Mitigation of electrical treeing at high temperature in nano-SiO_(2) doped epoxy resin

In this work,electrical tree inception and ageing experiments were conducted to investigate the initiation,growth characteristics and structural characteristics of electrical trees of nano-SiO_(2) doped epoxy resin at different temperatures.Electrical tree inception and ageing experiments were conducted using power frequency voltage of 15 kV at 20,40,60,80 and 100℃.Electrical breakdown properties of epoxy resin were investigated by applying AC and DC voltages at 20,40,80 and 120℃.There are two major observations based on the experimental results.With the increase of temperature,the morphological structure of the electrical trees changes from dendritic to plexiform,the tree inception voltage decreases and the growth rate of the electrical trees increases obviously.Either under DC or AC voltage,the breakdown strength of the nano-SiO_(2) doped epoxy resin decreases as the temperature increases.Finally,based on the space charge and the trap level measurement,the mechanism of the electrical tree propagation and breakdown in nano-SiO_(2) doped epoxy resin was analysed.

Surface Charging Phenomena on HVDC Spacers for Compressed SF_(6) Insulation and Charge Tailoring Strategies

Surface flashover of spacers is a key factor limiting the application of HVDC GIS/GIL,while the charge accumula-tion on the surface of the spacer could have a potential adverse effect on the surface flashover voltage.This paper discusses the laws regarding distribution patterns of surface charges and the related mechanisms.The field-dependent property is discussed in detail to comprehensively illustrate the charge transport mecha-nism and explain the research differences regarding different surface charge patterns obtained by previous researchers.In addition,the main surface charge control methods for epoxy resin are summarized and discussed.The potential research directions of charge control methods and key points in manufacturing of spacers used in HVDC GIS/GIL are also explored.

Significant enhancement in the electrical conductance properties of ethylene propylene diene monomer using the nano‐SiO_(2) particles

To study the effect of nano‐silica particles on the conductivity characteristics of ethylene propylene diene monomer(EPDM)insulation,EPDM nanocomposites with different mass fractions of nano silica were prepared via the melt‐blending method.Scanning electron microscopy was used to analyze the dispersion of nanoparticles in EPDM.Based on the Fourier‐transform infrared spectrum,the bonding properties between the nanoparticles and EPDM were analyzed.The steady‐state current at 30-100°C and 1-20 kV/mm was measured.Moreover,the conductivity characteristic was analyzed.The experimental results show that as the content of nano‐silica increases,the electrical conductivity of EPDM nanocomposite as well as the threshold field strength gradually decreases.Samples with a small number of nanoparticles will increase the activation energy of the carrier conductivity,and the interface region effect introduced by nanoparticles will reduce the carrier mobility and concentration,resulting in a lower conductivity.Comparing the calculated value of the dielectric constant with the measured value can conclude that the high‐field conductance process is a combination of the electrode effect and the body effect,which in this case,Schottky effect dominates.

Effects of ambient humidity and thermal aging on properties of Nomex insulation in mining dry‐type transformer

Temperature and humidity are the main factors that cause the decomposition of Nomex paper widely used as turn‐to‐turn insulation material of mining dry‐type transformers. In this study, for understanding the effect of ambient humidity and thermal aging on the properties of Nomex paper, specimens with the initial moisture content of 7% were aged in the oven for 168, 504, 672 h at 180°C, respectively. After thermal aging, corona discharge experiments of unaged and aged specimens with different aging degrees were carried out at different relative humidity (RH) levels from 10 to 70%. The characteristic parameters of corona discharge, such as the average discharge amplitude and the number of discharges, were measured. Then, the surface conductivity and the surface potential decay rate were measured. The morphology and attenuated total internal reflectance Fourier transform infrared spectroscopy were also analysed for explaining the difference of properties be-tween unaged and aged specimens. The experiment results indicate that the average discharge amplitude decreases with the increase of RH or the deepening of the aging degree of specimens, but the number of discharges shows the reversal tendency. The changes in the physical structure and chemical composition, which is induced by the hydrolysis reaction of Nomex paper, accelerate the surface potential decay and increase the surface conductivity.

Effect of long-term fluorination on surface electrical performance of ethylene propylene rubber

To investigate the effect of fluorination on surface electrical performance of ethylene propylene rubber(EPR),four pieces of EPR specimens are prepared and fluorinated for different duration ranging from 120 min to 480 min.The surface morphology and element compositions of experimental specimens are tested.The surface potential decay and complex permittivity are measured.The tracking discharge property and the erosion properties after tracking test are investigated.The surface charge transport and electrical property tailoring mechanism are discussed.The results show that when the fluorination duration is from 120 min to 360 min,the surface morphology gradually becomes flat and compact with time,and the surface resistance to electrical tracking is improved.While an excessive fluorination with the duration of 480 min leads to a significant increase in surface conductivity and a weakening in the resistance to electrical tracking,which is mainly due to a change in surface morphology.The recommended fluorination duration for an optimised surface property is between 240 and 360 min for EPR specimen.

Development and pattern identification of end-winding discharge under effect of relative humidity and temperature for HV motors

During operation of high voltage(HV)motors,different types of discharges,e.g.surface discharge,corona discharge and bar-to-bar discharge can occur at the same time,increasing the difficulty of partial discharge(PD)sources determination and PDs pattern identification.In this study,the end-winding of a 10 kV motor coil was artificially aged and the related PD was measured.The initiation and variation of different PDs under multi-factor stresses were studied.The localised fingerprints coexisting with multiple PDs were identified and analysed.The results confirm that the end-winding discharge process was significantly influenced by the experiment relative humidity(RH)and temperature.The bar-to-bar discharge was easily recognised at a lower voltage since the identification of bar-to-bar discharge pattern would be affected by the corona discharge pattern at a higher voltage.It is shown that it is more difficult for the corona discharge to be detected when RH exceeded 80%,while the surface discharge dominated at the higher RH.In addition,the PDs were more easily identified with the rise of the temperature.This study can provide a reference in PD identification test and be useful for the PD online monitoring of HV motors.