Research On The Characteristics Of Large Oil Gap Discharge In Natural Ester Under Lightning Impulse

As a new kind of environmentally friendly liquid dielectric,the natural ester has excellent physicochemical properties,together with high fire point and Low toxicity.So they are regarded as suitable insulating material for transformer.There already existed a large amount of distribution transformers using natural ester in service in the world,while the use of natural ester in power transformer is still under developing for the higher requirement caused by the high electrical field strength,large insulating structure and the complicated electrical field distribution within power transformer.So the research on the discharge characteristics of natural ester at large oil gaps is conducted in this paper to provide data of characteristic and influence factor,and then improve relevant theory,which could be the valuable reference for the design,manufacture and operation of the transformers filled with natural ester.The main contents are showed as follows.(1)The platform of lightning impulse test for long gap discharge in insulating oil and image recording for streamer was set up and improved.The most suitable method for confirming breakdown voltage is chosen by comparing the results of breakdown voltage obtained and estimated by different voltage applying method.The way to adjust temperature and moisture in oil is raised up.The results show that the dispersion of data is smaller when rising voltage method is used to estimate the 50% breakdown voltage of insulating oil in large gap length,the image of streamer can be obtained by the system successfully.(2)The long gap discharge experiment in natural ester under lightning impulse was carried out.The breakdown characteristics under different oil gap length and oil type are obtained.The influence to breakdown characteristic from oil types,polarity of voltage and the temperature of oil are analyzed.The empirical formulas are fitted on the basement of current test data and previous published research by other institution.The results show that with the increase of gap length,the positive breakdown voltage of natural ester become much lower than mineral oil.And the threat from positive lightning impulse become more and more serious to natural ester in large oil gap.While the accumulation of breakdown times which causes the 20% decrease of breakdown voltage of mineral oil has no obvious influence to natural ester.(3)The shape information of streamer is obtained by high speed CMOS by shadow graph technique.The propagation regulation of streamer of different oil types and polarities is analyzed by the axial propagation characteristics and shape.The results show that the initiation of streamer is mainly influenced by curve radius.The propagation mode is mainly influenced by oil type as well as the length of oil gap.Compared with the one generated in mineral oil,under the similar strength of positive electric field the longer streamer which means longer stop length of streamer will be generated in natural ester.