Investigation On Electrical And Physical Properties Of Nomex Insulation Paper In The Mining Dry Type Transformer During Multi-stress Aging

This thesis is one of the main research contents of the National Natural Science Foundation of China “Nomex insulation aging mechanism for mining dry type transformer”(51577123),aiming to solve the aging problem of main insulation of mine dry-type transformer.Nomex insulation paper has been widely used in the field of mining-dry type transformer insulation because of it’s excellent performance,such as favorable mechanical properties,great electrical characteristics,good thermal stability,high moisture resistance,and so on.However,the environment of coal mines is much severer than that on the ground.Under the combined stress of high temperature and moisture,which would cause the change of internal microstructure of Nomex insulation,carriers injected into the dielectric would be trapped to form space charge under the action of electric field.The space charge accumulated in the insulating paper would distort the original electric field leading to partial discharge in the dielectric,which could accelerate the process of insulation deterioration.Therefore,it is of great theoretical value and practical significance to study the space charge characteristics of Nomex insulation during hygrothermal aging process.At present,the research on space charge characteristics of transformer insulation paper by scholars at home and abroad is mainly focused on the space charge distribution characteristics and formation mechanism of oil-immersed cellulose paper under the influence of electric field,moisture,temperature and thermal aging.However,there are few reports on the space charge characteristics of Nomex insulating paper for mining dry type transformer during thermal aging.In this paper,the space charge characteristics of varnish-immersed Nomex paper with different aging degrees under varying electric field intensity has been measured.The relationship between space charge behavior,dielectric and physicochemical properties were analyzed.The research is as follows:The hygrothermal aging test was conducted,based on the accelerated aging test system(sealed reactor was used as the hydrolysis reaction vessel and the accelerating aging oven was used as the thermal aging device).Considering the IEC 60811 standards,combined with plenty of testing results of research team,160℃,180℃ and 200℃ were selected as the the temperature of thermal aging test to meet experimental requirements.During the aging process,the initial moisture contents of insulating paper were chosen as 3%,5% and 7%.Referring to the actual production process of mine dry-type transformer,the varnish-immersing test platform of aged Nomex specimens was established.The space charge distribution of varnish-immersed Nomex aging specimens at-5kV/mm,-10 kV/mm and-20 kV/mm was measured at room temperature(20℃).Simultaneously,the effect of space charge on the dielectric strength of Nomex was analyzed.The results show that with the increase of electric field,the amount of fast and slow space charges in Nomex increases because of the change of trap energy level within insulating paper.As the aging degree deepens,the charge dissipation rate decreases,resulting in a significant increase in the local electric field distortion rate.The space charge distribution and dielectric properties of Nomex insulating paper under varying aging temperatures and initial moisture contents were measured in this paper.Combined with the results of X-ray diffraction(XRD)and scanning electron microscopy,on the basis of trap theories,the evolution mechanism of space charge behavior during thermal aging process was further discussed(the total charge quantity,fast and slow moving charge,trap energy level distribution were chosen as characteristic quantities).The results show that the hetero-polar charge at the varnish-paper interface near the anode was derived from the migration of electrons injected from cathode;with the increase of aging temperatures and initial moisture contents,the phenomenon of space charge injection became more obvious,and the trap energy level further deepened,resulting in the slower charge dissipation rate;the decrease of relative dielectric constant εˊ would change the distribution of space charge in the insulating paper;during the process of hydrolysis,voids began to appear at the intersection of pulp and chopped fibers on the insulating surface,while the crystallinity of aged specimens and the size of the nucleus decreased.