Search Results (6)

Accurately detecting defect-induced photon emissions enables early defect detection and characterization. To address this, a defect evolution state recognition model based on phase-resolved photon counting and dimensionality reduction calculations is proposed under alternating current (AC) excitation. Initially, photon information from protruding metal defects simulated using needle–plane electrodes during partial discharge (PD) evolution is analyzed in SF₆. Subsequently, phase-resolved photon counting (PRPC) techniques and statistical analysis are employed to extract feature parameters for quantitative characterization of defect-induced photon responses. Finally, a t-distributed stochastic neighbor embedding (t-SNE) dimensionality reduction analysis is utilized to establish criteria for categorizing defect evolution states. The findings reveal that metal-particle-triggered optical PRPC maintains the obvious polarity effect, and the entire evolution of the discharge can be divided into three processes. These research findings are expected to advance the accurate assessment of operational risks in gas-insulated systems. Full article

Oil-impregnated paper condenser transformer bushings are an important part of transformer equipment, and partial discharge (PD) occurred when defects exist on the condenser aluminum foil layers. Firstly, to study the PD process of the oil-paper insulated capacitance graded bushing with the defect of broken aluminum foil, a defective oil-paper bushing discharge sample is constructed to study the PD parameters and capacitance, and to discharge carbonization traces at different voltage levels. Then, in order to verify the process of condenser aluminum foil layer discharge and the space charge variation in the oil-paper insulation system of a sample model, the surface flashovers of a needle-plane discharge model based on the bipolar charge transport model and the hydrodynamic model was built. The simulation, by Transport of Diluted Species physics of COMSOL Multiphysics software, points out the discharge process of aluminum foil electrode caused by space charge action and electric field distortion under an electric field at different voltages. The results of simulation and sample bushing experiments showed that the PD process of the defective condenser foil layer is mainly divided into three stages: tip corona discharge, streamer in oil, and surface flashovers. The voltage amplitude is larger the more electrical branches are discharged and the shorter the discharge time is. The findings of the article have important implications for the discharge of the foil layer inside the oil-paper bushing. Full article

Manufacturing or assembly defects in gas-insulated equipment can introduce field enhancements that could lead to partial discharge (PD). This paper examines the PD characteristics of SF₆ alternatives considered for potential application to retro-filling existing SF₆-designed equipment. The PD performance of the C₃F₇CN/CO₂ gas mixture and SF₆ were characterised adopting the ultra-high frequency (UHF) method and investigated for different defect configurations, pressures, and gas mediums. Hemispherical rod-plane and plane-to-plane configurations with needle on the high-voltage (HV) and ground electrodes were used to mimic conductor and enclosure protrusion defects, respectively. The results demonstrate that with a needle length of 15 mm, the 20% C₃F₇CN/80% CO₂ gas mixture had almost half the partial discharge inception and extinction voltages (PDIV/EV) of SF₆. For less divergent fields, the 20% C₃F₇CN/80% CO₂ gas mixture demonstrated a comparable PDIV/EV performance as SF₆. The phase-resolved PD patterns of the 20% C₃F₇CN/80% CO₂ gas mixture demonstrated a 3-stage transition phase that was not observed with SF₆, which could be due to the discharge mechanism of the weakly attaching CO₂ gas used within the mixture. Full article

Graphite microcrystals are the product of coal graphitization and widely exist in the graphitized coal of Yongan Coalfield, Fujian Province, China, which is direct mineralogical evidence for the transformation of coal to graphite. Optical microscopy, scanning electron microscopy (SEM) and micro-Raman spectroscopy were used to detect the morphology and microstructure of the in situ graphite microcrystals. The results show that the volume proportion of graphite microcrystals in graphitized coal samples is between 2.39% and 7.32%, and the optical anisotropy of graphite microcrystals is stronger than that of coal macerals. Graphite microcrystals show the occurrence of attached microcrack inner walls or infilling the cell cavity, with several forms of flakes, needles or aggregates. Under optical microscopy of polarized light and with a retarder plate of 1λ, graphite microcrystals show the color of primary yellow and secondary blue, and the two kinds of colors appear alternately when the microscope is rotating. Additionally, flake-like graphite microcrystals with an isochromatic zone diameter of 10−50 μm are the most widely distributed in graphitized coal. Under SEM, graphite microcrystals show a rough and irregular edge and are characterized by flow or bubble film structures along with several pores, which is the product of cooling crystallization after the softening and melting of carbon-containing substances. Moreover, flake-like graphite microcrystals developed interlayer pores with a clear outline of loose stacking and were almost entirely composed of pure carbon; a small amount of oxygen is related to oxygen-containing functional groups or structural defects. The micro-Raman spectra of graphite microcrystals in the first-order region are characterized by low-intensity D1 and D2 bands and a high-intensity G band, and the parameters R1 and R2 vary from 0.21–0.39 and 0.60–0.74, respectively. The second-order micro-Raman spectra of graphite microcrystals are characterized by a higher intensity of the 2D1 band and a lower intensity of the other three bands. The parameter R3, derived from the area ratio of the 2D1 band to all the bands in the second-order region, was proposed. The value of R3 ranges between 0.78 and 0.86, and both of them indicate a higher percentage of graphene plane with a highly internal crystallographic structure. Similar to the parameters R1 and R2 in the first-order micro-Raman spectrum, the parameter R3 is an effective parameter to characterize the ordering degree of the microstructure, which may be used to evaluate the graphitization degree of graphitization coal. Full article

Metal nanoparticles (NP) in minerals are an emerging field of research. Development of advanced analytical techniques such as Z-contrast imaging and mapping using high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) allows unparalleled insights at the nanoscale. Moreover, the technique provides a link between micron-scale textures and chemical patterns if the sample is extracted in situ from a location of petrogenetic interest. Here we use HAADF STEM imaging and energy-dispersive X-ray spectrometry (EDX) mapping/spot analysis on focused ion beam prepared foils to characterise atypical Cu-As-zoned and weave-twinned hematite from the Olympic Dam deposit, South Australia. We aim to determine the role of solid-solution versus the presence of discrete included NPs in the observed zoning and to understand Cu-As-enrichment processes. Relative to the grain surface, the Cu-As bands extend in depth as (sub)vertical trails of opposite orientation, with Si-bearing hematite NP inclusions on one side and coarser cavities (up to hundreds of nm) on the other. The latter host Cu and Cu-As NPs, contain mappable K, Cl, and C, and display internal voids with rounded morphologies. Aside from STEM-EDX mapping, the agglomeration of native copper NPs was also assessed by high-resolution imaging. Collectively, such characteristics, corroborated with the geometrical outlines and negative crystal shapes of the cavities, infer that these are opened fluid inclusions with NPs attached to inclusion walls. Hematite along the trails features distinct nanoscale domains with lattice defects (twins, 2-fold superstructuring) relative to hematite outside the trails, indicating this is a nanoprecipitate formed during replacement processes, i.e., coupled dissolution and reprecipitation reactions (CDRR). Transient porosity intrinsically developed during CDRR can trap fluids and metals. Needle-shaped and platelet Cu-As NPs are also observed along (sub)horizontal bands along which Si, Al and K is traceable along the margins. The same signature is depicted along nm-wide planes crosscutting at 60° and offsetting (012)-twins in weave-twinned hematite. High-resolution imaging shows linear and planar defects, kink deformation along the twin planes, misorientation and lattice dilation around duplexes of Si-Al-K-planes. Such defects are evidence of strain, induced during fluid percolation along channels that become wider and host sericite platelets, as well as Cl-K-bearing inclusions, comparable with those from the Cu-As-zoned hematite, although without metal NPs. The Cu-As-bands mapped in hematite correspond to discrete NPs formed during interaction with fluids that changed in composition from alkali-silicic to Cl- and metal-bearing brines, and to fluid rates that evolved from slow infiltration to erratic inflow controlled by fault-valve mechanism pumping. This explains the presence of Cu-As NPs hosted either along Si-Al-K-planes (fluid supersaturation), or in fluid inclusions (phase separation during depressurisation) as well as the common signatures observed in hematite with variable degrees of fluid-mineral interaction. The invoked fluids are typical of hydrolytic alteration and the fluid pumping mechanism is feasible via fault (re)activation. Using a nanoscale approach, we show that fluid-mineral interaction can be fingerprinted at the (atomic) scale at which element exchange occurs. Full article

Partial discharge (PD) behaviors of oil-paper insulation is distinctive in AC and DC combined electric fields in converter transformers from PD behaviors in pure AC or DC electric fields. The present study focuses on the PD developing processes and characteristics of oil-paper insulation systems with needle-plane defects under different AC/DC proportions. The degradation of oil-paper insulation can be accelerated by PD pulses incurred by needle-plane defects. AC-DC combined voltages are applied to the needle-plane defect model simultaneously in the established experimental platform, and the proportions of AC/DC voltages are decided according to the cases in actual converter transformers. The developing processes from the initiation of partial discharge until final breakdown were observed for each AC/DC proportion. PD parameters and patterns were acquired by a detector using the pulse current method. The test results indicate that the inception and breakdown voltages increase with the increase of the DC component in AC-DC combined voltages. However, pulse repetition rate and amplitude of PD shows a descending trend when AC/DC proportion decreases. Meanwhile, the PD recurrence rate in the phase between 180° and 360° becomes higher than that in the phase between 0° and 180° at the initial stage as the DC proportion increases; high-amplitude discharges mainly occur in the phase range between 180° and 360° when the pressboard is close to breakdown. The current study is useful in further research on fault diagnosis in converter transformers. Full article