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The mechanisms of lath decomposition and dispersion were hitherto rarely explained due to limited understanding into the heterostructure deformation between two phases and associated interactions. In this work, the lath decomposition and dispersion were systematically investigated in a wide subtransus processing range of 0.001 s−1–1 s−1 and 745 °C–820 °C with the consideration into the heterostructure deformation and associated interactions. The relationship between flowing behavior and microstructure evolution were discussed. Results indicated that the initial buckling of α laths caused high peak strength. The rotation and twist of α laths are helpful to decrease the flow stress, but it caused serious extra deformation and dislocation storage in the β matrix. The lath decomposition dominated by the ablation was greatly contributed by the adiabatic temperature rise (ATR) near boundaries with high energies. The α laths could rotate under low strain rates, and numerous residual laths (RLs) could be obtained. The increment in strain rates to 0.1 s−1 resulted in earlier lath decomposition and softening, as well as to disperse α p particles and homogenize the β matrix. But an overhigh strain rate of ∼1 s−1 caused adiabatic shear bands. The low temperatures are unfavorable to the lath decomposition, while over-high temperatures diminished the extra deformation and ATR. On this basis, a deformation mechanism map for the subtransus processing of lamellae Ti-55531 alloy was established. This investigation sheds deeper insight into the tailoring of microstructure and prediction of flowing behavior for the subtransus processing of lamellae metastable-β Ti alloys. [ABSTRACT FROM AUTHOR]

Polyimide film (PI) is widely used as dry-type smoothing reactor insulation for ultra-high voltage direct current (UHVDC) projects. In practice, the temperature of a central conductor can reach 80 °C to 100 °C at the rated power flow. Thus, the inner insulation composed of PI sustains these high temperatures. In this study, the space charge characteristics of PI at different temperatures (40 °C, 60 °C, and 80 °C) and thermal aging degrees (336, 500, 1000, and 1500 h) were examined with an improved pulsed electro-acoustic (PEA) measurement system to investigate the effects of temperature and thermal aging on dielectric performance. The apparent trap depths corresponding to various aging degrees were obtained according to the space charge decay dynamics at the interface. Results indicate that positive charges likely accumulate within the bulk of the material because of the thermal aging of homogeneous materials. The polarity of space charge at the interface also changes from positive to negative as aging degree increases. The space charge decay dynamics at the interface further shows that the trap energy increases as the thermal aging degree increases. The amount of residual space charge also increases remarkably as the electric strength and aging degree increase. The electric field distortion is closely related to temperature, thermal aging degree, and electric strength. [ABSTRACT FROM AUTHOR]

The operational stability of a power transformer exerts an extremely important impact on the power symmetry, balance, and security of power systems. When the grid load fluctuates greatly, if the load factor of the transformer cannot be maintained within a reasonable range, it leads to increased instability in grid operation. Adjusting the transformer capacity based on load changes is of great significance. The existing control methods for on-load capacity-regulating (OLCR) transformers have low timeliness, and the daily switching frequency of the capacity-regulating switch is not controlled. To ensure the safe and stable operation of transformers, this paper proposes a control method for OLCR transformers based on load prediction and fuzzy control. Firstly, the operating principle of OLCR transformers is analyzed, and a multi-strategy enhanced dung beetle optimizer (MSDBO) combined with a CNN−LSTM model is proposed for load forecasting. On this basis, the daily switching frequency of the capacity-regulating transformer is introduced, and hesitant fuzzy control is used to select the optimal capacity-regulating strategy relying on three factors: loss, economy, and switching frequency. Finally, simulation models are constructed using the MATLAB/SIMULINK platform and simulation analysis is conducted to verify the effectiveness and superiority of the proposed control method. For the three scenarios in this paper, the method reduces daily power loss by 28.5% to 56.3% and daily operating costs by 25.4% to 50.8%. The method used in this paper can sacrifice 3.5% to 9.2% of the loss reduction capability in exchange for reducing the number of switch operations by 28.6% to 57.1%, significantly extending the lifespan of the switches and thereby increasing the operational lifespan of the transformer. [ABSTRACT FROM AUTHOR]

Nanocomposite (NC) polymers are commonly employed because of their high breakdown strength, but the underlying mechanism of this phenomenon is not well understood. In the present study, specimens of polyethylene terephthalate (PET) films coated with epoxy resin (EP) and filled with nano-SiO 2 and Si 3 N 4 at concentrations of 0, 0.5, 1, 2, 5 and 10 wt % were subjected to DC and impulse voltages to breakdown. The effects of the types of nano-fillers and concentrations on the dielectric properties, crystallinity, and space charge trap characteristics were investigated on the breakdown strength. The experimental results show that improved DC breakdown strength of a sandwich structure of PET/nano-EP film is obtained even at 10 wt%, which is attributed to the effect of increased trap density for deep traps. In addition, to experimentally establish the dominant factors during impulse breakdown, a systematic theoretical model was used to account for the variations of impulse breakdown performance. It is concluded that the frequency-dependent dielectric characteristics plays an important role in determining the absorption of activation energy for trapped electrons, which affects the instantaneous insulating performance of NC polymers. [ABSTRACT FROM AUTHOR]

As the core component of composite insulation for dry-type high-voltage equipment, polytetrafluoroethylene (PTFE) films are subjected to high temperatures and complex electric fields. Therefore, this study explores the effects of electric field and temperature on the space charge characteristics of double-layered PTFE films based on the measurement of space charge distribution in double-layered PTFE films under different DC voltages and temperatures using the pulsed electro-acoustic measurement system. Results show that more charges accumulated in the PTFE sample subjected to a positive electric field during polarization and the space charges dissipated more rapidly during depolarization compared with those subjected to a negative electric field. The existence of a PTFE/PTFE interface increased the density of trap and deepened the trap. The positive charges that accumulated at the PTFE/PTFE interface during polarization gradually held a dominant position with the increase of temperature. Finally, the formation of charge in the PTFE/PTFE interface mainly depended on two factors. When the applied DC voltage was not large, the charge formation was mainly dominated by the Maxwell-Wagner polarization effect. When the voltage increased to a certain threshold, the charge formation was mainly caused by the charge injection and migration characteristics. [ABSTRACT FROM AUTHOR]

The polyethylene terephthalate (PET) is widely used in dry-type insulation application because of its excellent insulation properties. In practice, thermal aging of PET is often encountered, but the related mechanisms and effect of thermal aging on the PET dielectric properties still remain unclear. In the present paper, we presented a systematic and comparative study on the effect of thermal aging on surface microscopic morphological characteristics, DC breakdown strength, trap depths and the apparent mobility. The results indicate that when aged at 130?C, the surface topography becomes coarser than that aged at 80?C. The polarity reversal at the PET/PET interface during the polarization indicates that the injection abilities of Al and Sc electrodes are gradually enhanced when aged at 80?C and 130?C, respectively. Moreover, the measured isothermal discharge currents during depolarization show that the trap depth corresponding to the maximum trap density increases significantly in the beginning of aging, and then it decreases gradually when aged at 80?C, but it increases evidently when aged at 130?C with increasing the aging degree. It also has been found that the apparent trap-controlled charge mobility is greatly enhanced when aged at 80?C with the increase in thermal aging degree. This enhancement is proved to be associated with the decrease in trap energy level at the PET/PET interface. Also, it can be preliminary concluded that the enhancement of DC breakdown strength (at 1500 h) is closely related to the increase in electron type trap energy and its density when aged at 130?C. [ABSTRACT FROM PUBLISHER]

Natural ester is viewed as the most promising future alternative to mineral oil due to its high degradability, but its inferior breakdown characteristics prevent its application of large power equipment. The purpose of this research is to investigate the breakdown characteristics of natural ester. First, the natural ester's ionization potential and molecular number density are calculated. Then, an improved natural ester streamer discharge model is developed using the Poisson's equation and the carrier continuity equation, taking into account the effect of temperature on carrier mobility and incorporating the computed ionization potential and other factors. The results for different lightning impulse voltages were analyzed by COMSOL software. Comparing the mineral oil, the results indicate that the natural ester breakdown voltage is close to and slightly lower than that of the mineral oil but the acceleration voltage is very similar to the breakdown voltage. The lower ability of esters in prevention against the development of fast and energetic streamers is consistent with the description of the experimental results. In addition, the model takes into account the effect of temperature on the carrier mobility. As the applied voltage increases, the change of temperature in the streamer channel accelerates the carrier motion and promotes the propagation of the streamer. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

To understand the effect of thermal ageing mechanism on space charge formation and dissipation in double-layered polyester films (PET), the time dependence of space charge characteristics at physical interface and in the bulk of PET films have been investigated. The measurements of space charge characteristics in specimens of different thermal ageing conditions (80 and 130 °C) were conducted under different DC field strengths (polarization) and short circuit conditions (depolarization) using the Pulsed Electro Acoustic (PEA) measurement system. The influence of electric field distortion caused by the space charges on average AC breakdown strength of PET specimens was also investigated. Results indicate that the thermal ageing leads to polymer interface acts as positive charge traps at ageing temperature of 80 °C, but acts as electron traps at ageing temperature of 130 °C and provides deeper trapping level for positive charge carriers. Besides these, average breakdown strength of the composite decreases due to the electric field distortion caused by the space charges. A preliminary study of this phenomenon is given and further investigation is needed. [ABSTRACT FROM AUTHOR]

A detailed research on the streamer discharge local mechanism is necessary to enhance and develop the streamer theory. In the present paper, the characteristics of a streamer discharge in air at an atmospheric pressure are investigated using a hydrodynamic model based on plasma chemistry. Aside from continuity equations for electrons and ions, as well as Poisson?s equation for the electric field, the model includes the electron mean energy density equation and the neutral particle density continuity equation. The electric field strength and the velocities of streamer propagations obtained from proposed model are in good agreement with experimental and simulation results in literature. The mean electron energy is nearly constant in the streamer channel, except for a peak value at the streamer tip. Although the mole fraction of oxygen is only about 0.2 in air, oxygen ions are the dominant charged particles in the streamer discharge. At the electrode surface, the charged particle densities significantly decrease in the direction of the electrode because of surface reactions. [ABSTRACT FROM AUTHOR]

Abstract: A diode laser-pumped acoustic-optic Q-switched Nd:YAG master-oscillator power amplifier laser is presented. The laser is quasi continuously pumped at 1.1kHz with a pulse width of 172μs, and the ultrasonic frequency of the AO Q-switcher is set at a higher value (53kHz). The master oscillator is designed as a thermally near-unstable-resonator, which presents an average output power of 48W with a beam quality value of M 2 =1.41 and a Q-switching pulse duration of 121ns. The maximum average power of the MOPA system is 654W, and the beam quality is M 2 =6. [Copyright &y& Elsevier]

In this work, an (α + β) TRIPLEX heat treatment process was designed, as well as its impact on the microstructures and high–temperature mechanical properties of 2.5 vol% (TiB + TiC)/Ti–6Al–4Sn–7Zr–1Nb–1Mo–1W–0.2Si (wt.%) was carefully investigated. The results show that the microstructures of heat–treated TMCs consist of the primary α phase (α p) and the transformed β (β t) with a large number of nano–scale secondary α phases (α s). The TRIPLEX heat treatment encourages the effect of structural heredity, causing the residual α p lamellae to develop adequately and even mature into lamellae clusters with the same orientation, which induces a visible strengthening of the variant selection for the α phase. After the TRIPLEX heat treatment, the increase in content and size of silicides is mainly due to the process providing favorable sites and sufficient time for nucleation and growth. The longitudinal axes of some silicides exhibit an approximate 40° angle or parallel arrangement with the β/α p interfaces, which means that the silicides will grow preferentially along a specific orientation to minimize the strain energy. The (α + β) TRIPLEX heat treatment significantly increases the high–temperature strength of the TMCs with no appreciable elongation loss (as–cast TMCs: UTS = 487.6 MPa, EL = 17.4%; HT–4#: UTS = 558.1 MPa, EL = 11.5%). The strength improvement of HT–4 # is mainly related to the combined effects of α p content reduction, α s precipitation, obstruction of dislocations by the β t /α p interface, hetero–deformation strengthening, silicides precipitation strengthening, and α 2 dispersion strengthening. [ABSTRACT FROM AUTHOR]

Grounding resistance is an important basic index used to ensure the safety and reliability of grounding systems. In this paper, a method for grounding resistance measurement based on the fall of potential curve is proposed that can be performed using a 2–2.5D lead wire length. In this method, the potential distribution near the auxiliary current electrode is tested and is used as the main analysis and measurement parameter. Based on the relationship between the potential distribution generated by the grounding grid and the current electrode, the grounding resistance of the grounding grid can be deduced. First, the voltages between the grounding grid and the earth surface potential near the current electrode are measured; then, the ground potential rise (GPR) of the grounding grid is calculated through theoretical analysis and numerical calculation; and finally, the grounding resistance is calculated by dividing the potential by the test current. This method avoids the complex task of accurate compensation point location required for traditional measurement methods, and can be performed easily and quickly using a short lead wire. Measurement results from spot testing show that the proposed method can meet the accuracy requirements of engineering testing. [ABSTRACT FROM AUTHOR]

The MIE (Minimum Ignition Energy) characterizes the danger of a kind of combustible gas and its ignition behavior, and the antistatic test for the non-metallic materials is of great value to mining industry. This paper describes an antistatic level testing system for the non-metallic materials used in mines, and the methods to calculate the MIE of the ignition system, electromagnetic energy and scale the antistatic grade are also presented in details. According to the structure of the antistatic level testing system, the model to calculate the electromagnetic energy of the ignition system is set up firstly, then the MIE and electromagnetic energy before and after ignition are computed respectively. The ignition energy coupling coefficient is defined to indicate the ratio between the energy used for ignition and the total electromagnetic energy, based on which the quantity of triboelectric charge produced by the testing material can be obtained. The numerical relation between the ignition energy coupling coefficient and electrode gap is investigated, for the system described in the paper the threshold of ignition energy coupling coefficient increases from 0.42 to 0.77 when the gap distance increases from 1 mm to 5 mm. The system designed in the paper can be used to test the possibility of ignition and explosion for non-metallic devices used in mine, and the safety limit space for different material can be determined. In addition, it also offers reference for the scaling of triboelectric charge. [ABSTRACT FROM PUBLISHER]

Estimation of the degree of aging and the remaining lifetime of composite insulators is an important issue. In this paper, one shed of a 500 kV AC composite insulator that has operated for three years on a transmission line was studied. The insulator shed was cut longitudinally into four layers from the upper surface to the bottom surface and tested by the thermally stimulated current (TSC) method, and each layer was also tested by Fourier transform infrared (FTIR) spectroscopy. The results show that the first layer has fewer trap charges than the other layers in the TSC test, but it also has the highest trap level, and the trap depth of the first layer is deeper than that of the other layers. The trap levels and the quantities of shallow traps decrease successively from the second to the fourth layer. In the FTIR spectra, the absorption peaks of the first layer are lower than those of the other layers, and the other three layers have similar absorption peaks. The aging state of the first layer is more serious than that of the other layers. The study indicates that long-term contamination and ultraviolet (UV) radiation are the main reason for the aging differences among the different insulator shed layers. [ABSTRACT FROM AUTHOR]

Under operational conditions of high electric fields and elevated temperatures, the accumulation of space charges at multilayer insulation interfaces is often considered as an important factor affecting insulation performance. This study experimentally explored the influence of different thermal aging degrees (110 °C for 0, 720, 1600, 2100, and 2900 h) on physicochemical characteristics. The space charge dynamics in two-layered thermally aged PET-PET films were measured using the pulsed electro-acoustic (PEA) method and simulated on the basis of a one-dimensional modified bipolar charge transport model. The parameterization for key parameters involved in the model was analyzed through parameter sensitivity. Results indicated that the molecular structure, crystallinity, and dielectric spectra of the PET films are affected by thermal aging. The thermalization process also has noticeable effect on the surface state characteristics, which are characterized by deeper trap depth and larger trap density. Several experimental phenomena measured by the PEA method were observed on the basis of numerical simulation. [ABSTRACT FROM AUTHOR]