Evolution Of Space Charge Behavior In Two-layered Polyethylene Terephthalate Films Under Thermal Action And Its Influence On Breakdown Characteristics

The effect of space charge is an important factor that leads to the aging and failure of insulation in electrical equipment,and the thermal aging process have a significant impact on the space charge characteristics.To reveal the breakdown mechanism of multilayer composite dielectrics,a thorough and systematic study on the evolution of space charge characteristics under electrical and thermal stress has important academic significance and engineering value.In this paper,take the two-layered insulation of dry air-core reactor(Polyethylene Terephthalate film,PET)as the research object,through Pulsed Electro-acoustic method(PEA)and isothermal relaxation current method(IRC),the space-time distribution of space charges and the variation of trap characteristics in two-layered PET film under different thermal temperatures and times are systematically studied.The Bipolar Charge Transfer(BCT)model of two-layered PET films under thermal aging is established.The influence mechanism of the test temperature and the thermal temperature on the interface charge characteristics is simulated,the influence mechanism of surface state on AC and DC breakdown characteristics is revealed.A frequency-dependent model of energy absorption in the thermal excitation process of localized charge is established,and the mechanism of thermal action on impulse breakdown characteristics is revealed.The main achievements of this paper are as follows:(1)The space-time distribution of space charges and the variation of trap characteristics of two-layered PET films under different thermal temperatures and times were studied.The results of charge space-time distribution characteristics show that,when the test temperature is 25 ?,with the increase in aging time,the charge in the bulk accumulates and dissipates rapidly,and the charge density at the interface increases first and then decreases.After increasing the thermal temperature(130 ?),positive and negative charges at the interface dissipate simultaneously,its distribution region slightly increase.After the test temperature is increased(60 ?),the glass transition process makes the interfacial charge density and its distribution region obviously increase.IRC results show that the evolution of the surface state during thermal aging causes the trap density and normalized trap depth to increase first and then decrease.(2)Based on the space-time distribution characteristics of space charges,a BCT model was established,which takes into account the asymmetric surface states and charge behaviors of thermally acted two-layered PET films.The influence of thermal action and test temperature on the interfacial charge characteristics was revealed.With the increase in action time,the evolution of surface state trap density leads to the interfacial charge density increasing first and then decreasing;After increasing the thermal temperature,the enhanced asymmetric surface state of the two-layered dielectric leads to the phenomenon of positive and negative charges dissipate simultaneously in the interface region;After increasing the test temperature,the increase in surface state trap density and extension depth results in the increase of interfacial charge density and its distribution range,and the difference of surface states' ability to capture electrons and holes leads to the interfacial charge density increase first and then decrease during depolarization.(3)AC and DC breakdown characteristics of two-layered PET films under different thermal temperatures and times were obtained,and the evolution mechanism of breakdown characteristics was revealed based on the surface state characteristics.The results show that the DC breakdown strength increases first and then decreases with the action time,while the AC breakdown strength decreases first and then increases.Considering the evolution of surface state during thermal action,a simulation model of AC/DC breakdown of two-layered dielectric is established.It is found that,breakdown occurs in the dielectric bulk under DC field,while it occurs on the dielectric surface under AC field.In the initial stage of thermal action,the increase in surface state trap density and extension depth results in a large number of electrons/holes accumulating on the surface under DC voltage,weakening the field strength near the electrode and inhibiting the electron/hole injection process,thus increasing the DC breakdown field strength.However,due to the frequent charge injection and extraction process under AC voltage,the increased trap density on the dielectric surface resulting in an increase in the charge density and field strength distortion on the surface.Therefore,the AC breakdown strength decrease.The decrease of surface state trap density in the later stage of thermal action is beneficial to the injection of charges,resulting in an increase in the distortion of electric field in the bulk under DC field,but also enable surface charges to be more easily extract under AC fields,resulting in a decrease in the distortion of electric field on the surface.Therefore,the DC breakdown strength decreases while the AC breakdown strength increases.(4)The breakdown characteristics of two-layered PET films at different thermal temperatures and times under impulse voltages were obtained through experiments.A frequency-dependent model of local charge absorption energy was proposed.The influence mechanism of dielectric and trap characteristics evolution of two-layered dielectric on impulse breakdown characteristics was analyzed.The results show that the breakdown voltage of the two-layered dielectric increases with the action time,and the breakdown voltage under standard switching wave is larger than that under standard lightning wave.In view of this phenomenon,an energy absorption model for local state charge thermal excitation desorption process is established,which takes into account the frequency spectrum distribution characteristics of chopping wave and the frequency dependence characteristics of charge absorption energy.Through numerical analysis,it is found that the decrease in dielectric loss tangent tan? and the increase in normalized trap depth lead to the gradual increase of breakdown voltage during thermal action.Due to the equivalent frequency of the standard lightning chopping wave is higher than that of the standard switching chopping wave,and the frequency-dependent characteristic of absorption energy determines that the charge is more inclined to absorb high-frequency energy,so the breakdown voltage under standard lightning wave is lower.