Study On Space Charge Characteristics Under Alternating Stress Of Oil-paper Insulation After Thermal Ageing

The converter transformer is one of the most important equipment in high voltage direct current(HVDC)transmission system.Unlike general power transformers,the valve side winding of the converter transformer(secondary winding)is not only subjected to alternating current(AC)stress but also to the component of direct current(DC)stress,the insulation problem is quite prominent.In this paper,the oil-paper insulation samples of converter transformer after different degrees of thermal ageing are investigated.The space charge characteristics are investigated under DC stress,AC stress and AC-DC combined stress based on automatic equipartition phase shift principle.Combined with physicochemical experiments including microscopic observation and infrared spectrum and electrical test including frequency domain spectrum and AC breakdown test,information inflected by space charge measuring experiment is study comprehensively to explore the relationship between ageing characteristics and space charge characteristics.The purpose of this paper is to provide a theoretical basis for the integrated insulation state evaluation method for oil-paper insulation of converter transformer.The result of microscopic observation shows that as the thermal ageing progresses,the molecular chains of the cellulose gradually break,the fibers tear and peel off and the surface of insulating paper becomes loose and scattered.The result of infrared spectrum shows that thermal ageing does not change the main functional groups in the cellulose insulation paper.With the aging time progresses,the functional groups are damaged to a certain extent.The result of frequency domain spectrum shows that the dielectric loss curve reflects the influence to the dielectric properties of oil-paper samples by thermal ageing.It is presumed that the thermal ageing effect is not obvious at the early stage of thermal ageing.The result of AC breakdown test shows the breakdown strength of the samples with different thermal ageing degrees decrease with the ageing time goes on,and the thermal ageing has a significant effect on the breakdown properties of the oil paper insulation.The space charge contribution shows that the charge injection field of the oil-paper sample is quite low under DC stress.The charge peak of the upper electrode is shifted to the inside of the sample with the polarization time,and the maximum value of the thermal ageing sample is 96 h;The hetero-charge accumulates at locations of both electrodes while short circuit,and it increases with the increase of the polarizing stress;The space charge decays gradually during short-circuit;Under high stress(50 kV/mm),the phenomena of serious bipolar space charge injection from both electrodes occurs and there is still a large amount of accumulated charge after the removal of polarization stress for 1800 s and the sample of 96 h is most serious.Under AC stress,the injected stress of space charge is obviously higher than that of DC stress,and the accumulation of the space charge is quite small even under higher stress.The distribution of space charge in the entire AC frequency cycle is with a symmetry to phase 180° axis of symmetry.The mean charge density of the samples under AC stress reflects the length of the thermal ageing stress of the samples to a certain extent.The mean charge density of the samples increases with the increase of the thermal ageing time.Under AC-DC combined stress,the space charge distribution do not make much difference at 30 min and 60 min.The effect of AC-DC combined stress on oil-paper insulation is not simple,and it involves more complex effects.With conclusions above,it shows that thermal ageing does cause thermal degradation of cellulose of insulation paper,including the breakage of cellulose molecular chains,hydroxyl radicals,primary alcohol radicals oxidized to water,organic acids and ionizable impurities.On the other hand,the evaporation of water and the loss of organic acids and ionizable impurities will be accelerated at high temperature.The ageing of the insulation can be divided into two stages according to the aging time.In the first stage,the rate of cellulose molecular chain pyrolysis and the formation of ionizable or polar by-products is lower than the accelerating loss of water and organic acids caused by high temperature,so that the number of traps increases slightly while the number of carriers decreases much.The mean charge density of sample thermal ageing 48 h is abnormally lower than that of no-ageing.In the second stage,due to more cellulose molecular chain breakage,the increase of glycosidic by oxidation and more oxygen and acid gas dissolved in insulting oil,so that number of traps increases much more while the number of carriers becomes increased.The mean charge density of sample thermal ageing 96 h is more than that of no-ageing and 48 h.The effect of thermal ageing tends to be obvious.