Stress Concentration Effect in Interface Between Conductor and Epoxy Resin in Coaxial Components of Power Equipment

The spacer used in gas insulated switchgear(GIS)plays a vital role in its safe operation.The mismatch of mechanical properties between conductor and insulating material in spacers results in stress concentration at the interface between them.During a hydrostatic test,mechanical damages are often initiated from the interface.In this paper,the stress between aluminum conductors and epoxy resin is studied by using a simplified coaxial structure specimen.The measurement system and finite element simulation are used to explore the generation of stress and strain during curing and shrinkage.The real-time measurement of strain at the interface shows that the residual strain in the entire process is primarily generated during the cooling stage.Consequently,the temperature and stress/strain distribution at the interface during the cooling stage are further studied,and combined with experiments and simulations.The consistency between the finite element simulation and the experimental measurement results verifies the validity of the simulation method used.The results show that the maximum of the first principal stress generated during the cooling stage of the production process is concentrated on the upper and lower edges of the interface which is critical to the impact of the production quality of the spacer.

Acoustic carrier signal transmission technology and its potential for in-site monitoring of sliding electrical contact used in gas-insulated switchgear/gas-insulated transmission line

A wireless signal transmission technology based on acoustic carrier is proposed,which overcomes the limitation of electromagnetic signal shielding and shows great potential for in-site monitoring of sliding electrical contact used in gas-insulated switchgear(GIS)/gasinsulated transmission line(GIL).Here,the state parameter of the sliding electrical contact is modulated onto the frequency domain of an ultrasound wave.As a mechanical wave,the ultrasonic wave is immune to electromagnetic shielding,so that it could carry the sensing signal to penetrate the metal shielding layer and transmit to the external terminals of GIS/GIL.The principle and signal modulation process of the acoustic carrier based transmission system have been demonstrated in detail.Both simulation and experiment have been conducted to analyse the system characteristics as well as optimise the system configuration.As a proof-of-concept application,the in situ and on-line monitoring of the thermal rise of a slide electrical contact both in a current loading equivalent model and a prototype of GIS is demonstrated.Experimental results fit well with the physical process,and show a good measurement accuracy of 0.6%and tem-perature sensitivity of 400 Hz/℃.

In-situ observation of electrical tree evolution in epoxy dielectrics with internal cracks

Gas-filled internal crack might appear in thermoset materials like epoxy resin during the equipment manufacturing,which would become a vulnerable local region to initiate the electrical tree,thus prone to cause insulation failure.The withstand voltage test was carried on epoxy samples with artificial cracks based on a rod-plane electrode arrange-ment.Simultaneously,surface state variation and tree evolution with crack were observed by an optical microscope in conjunction with a charge-coupled device camera.The changes in morphology and chemical status of the crack surface were characterized by scanning electron microscopy,laser Raman spectrometer and energy dispersive spec-trometer,respectively.It was found that the erosion and tree started from the borderline of crack under a relatively low electric field strength;however,the area near the electrode had relatively little damage.The breaking of epoxy molecular chains coarsens the crack surface and further forms deep channels on a micro-level,which is the forerunner of the electrical tree inception.Based on these,the initiation mechanisms of the crack-induced electrical tree and the reasons for the erosion near the borderline have been revealed.This study provides a train thought for the polymer degradation opening up into the initial tree channel during tree evolution processes.

Space charge dynamics in epoxy resin under voltage polarity reversal at various temperatures

Epoxy resin has been used to cast the core of the dry‐type high voltage direct current bushings and can accumulate space charge under high electric fields at high temperatures,which is believed to be a potential threat to the safe operation of dry bushings,especially under polarity reversal.In this study,results of a study of the evolution of space charge and electric field distribution in epoxy resin,under polarity reversal at 10 and 20 kV/mm,and at temperatures between 40 and 100°C,are presented.The results show that the space charge dynamics in epoxy resin during the test were different at various temperatures.At tem-peratures no more than 60°C,space charge accumulation within the samples was not obvious.Instead,a low‐frequency dielectric relaxation process maybe occurs during the polarity reversal,which made the charge peaks on the electrodes after the reversal were slightly smaller than those before the reversal.At temperatures no less than 80°C,considering that the applied electric field has little effect on the barrier reductions of the traps within the samples,the space charge dynamics were mainly governed by a thermally activated process,and the depths of the deepest occupied traps were obtain by analysing the transient processes.

Research on the dynamic characteristics of electric field distribution of the±1100 kV Ultra high voltage converter transformer valve‐side bushing using weakly ionised gas conductance model

Ultra high voltage(UHV)converter transformer valve‐side bushings are the key equipment indirect current(DC)transmission projects,and are subjected to high voltage and large current with large number of harmonics simultaneously.The electric field distribution of converter transformer valve‐side bushings is time‐dependent under DC voltage,polarity reversal voltage and actual voltage waveforms,and the electric and temperature field are coupling with each other in actual operation.The dynamic char-acteristics of the surface charges and the electric field distribution of China's first�1100 kV epoxy/paper-SF6 composite insulation converter trans former valve‐side bushing under 2 h DC withstand test voltage and polarity reversal voltage were analysed considering the natural ionisation,drifting,diffusion and recombination processes of positive and negative ions in SF6 gas and air.The time‐dependent electro‐thermal coupling field of the bushing under the actual voltage waveform in one cycle was ob-tained.The rationality of the simulation method adopted was verified through type tests of the UHV converter transformer valve‐side bushing.The simulation results of the paper can provide a theoretical foundation for the design of converter transformer valve‐side bushings and it is of great significance to ensure the safe operation of converter stations.

Numerical simulation of the interfacial stress between epoxy resin and metal conductor of power equipment during epoxy curing

There are many metal and epoxy resin interfacial structures in power equipments,which can generate interfacial stress concentrations and may induce accidents due to epoxy curing.In this paper,through simplifying interface structure into a coaxial cylindrical model,the formation mechanism of interfacial stress concentration of this interface structure was studied during epoxy resin curing process.The strain and temperature at the interface were measured by the strain and temperature measurement system.Then a model for calculating the interface stress distribution was established,including the heat conduction module,curing dynamics module and curing deformation module.The validity of the model was verified by comparing the calculated results with the measured results.In addition,the air gap defects at the interface,appearing after the curing process,were simulated.The results show that,after the curing process,the first principal stress at the centre of the interface gradually increases from the centre to the edge,reaching 41.06 and 40.20 MPa at the upper and lower edges respectively.The existence of air gap leads to partial discharge at low voltage.The calculation model can be extended to the stress prediction between the metal and epoxy resin in other power equipments.

A satellite-derived,ground-measurement-independent monthly PM_(2.5)mass concentration dataset over China during 2000-2015

Following the accelerated development of urbanization and industrialization,atmospheric particulate matter has become a significant threat to public health globally.Environmental health studies usually use the mass concentration of fine particles(PM_(2.5))as a base data to predict the health risks of particulate exposure.However,PM_(2.5)data from ground monitoring stations in China has not been provided until January 2013 by the Ministry of Environmental Protection of China.Hence,an alternative dataset of PM_(2.5)spatiotemporal distributions extending to years earlier than 2013 is urgently needed,which is of great significance to atmospheric environment assessment and pollution prevention and control.Atmospheric aerosol products by the moderate-resolution imaging spectroradiometer(MODIS)have been released since 2000,which provides the possibility to reconstruct historical PM_(2.5).However,most current methods do not have the ability to estimate PM_(2.5)mass concentration independently of ground observations.The PM_(2.5)mass concentration data set produced by PM_(2.5)remote sensing(PMRS)model based on physical processes does not depend on the ground observations,and also is not affected by the uncertainty of model emission sources or the completeness of chemical reaction mechanism.These ensure that the point-by-point validation for PM_(2.5)mass concentration data is more convincing,and the dataset can also be further used for model assimilation and artificial intelligence training to improve their predictions.In this study,we calculate the monthly PM_(2.5)mass concentration near the ground over land of China using aerosol inversion products(aerosol optical depth and fine-mode fraction)of MODIS and meteorological data(boundary layer height&relative humidity)provided by the Modern-Era Retrospective Analysis for Research and Applications Version 2(MERRA-2)data set.The results show that,in China,6 pollution centers mainly concentrated in the central and eastern regions.The highest PM_(2.5)mass concentration occurred in winter,whereas the pollution range was larger in summer.There are 63.4%of validation sites with biases within±20μg m−3,and the expected error is as±(15μg m−3+30%)enveloped by the monthly mean PM_(2.5)mass concentrations.The monthly PM_(2.5)is stored as NETCDF format,with a spatial resolution of 1°×1°.The published data is available in http://www.dx.doi.org/10.11922/sciencedb.j00076.00061.

Satellite remote sensing of atmospheric particulate matter mass concentration:Advances,challenges,and perspectives

Mapping the mass concentration of near-surface atmospheric particulate matter(PM)using satellite observations has become a popular research niche,leading to the development of a variety of instruments,algorithms,and datasets over the past two decades.In this study,we conducted a holistic review of the major advances and challenges in quantifying PM,with a specific focus on instruments,algorithms,datasets,and modeling methods that have been developed over the past 20 years.The aim of this study is to provide a general guide for future satellite-based PM concentration mapping practices and to better support air quality monitoring and management of environmental health.Specifically,we review the evolution of satellite platforms,sensors,inversion algorithms,and datasets that can be used for monitoring aerosol properties.We then compare various practical methods and techniques that have been used to estimate PM mass concentrations and group them into four primary categories:(1)univariate regression,(2)chemical transport models(CTM),(3)multivariate regression,and(4)empirical physical approaches.Considering the main challenges encountered in PM mapping practices,for example,data gaps and discontinuity,a hybrid method is proposed with the aim of generating PM concentration maps that are both spatially continuous and have high precision.