Study On AC Space Charge Characteristics Of Thermal Aging Polymer Insulation Materials In Oil

The electrical and mechanical properties of oil-paper insulation system always decline under the stress of electro-thermal aging in the long-term operation of transformer.With the advantages of superior insulation performance,high temperature resistance and light weight,the potential use of polymer insulation material as a substitute for the traditional insulation paper can greatly improve the reliability and service life of transformer.The aging state of transformer insulation is closely related to the accumulation of the internal space charge which will cause the distortion of electric field and thus affect the insulation performance of the material.In this paper,polycarbonate(PC)and polyester film(PET)were chosen and aged for 300 days in oil respectively at 90℃,110℃ and 130℃.The samples were taken out at different aging stages and then the AC space charge was tested using pulsed electro-acoustic method,meanwhile a new phase matching method was proposed which improved the accuracy of space charge test results.The experimental results show that the aging degree is higher,the time for peak charge to reach a maximum is less,that is,the charge accumulation is faster,and the peak value of charge is larger.The charge accumulation rate of PET is higher than that of PC.Under the same temperature,the longer aging days are,the more space charge is,the more easily the charge injects.The higher the aging temperature is,the faster the charge injection is,the deeper the charge migration is.It is more easy for the charge of PET to inject into the test sample than that of PC in the same degree of aging.With the increase of depolarization time,the space charge quantity of different aging stages all showed obvious decreasing trend and slowed.With the increase of aging time,the space charge dissipation becomes slow.The longer the aging time,the more the residual charge left.As for the sample aged under higher temperature,the dissipation rate of charge generally becomes slower and more residual charge left.Compared with the space charge characteristics of aging polymer under DC field,it is much more difficult for charge under AC electric field to inject inward.Space charge injection and dissipation characteristics show that the aging resistance of PC materials is better than that of PET,while PET is better than PC when it comes to mechanical properties at 130℃,but the actual operating temperature of transformer rarely reaches this level,therefore,after the comprehensive evaluation,the aging resistance of PC is better.