BCC and Cr diffusion in forming protective oxide layers, enhancing HT oxidation resistance. The study on diffusivities, phase structures, and oxidation behavior in the FeCrNi alloy establishes a strong foundation for advancing HT alloys with enhanced oxidation resistance. The hot corrosion resistance primarily depends on the thermally grown oxide layer (TGO) and the phase stability of the structural base alloy. Therefore, this study also explores the corrosion resistance and structural stability of TG-CCOs on a novel AlCrTiVNi5 alloy. These TG-CCOs demonstrate remarkable thermodynamic stability and corrosion resistance, with a significant structural transition from rock-salt to corundum oxides under HT conditions. Moreover, lattice distortion assessments suggest that these structural changes enhance the durability of the oxides. These findings establish TG-CCOs as promising candidates for next-generation protective coatings.
This comprehensive study addresses critical gaps in materials science by providing insights into the design, performance, and optimization of CCAs and TG-CCOs. The outcomes contribute to advancing HT structural materials, offering significant potential for improving energy efficiency, reducing emissions, and extending the service life of components in extreme environments
Numerical Approximation of Elasticity Tensor Associated With Green-Naghdi Rate
No full textAbstract
Objective stress rates are often used in commercial finite element (FE) programs. However, deriving a consistent tangent modulus tensor (also known as elasticity tensor or material Jacobian) associated with the objective stress rates is challenging when complex material models are utilized. In this paper, an approximation method for the tangent modulus tensor associated with the Green-Naghdi rate of the Kirchhoff stress is employed to simplify the evaluation process. The effectiveness of the approach is demonstrated through the implementation of two user-defined fiber-reinforced hyperelastic material models. Comparisons between the approximation method and the closed-form analytical method demonstrate that the former can simplify the material Jacobian evaluation with satisfactory accuracy while retaining its computational efficiency. Moreover, since the approximation method is independent of material models, it can facilitate the implementation of complex material models in FE analysis using shell/membrane elements in abaqus.</jats:p
Air Pollution Characteristics and Health Risks in the Yangtze River Economic Belt, China during Winter
No full textThe air pollution characteristics of six ambient criteria pollutants, including particulate matter (PM) and trace gases, in 29 typical cities across the Yangtze River Economic Belt (YREB) from December 2017 to February 2018 are analyzed. The overall average mass concentrations of PM2.5, PM10, SO2, CO, NO2, and O3 are 73, 104, 16, 1100, 47, and 62 µg/m3, respectively. PM2.5, PM10, and NO2 are the dominant major pollutants to poor air quality, with nearly 83%, 86%, and 59%, exceeding the Chinese Ambient Air Quality Standard Grade I. The situation of PM pollution in the middle and lower reaches is more serious than that in the upper reaches, and the north bank is more severe than the south bank of the Yangtze River. Strong positive spatial correlations for PM concentrations between city pairs within 300 km is frequently observed. NO2 pollution is primarily concentrated in the Suzhou-Wuxi-Changzhou urban agglomeration and surrounding areas. The health risks are assessed by the comparison of the classification of air pollution levels with three approaches: air quality index (AQI), aggregate AQI (AAQI), and health risk-based AQI (HAQI). When the AQI values escalate, the air pollution classifications based on the AAQI and HAQI values become more serious. The HAQI approach can better report the comprehensive health effects from multipollutant air pollution. The population-weighted HAQI data in the winter exhibit that 50%, 70%, and 80% of the population in the upstream, midstream, and downstream of the YREB are exposed to polluted air (HAQI > 100). The current air pollution status in YREB needs more effective efforts to improve the air quality
Non-symmetric hybrids of noble metal-semiconductor: Interplay of nanoparticles and nanostructures in formation dynamics and plasmonic applications
No full textQuantifying the Impact of Aerosols on Geostationary Satellite Infrared Radiance Simulations: A Study with Himawari-8 AHI
No full textAerosols exert a significant influence on the brightness temperature observed in the thermal infrared (IR) channels, yet the specific contributions of various aerosol types remain underexplored. This study integrated the Copernicus Atmosphere Monitoring Service (CAMS) atmospheric composition reanalysis data into the Radiative Transfer for TOVS (RTTOV) model to quantify the aerosol effects on brightness temperature (BT) simulations for the Advanced Himawari Imager (AHI) aboard the Himawari-8 geostationary satellite. Two distinct experiments were conducted: the aerosol-aware experiment (AER), which accounted for aerosol radiative effects, and the control experiment (CTL), in which aerosol radiative effects were omitted. The CTL experiment results reveal uniform negative bias (observation minus background (O-B)) across all six IR channels of the AHI, with a maximum deviation of approximately −1 K. Conversely, the AER experiment showed a pronounced reduction in innovation, which was especially notable in the 10.4 μm channel, where the bias decreased by 0.7 K. The study evaluated the radiative effects of eleven aerosol species, all of which demonstrated cooling effects in the AHI’s six IR channels, with dust aerosols contributing the most significantly (approximately 86%). In scenarios dominated by dust, incorporating the radiative effect of dust aerosols could correct the brightness temperature bias by up to 2 K, underscoring the substantial enhancement in the BT simulation for the 10.4 μm channel during dust events. Jacobians were calculated to further examine the RTTOV simulations’ sensitivity to aerosol presence. A clear temporal and spatial correlation between the dust concentration and BT simulation bias corroborated the critical role of the infrared channel data assimilation on geostationary satellites in capturing small-scale, rapidly developing pollution processes
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