| Polytetrafluoroethylene(PTFE)insulating dielectric is the main insulation material for the key high-voltage equipments such as dry-type current transformer,dry-type wall bushing and tubular bus.The insulation performance of PTFE is very important for the stable and reliable operation of power grid.During the operation of power equipment,PTFE materials suffer high temperature,humidity,strong electric field or other factors,which cause insulation strength degradation,insulation aging or other problems,and finally might lead to equipment failure.Studies have shown that space charge has great influence to the insulation state of polymer dielectric.space charge leads to a distortion of electric field and accelerates the aging of insulation materials.In addition,space charge characteristics can be used to reflect the insulation state of materials.Therefore,the research of space charge in PTFE dielectric has an important reference value for improving the insulation performance of PTFE insulating materials and the service life of dry-type high-voltage equipments.However,the problem of space charge accumulation and damage to PTFE as an insulating material for dry power equipment has been ignored.The main work are as follows:The experimental study on space charge characteristics of unaged PTFE dielectric under different test temperatures and different external DC electric fields is carried out by electro-acoustic pulse method.The effects of temperature and applied electric field on the space charge characteristics and electric field distribution characteristics are analyzed.The results show that,compared with positive electric field,space charge transfer rate in PTFE sample is faster after the polarization under negative polarity electric field,and the residual charge is less.With the increase of the applied electric field intensity,the space charge at PTFE/PTFE interface increases and the polarity of the charge changes,besides,the distortion of the electric field inside PTFE deepens.The polarity of PTFE/PTFE interface charge changes with the increase of test temperature.Compared with other temperatures,space charge accumulation at PTFE/PTFE interface is the least when temperature is 60 ?.Finally,it is proved that test temperature has little effect on the electric field distortion in PTFE dielectricThe space charge characteristics in PTFE films with different thermal aging states are investigated.The effects of thermal aging time and thermal aging temperature on the space charge characteristics in PTFE films are analyzed.It is found that PTFE dielectric is easy to accumulate negative space charges.With the increase of thermal aging time,the total amount of space charge accumulation decreases in the beginning and then increases.Compared with other aging stages,the accumulation of space charge and the residual charge after depolarization are the least in the PTFE sample with thermal aging for 30 days.The thermal aging temperature has little effect on the space charge distribution characteristics of PTFE dielectric,but has a certain effect on the total space charge attenuation characteristics,which is closely related to the thermal aging time.The breakdown characteristics,dielectric spectrum characteristics and infrared spectrum characteristics of PTFE films under different thermal aging conditions are studied.Effects of thermal aging on breakdown characteristics,dielectric properties and microstructure are analyzed.The results show that,as the degree of thermal aging deepens,the AC breakdown field strength decreases at first and then increases,and the DC breakdown field intensity decreases monotonically,while the relative dielectric constant and dielectric loss factor increase in varying degrees.In addition,the reason for the change of electrical properties is the overall bending or movement of PTFE molecular chains and the decomposition of residual organic impurities to produce derivatives under high temperature thermal aging. |
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