| Moisture is one of the most important factors leading to the accelerated aging of oil-paper insulation,the decrease of electrical and mechanical properties,and the increase of dielectric loss.The moisture balance characteristics of oil-paper insulation are studied,which can be used to evaluate the moisture content in insulation paper.However,the oil-paper insulation degrades gradually with the growth of service time of transformer and its capability of moisture absorption changes.This results in the moisture distribution differs greatly for oil-papers of different ageing stages and affects the accuracy of moisture evaluation in transformer insulation paper.At present,the influence of thermal ageing on capability of moisture absorption of vegetable insulating oil is not clear up to now and there is a lack of research on moisture distribution characteristics of vegetable oil-paper insulation under thermal ageing conditions.Therefore,it is difficult to establish a method to evaluate moisture content in vegetable oil-paper insulation in transformers.Therefore,the moisture absorption characteristics of thermal aging vegetable oil-paper insulation were studied,and the primary content of the dissertation includes:(1)The difference of the typical aging behaviors between vegetable and mineral oil-paper insulation,and moisture generation rule in aging process were studied.The moisture absorption mechanism of thermal aging vegetable oil-paper insulation was revealed.The results show that the acid value and moisture content of vegetable oil were higher,but the moisture content in paper was always lower than that of mineral oil-paper insulation.At the end of aging,the DP of vegetable oil-paper system was higher,which showed better thermal protection effect on insulating paper.The main reason for the enhancement of vegetable oil moisture absorption capacity was the accumulation of acid degradation products.The moisture absorption ability of insulation paper after aging was weakened due to the increase of the amorphous region,the increase of the inner pore and the decrease of the specific surface area.(2)The moisture equilibrium experiments were conducted.The equilibrium curves considering thermal aging were plotted.The moisture steady distribution rule in oil-paper insulation at different aging stages was obtained.A mathematical model of moisture equilibrium with thermal aging factors was established.The results show that the main reason for the migration of equilibrium curve was the change of moisture absorption ability of oil-paper insulation caused by temperature and thermal aging.And the higher temperature,the more serious aging,the stronger the moisture absorption capacity of insulating oil,the weaker the moisture absorption capacity of insulation paper.The moisture absorption ability of vegetable oil was more sensitive to temperature and aging than that of mineral oil.The moisture equilibrium mathematical model could take into account the synergistic effects of temperature and thermal aging,which described the moisture equilibrium of vegetable and mineral oil-paper insulation under different conditions.(3)Moisture balance and diffusion experiments of different aged oil-paper were conducted.The effects of thermal aging on the moisture equilibrium characteristics and the effects of thermal aging on the moisture equilibrium and diffusion characteristics were obtained.The results show that in the early stage of aging,the moisture absorption capacity of vegetable oil was increased slightly,but the moisture absorption capacity of insulation paper decreased sharply and the relative ability was stronger.In the middle and later stage of aging,the moisture absorption ability of vegetable oil increased rapidly,and the moisture absorption ability of insulation paper decreased slowly and gradually.The moisture equilibrium curve was initially caused by the aging of the insulation paper,and the latter has the common influence of vegetable oil-paper aging.In the vegetable oil paper system,the worse the insulating paper degraded,the greater the rate of moisture diffusion was,the shorter the time to reach a steady-state was,and the lower the moisture content in a steady state was. |
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