Study On Discharge Characteristics And Its Effect On Insulation Detorioration Of Thermal-aged Vegetable Oil-paper Insulation

Vegetable insulating oils are fire-resistant,biodegradable and renewable liquid dielectrics with good propertises.Nowadays,the transformer industry is experiencing a good application of vegetable insulating oils in distribution transformers.However,the use of vegetable insulating oils in power transformers is still under developing.The main reason is that the molecular composition of vegetable insulating oil is quite different from that of mineral insulating oil,which leads to the remarkable differences in degradation rate and products of these two types of oil-paper insulating systems under the same operating conditions.Moreover,the developing characteristics would be different from each other under the same discharge faults.Thus,the traditional detection techniques and rules cannot be applied to the status assessment and fault diagnosis of vegetable-oil power transformers without any modification.The investigations on the thermal aging characteristics and partial discharge(PD)characteristics are still in their infancy.The existing research results are not sufficient to guide the safe and stable operation of vegetable-oil power transformers.Therefore,it has signifacent importances in theory and engineering to investigate the discharge characteristics and its effect on insulation detorioration of thermal-aged vegetable oil-paper insulation.For solving the problems mentioned above,rapeseed oil impregnated paper and soybean oil impregnated paper were selected as the main research objects in this thesis,while traditional mineral oil impregnated paper is taken as the reference object.The differences in aging characteristics of different oil-paper insulation systems under thermal stress were studied comparatively.The effect of thermal stress on the property degradation and structural damage of vegetable oil impregnated paper was analysed systematically from the molecular and microscopic level.The phenomenon of accelerated electrical aging within the air-gap defects of vegetable oil impregnated paper was discussed.The PD characteristic parameters closely related to the electrical aging state were extracted,which provide necessary support for the deep analysis of PD developing characteristics of vegetable oil impregnated paper.The evolution rule of PD patterns and inslation deterioration of thermal-aged vegetable oil-paper insulation under electrical stress were obtained.Furthermore,the correlation mechanism between the PD characteristics and the microscopic characteristics were revealed.The main innovative achievements of this thesis are as following:(1)A large number of accelared thermal aging tests were carried out.The property deterioration of vegetable oil impregnated paper and mineral oil impregnated paper in the process of thermal aging were compared,and reason for these differences were discussed.The results show that,during the process of thermal aging,the strong hydrophilicity and the water comsumption because of the specific hydrolysis reaction of vegetable oil impregnated paper weaken the promoting effect of water on the cellulose degradation.Vegetable oil impregnated paper exhibits much better thermal stability,and the energy barrier needed to overcome during the pyrolysis process of vegetable oil impregnated paper would be reduced with the increase of thermal aging time.The relaxation polarization of thermal-aged vegetable oil impregnated paper would be more significant under the stress of external electrical field,and the vegetable oil impregnated paper with higher thermal aging degree exhibits higher dielectric loss at a lower frequency band.(2)The changes of function groups,supramolecular structure,and surface morphology of thermal aged vegetable oil impregnated paper with the variation of thermal aging time were investigated systematically.The influence mechanism of thermal stress on the structural damage of vegetable oil impregnated paper was revealed.Hence,the deterioration process of vegetable oil impregnated paper under thermal stress could be explained from the molecular and microscopic levels.The results show that thermal stress could promote the occurrence of esterification reaction in vegetable oil impregnated paper,leading to the formation of ‘water barrier'.The degradation process of amorphous region is inhibited during the thermal aging process of vegetable oil impregnated paper.The crack process of the second wall in the fibre of vegetable oil impregnated paper is relatively slower under the same stress,which indicates that better machinery intensity is maintained.(3)The accelerated electrical aging test under the air-gap defect of vegetable oil impregnated paper was carried out by the constant voltage method.The initial discharge characteristics,the characteristics of PD current pulse waveform and the characteristics of ?-q-n plot,which are closely related with electrical aging state,were extracted.The differences in the PD characteristic parameters during the process of electrical aging were analyzed from the discharge mechanism.The study found that,compared with mineral oil impregnated paper,the initial discharge voltage of vegetable oil impregnated paper is lower by about 13%,the initial discharge is smaller,and the initial phase angle is smaller.The higher content of electronegative gases and moisture within the air-gap defect of vegetable oil impregnated paper leads to more remarkable ‘rabbit ear' phenomenon and the disappear of current pulse overlapping phenomenon.The higher generation rate of free electron in vegetable oil impregnated paper results in more obvious shift of pulse phase.The significant increase of pulse number in the later stage of PD is the main factor causing the final breakdown.These results provide a valuable reference for identifying the electrical aging stages of vegetable oil impregnated paper.(4)PD tests of vegetable oil impregnated paper and mineral oil impregnated paper with different thermal aging condition were carried out systematically.The evolution of PRPD patterns of thermal-aged oil impregnated paper under electrical stress was obtained.Moreover,the effect of electrical stress on the microstructure of thermal-aged oil impregnated paper was analyzed.The study found that the oil impregnated paper with higher thermal aging degree exhibits smaller initial phase angle,larger phase width,higher amplitude and earlier appearance of ‘rabbit ear'.The pulse phase characteristics of vegetable oil impregnated paper nearing the end of its life are much more remarkable.In the early stage of PD,the initial phase of discharge pulse has been ahead of 0° and 180°,and the phase width even exceeds 100°.The esterification reaction in thermal-aged vegetable oil impregnated paper is promoted significantly under the electrical stress.Compared with mineral oil impregnated paper,thermal-aged vegetable oil impregnated paper remains larger amorphous region during the PD process.The damage degree is lower and more ‘droplet' and ‘crystal' are observed on the surface of vegetable oil impregnated paper.(5)The dielectric properties and space charge propertises of thermal-aged oil impregnated paper with different PD stages were obtained.Furthermore,the correlation mechanism of PD developing characteristics and insulation deterioration was established.It is found that the supramolecular structure of oil impregnated paper would affect the formation process of trap on the air-gap wall.The larger amorphous region leads to more shallow traps by particle bombardment,which results in more obvious phase shift and larger number of discharge pulses.Compared with mineral oil impregnated paper,the electrical field distortion of vegetable oil impregnated paper is lower,leading to smaller discharge amplitude,and ‘rabbit ear' pattern is promoted with the formation of ‘droplet' and ‘crystal'.These results provide theoretical support for diagnosing the insulation state in vegetable oil transformers by PD signals.