Phase field model for electric-thermal coupled discharge breakdown of polyimide nanocomposites under high frequency electrical stress

In contrast to conventional transformers, power electronic transformers, as an integral component of new energy power system, are often subjected to high-frequency and transient electrical stresses, leading to heightened concerns regarding insulation failures. Meanwhile, the underlying mechanism behind discharge breakdown failure and nanofiller enhancement under high-frequency electrical stress remains unclear. An electric-thermal coupled discharge breakdown phase field model was constructed to study the evolution of the breakdown path in polyimide nanocomposite insulation subjected to high-frequency stress. The investigation focused on analyzing the effect of various factors, including frequency, temperature, and nanofiller shape, on the breakdown path of Polyimide(PI) composites. Additionally, it elucidated the enhancement mechanism of nano-modified composite insulation at the mesoscopic scale. The results indicated that with increasing frequency and temperature, the discharge breakdown path demonstrates accelerated development, accompanied by a gradual dominance of Joule heat energy. This enhancement is attributed to the dispersed electric field distribution and the hindering effect of the nanosheets. The research findings offer a theoretical foundation and methodological framework to inform the optimal design and performance management of new insulating materials utilized in high-frequency power equipment.

Long-term corona behaviour and performance enhancing mechanism of SiC/epoxy nanocomposite in SF6 gas environment

Surface coating technology is an effective way to solve the interface insulation problem of DC GIS/GIL basin insulators, but the performance of the coating will change greatly, and the insulation strength will be completely lost, after long-term use in the extreme conditions of corona erosion. In this research, the multi-needle-plate electrode platform was constructed to explore the long-term use performance of Si C-doped nanocomposite exposed to corona discharge in SF6gas. Samples with a high Si C content have advantages in maintaining physical and chemical properties such as elemental composition, erosion depth, surface roughness and mass loss. The nanocomposite doped with 6 wt.% Si C has prominent surface insulation strength after long term exposure to corona, and the others are close to losing, or have completely lost,their insulating properties. Furthermore, the degradation mechanism of physicochemical properties of composite exposed to corona discharge was investigated with the proposed Reax FF MD model of energetic particles from SF6decomposition bombarding the epoxy surface. The reaction process of SF particles and F particles with the cross-linked epoxy resin, and the Si C nanoparticles providing shelter to the surrounding polymer and mitigating their suffering direct bombardment, have been established. The damage propagation depth, mass loss and surface roughness change of nanocomposite material bombarded by SF6decomposition products is reproduced in this simulation. Finally, the deterioration mechanism of insulation properties for the Si C-doped composite was elucidated with DFT analysis. The band gap of the molecule containing S drops directly from the initial 7.785 e V to 1.875 e V, which causes the deterioration of surface electric properties.

Influence of Repetitive Impulse Waveforms on Surface Discharge Characteristics and Insulation Life for Polyimide Film

Accelerated insulation aging problems under high frequency repetitive impulses in power electronic transformers are drawing more and more attention in modern power systems. Partial discharge (PD) characteristics including discharge inception voltage, phase distribution and statistical characteristics on polyimide (PI) surface under different impulse waveforms and the insulation life of PI films are studied in this paper. We carry out experiments based on PD and insulation lifetime test systems, using five different types of repetitive impulses, including three bipolar waves and two unipolar waves. The experimental results show that there is little variation in discharge inception voltage under different waveforms, but great variation in phase distribution and statistical characteristics of PD. In addition, insulation life is approximately the same under different waveforms with the same polarity, and the aging rate under bipolar waveforms is larger than that under unipolar waveforms. We explain the differences between the bipolar and unipolar waveforms on insulation life, which can be concluded that the surface charge under unipolar waveform accumulates more significnatly compared with bipolar waveform and decreases the electric filed strength in discharging the air gap and inhibits surface discharge from occurring.

High wall-plug efficiency 808-nm laser diodes with a power up to 30.1 W

A very highly efficient InGaAlAs/AlGaAs quantum-well structure was designed for 808 nm emission,and laser diode chips 390-μm-wide aperture and 2-mm-long cavity length were fabricated.Special pretreatment and passivation for the chip facets were performed to achieve improved reliability performance.The laser chips were p-side-down mounted on the AlN submount,and then tested at continuous wave(CW)operation with the heat-sink temperature setting to 25℃using a thermoelectric cooler(TEC).As high as 60.5%of the wall-plug efficiency(WPE)was achieved at the injection current of 11 A.The maximum output power of 30.1 W was obtained at 29.5 A when the TEC temperature was set to 12°C.Accelerated life-time test showed that the laser diodes had lifetimes of over 62111 h operating at rated power of 10 W.

Impact of SF_(6) Decomposition Products on Epoxy Resin Chemical Stability and Doping-nano-Al_(2)O_(3)-based Enhancement Using the ReaxFF-MD Method

Chemically active by-products formed by corona discharge in SF_(6) gas are prone to damaging the exposed epoxy resin,or even leading to an entire insulation failure of the operational GIS/GIL power equipment.In this proposed research,reactive force field molecular dynamic simulation methodology is applied to investigate the chemical reaction kinetics of epoxy polymer under the impact of highly energetic particles(F,S,SOF,SF,OH and O)so as to explain the degradation mechanism.Among all cases,SF particle-impacted epoxy resin suffers the most serious surface erosion with the lowest remnant mass of 9%and deepest damage penetration of 32.6Å,to which the S particle-caused damage showed similar results.Due to high reactivity of the S atom,it can merge into the epoxy molecules to promote long chain breaking,causing a six-membered ring opening and further dissociation of short carbon chains,which makes the epoxy resin molecules undergo faster spontaneous dissociation with increased temperatures.The changes of small molecular gas products,such as CO_(2),H_(2)O and CH_(2)O,as well as that of the characteristic products,such as HF,CS_(2),SO and H_(2)S,are also evaluated under the impact of different particles.The presented research indicates that enhancing the resistance strength of epoxy polymer against S and SF particles'corrosion is the key approach to improving chemical stability in the SF_(6) environment.Further studies were implemented to optimize the concentration and diameter of nano-Al_(2)O_(3) doped in the composites.According to this paper,aluminum nanoparticle with a diameter of 1nm could significantly reduce the erosion caused by SF and S particles.The micro-scale mechanism lies primarily within two aspects:the nanoparticles improve the surface heat transfer efficiency as to reduce temperature rise,and also provide an effective protection area by balancing distribution and self-exposing,which finally slows down the pyrolysis process of epoxy resin,as well as the reaction intensity with the incident particles.

Correlation between surface charge and creepage discharge on polymeric dielectrics under high-frequency high-voltage stress

In the present work,creepage discharge characteristics,i.e.amplitudes,phases,and repetitiveness,and surface charge dynamic behaviors under a 20 kHz high-frequency sinusoidal waveform high-voltage electrical stress were captured in a discharge chamber with temperature and humidity control.The results showed that the creepage discharges mostly occurred in the positive half phase,whose maximum amplitude increased with the development of discharge.The inception voltage of the creepage discharge is independent of the frequency of the external electrical stress.Once the discharge occurred,there were a large number of positive and negative particles ionized by a high electric field.Because of the much higher velocity of electrons than positive ions,the energetic discharge-produced electrons are likely to disperse away along the surface and be accumulated through adsorption,collision,and reactions.Moreover,the positive ions join the high-conductive discharge channel and disappear though the ground electrode.Thus,after high-frequency creepage discharge,only negative charges remained on the dielectric surface,as measured.Particularly,the creepage discharges mostly occurred in the positive half phase,owing to the reverse electric field induced by the accumulated negative charges.With the development of creepage discharge,some large-amplitude discharges began to occur in the positive-peak-phase region.The research concluded that the synergistic effect of negative surface charges and large-amplitude discharges eroded the dielectrics and excited the streamer to creep toward the ground electrode until flashover along the surface.Therefore,the correlation between high-frequency creepage discharge and surface charge is preliminarily revealed.

Effectiveness of Low-Temperature Plasma Adenoidectomy Combined with Bilateral Tonsillectomy in the Treatment of Snoring in Children and Its Effect on Sleep Quality

Objective:To evaluate the therapeutic effect of low-temperature plasma adenoidectomy(abbreviation:adenoid low-temperature plasma excision)coupled with tonsillectomy(bilateral)on snoring in children.Methods:Seventy-six cases of snoring children treated in the hospital between November 2020 and November 2023 were selected.38 cases of children in Group A agreed to surgery and were admitted to the hospital for surgical treatment and were treated with adenoid cryo-plasma excision combined with tonsillectomy(bilaterally),and 38 cases of children in Group B did not agree to outpatient conservative treatment for surgery,and the results of the treatment were compared.Results:Before treatment,there was no difference between the ventilation function indexes,sleep quality scores,and quality of life scores of the two groups compared with each other(P>0.05).After treatment,the ventilation function indexes of Group A were better than Group B,and the sleep quality score and quality of life score were higher than that of Group B.The total effective rate of Group A was higher than that of Group B(P<0.05).Conclusion:Adenoid cryo-plasma excision combined with tonsillectomy(bilateral)can improve the quality of life,ventilation function,and sleep quality of children with snoring,and can enhance the efficacy.

Metal Particle Contamination in Gas-insulated Switchgears/Gas-insulated Transmission Lines

Metal particle contamination in a gas-insulated switchgear(GIS)or a gas-insulated transmission line(GIL)is an important factor leading to the decline of insulation performance.Exploring the deterioration mechanism and suppression measures of metal particles on insulation is a key technical problem in enhancing the dielectric strength of GIS/GIL equipment.In this paper,the charge and motion characteristics of metal particles are first introduced.The gas gap breakdown caused by free metal particles and the surface flashover caused by metal particles near or adsorbed on the insulator are then analyzed according to different particle motion patterns and spatial locations.Subsequently in terms of operation managements,the existing methods of particle detection are analyzed.In addition,the main inhibition methods of metal particles are introduced from three aspects:particle trap,insulator surface treatment and electrode coating.Finally,the prospects in the future research on particle pollution in GIS/GIL are also pointed out.

Research on discharge phenomenon caused by cross‐adsorption of linear insulating fibre and metal dust under DC voltage

A large amount of metal dust is often generated during production and operation of Gas Insulated Switchgear/Gas‐Insulated Transmission Line,and insulating fibres are very easy to mix in during assembly.The discharge phenomenon induced by these two impurities remains unknown.By setting up a test rig with parallel electrodes,the discharge characteristics of metal dust and linear insulating fibres under the negative DC voltage were studied separately and jointly.The obtained research results indicate some interesting phenomena that the metal dust may render‘sand storm’motion characteristics under the action of the electric field,and the linear insulating fibres appear to show‘standing’,‘jumping’,‘flying fire’and other phenomena.A small amount of dust and fibres alone present little effect on the breakdown voltage of the air gap.However,while the insulating fibres and metal dust coexist,the metal dust will be adsorbed on the surface of the insulating fibres,then the surface charge of the fibres is reallocated,and the discharge phenomenon at both ends of the fibres is enhanced,so that the insulating fibre exhibits similar movement and discharge characteristics to the linear metal particles.At the same time,the fibres with dust absorbed on will result in an air gap breakdown voltage drop of approximately 25%-40%.What's more,if the discharge of fibres and the‘sand storm’movement of the dust coexist,it may directly lead to‘explosion’of the moving dust,causing severely ablation of the plate surfaces,and thereby the degree of destruction increases dramatically.

Surface Charge Accumulation Characteristics on DC GIL Three-post Insulators Considering the Influence of Temperature Gradient

Surface charge accumulation is considered to be a critical factor in flashover failure of three-post insulators.However,surface charge accumulation characteristics on threepost insulators with complex structures and uneven electric fields are still unclear.Furthermore,the temperature gradient field makes charge accumulation more complicated.In this presentation,surface charge profiles of DC three-post insulators under electro-thermal coupling stress are studied by establishing a multi-degree-of-freedom movement measurement system.The abdominal area of the three-post insulator accumulaftes charges of identical polarity as the DC voltage,while the leg area accumulates heteropolar charges.Charge density from the bottom of the leg to the center of the abdomen presents a trimodal distribution pattern,including two homopolar charge peaks and one heteropolar charge peak.Asymmetrical surface conductance distribution arising from the temperature gradient leads to a significant increase in amplitude and distribution range of the homopolar charge peak at the legs of insulator.Increase of the temperature gradient will further magnify the homopolar charge peak at the legs.When DC voltage is 100 kV and conductive pole temperature is 70℃,surface charge density of the three-post insulator can reach 100μC/m^(2).Therefore,surface conductance regulation of the leg region is the key to charge regulation and insulation optimization design of DC three-post insulators.

Finite-element analysis for surface discharge on polyimide insulation in air at atmospheric pressure under pulsed electrical stress

A surface discharge non-equilibrium plasma model of air–polyimide under pulsed electrical stress is established, byconsidering the reaction of charged particles on the dielectric surface and the secondary electron emission caused by thecondition that high-energy particles bombard the material surface. The model defines the chemical reaction of air discharge byusing simplified set of reactions, which greatly reduces the complexity of the model. To avoid the negative value of particledensity in the process of solution, the logarithmic finite-element method is used to solve the model established, so as toimplement the dynamic simulation of the surface discharge process. Also, the temporal and spatial evolution of themicroparameters such as charge and electric field distribution during discharge are obtained, and the reliability of the model isverified by experiments in terms of discharge development pattern and surface charge accumulation. By comparing thedevelopment process of surface discharge under single pulse and repetitive pulses, it can be seen that surface dischargedevelops from needle electrode to ground electrode under both repetitive pulses and single pulse stress, but the relationshipbetween the discharge propagation time under repetitive pulses and pulse repetition rate is a ‘u’ curve, and the inflection pointmoves to higher repetition rate region with the increase of voltage.

Research on numerical simulation for partial discharge of epoxy interface excited by high-frequency sinusoidal voltage

High-frequency sinusoidal voltage excitation is proposed as a feasible method to solve the difficulty of partial discharge detection at the epoxy interface.In order to study the development process of partial discharge under high-frequency stress,a two-dimensional plasma simulation model of partial discharge for a needle-plate electrode structure is established by coupling particle transport equation,Poisson equation and plasma chemical reaction.The model adopts a reaction set to reduce the difficulty of modelling generation,transport and disappearance of charged particles.It realises dynamic simu-lation of the partial discharge at the epoxy interface.The spatial-temporal distribution characteristics of microscopic parameters such as electron density,electron temperature and surface charge are obtained.Based on this simulation model,the partial discharge characteristics of the epoxy interface at different frequencies are further studied in this work.The results show that with the increase of applied voltage frequency,the electron temperature shows an increasing trend,but its increasing rate gradually slows down;while surface charge accumulation decreases uniformly with the increase of applied voltage frequency.These two factors make the partial discharge severest under the frequency of 10 kHz.The results of the model are verified from two aspects of discharge form and discharge intensity by experimental means.

Reviews on sulphur corrosion phenomenon of the oil-paper insulating system in mineral oil transformer

The active sulphur compounds in transformer oil would react with copper wires and produce cuprous sulphides,which then attach themselves to the surface of copper wires and insulating paper,thus reducing the electrical performance of oil-paper insulation and causing insulation failure.This article aims to make a comprehensive summary on the field of sulphur corrosion of oil-paper insulation in the mineral oil transformer,including the corrosion mechanism of sulphide on copper sheets,the deterioration mechanism of sul-phide on oil-paper,the factors influencing sulphur corrosion reaction,the method of characterising the degree of sulphur corrosion,the protection technology of sulphur corrosion and the field operation measures for sulphur corrosion.Moreover,four key problems that need to be further solved were sorted out:the synergistic deterioration mechanism of sulphide and its transformation mechanism,the micro-process and forming path of sulphur corrosion reaction,the characteristic parameters and quantitative charac-terisation methods of sulphur corrosion degree,and the effective protection technology of sulphide corrosion.To solve these problems,efforts will be made for deeper understanding of the sulphur corrosion phenomenon and its deterioration mechanism based on theo-retical background,the assessment methods and protection technology,endeavouring to put forward solid theoretical basis and feasible technical means for preventing sulphur corrosion insulation failure.

Effect of sinusoidal waveform and frequency on space charge characteristics of polyimide

The charge phenomena under sine and half‐wave sine voltages within the frequency range of 500 Hz are studied here.Based on the pulsed electro‐acoustic method,the traditional circuit design under high-frequency voltages is first analysed.It is found that the selection of a 186 pF protection capacitor and a 333 kΩprotection resistor can ensure that the actual voltage applied to the sample is consistent with the expected input.Based on this design,experimental results show that the polarity of the charge accumulated in the depth of the sample is determined by that of the upper electrode.Comparison results under special voltages with different amplitudes and frequencies indicate that the amount of accumulated charge under sine voltages are larger than those under positive and negative half-wave sine and DC conditions,and the samples under lower-frequency conditions show more charge accumulation.The maximum electric field strengths appear at 90 and 270 degrees of the sine voltage with a frequency of 10 Hz,and their values are 68.55 and 81.82 kV/mm,respectively.Therefore,the charge characteristics are easily affected by the voltage’s waveform and frequency parameters.The results obtained here can provide guidance for the application of insulating materials under special voltage environments.

Micro‐mechanism study on synergistic degradation of the oil‐paper insulation with dibenzyl disulfide,hexadecyl mercaptan and benzothiophene

In recent years,there have been many cases of transformer failures caused by corrosive sulfides.At present,research mainly focuses on the single sulfide dibenzyl disulphide,but few research on multiple sulphur or the oil‐paper hybrid insulation.In this study,three typical sulfides dibenzyl disulfide,hexadecyl mercaptan and benzothiophene were selected to form seven sulfide‐oil‐paper models.Then relaxation calculations were carried out,and through molecular dynamics simulation,the synergistic effect of different sulfides on the properties of insulating paper and insulating oil was discussed.Results show that the coexistence of the three sulfides has the most severe weakening on the mechanical properties of cellulose,and it also causes great damage to hydrogen bonds.Hexadecyl mercaptan has a weaker effect on hydrogen bond destruction,but it will greatly aggravate the cellulose chain movement.The viscosity of insulating oil is generally increased by the influence of sulfide.Hexadecyl mercaptan is the main factor affecting the viscosity.Thiophene has little effect on the viscosity,ctive protection technology on sulphur corrosion.

Micro-mechanism Study on the Different Antioxidants Protective Effect of Vegetable Oil by ReaxFF Molecular Dynamics

The aging problem of vegetable oil has severely restricted the popularization and application of vegetable oil transformers.Adding antioxidants is the most recommended solution.At present,there is insufficient research on the micromechanism application of the protective effect of antioxidants on vegetable oil.In this paper,multiple vegetable oil models with different types and concentrations of antioxidants are established.Then the molecular dynamics simulation based on ReaxFF is carried out at different temperatures and oxygen content.Results show that the protective mechanism of antioxidants is to release H and combine this with free radicals generated by the decomposition of unsaturated fatty acids(UFA)in the oil.In addition,the protective effects of four antioxidants show different results.The stronger ones are tert-butyl hydroquinone(TBHQ)and butylated hydroxytoluene(BHT),while the weaker ones are butylated hydroxyanisole(BHA)and propyl gallate(PG).TBHQ has a better protective effect at lower concentrations,but the decomposition of UFA is promoted at higher concentrations.When the temperature rises,the decomposition of UFA is promoted.With the addition of oxygen,smaller molecular compounds are easily oxidized,and the decomposition of UFA is accelerated.The above research could reveal the microscopic mechanism of antioxidant protection on vegetable oil,providing theoretical references for further exploration of effective vegetable oil aging protection technology.