Research On Dynamic Characteristics Of Streamer Discharge And Product Analysis In Natural Ester

Natural ester insulating oil has excellent physical and chemical properties,along with high ignition point and biodegradability.It is an ideal substitute for mineral insulating oil.Nowadays,the application of natural ester oil-immersed distribution transformers has been quite extensive,and it is being promoted to power transformers.At the same time,natural ester oil-paper has excellent insulation coordination and high overload capacity.Under the condition of high overload,the breakdown characteristics and product law of natural ester insulating oil needs to be studied.Therefore,the author conducted research on the streamer dynamic characteristics of natural ester and the breakdown products at high temperature.The main contents are:(1)Test tanks were designed and manufactured to conduct the long and short gap discharge of insulating oils under normal and high temperatures.The shadowgraph method was used to record the dynamic characteristics of streamer in the insulating oil,and two sets of optical recording systems with long and short oil gaps were established.High-speed trigger recording of the streamer of insulating oil is realized using an oscilloscope,a pulse delay generator,and a high-speed imaging device.Based on the streamer discharge recording system,the lightning impulse discharge test platform and the AC partial discharge test platform were built.The step-up method was selected as the test method for lightning impulse and AC breakdown test.(2)The lightning impulse discharge test of natural ester insulating oil was carried out,and breakdown characteristics of the insulating oil under different oil gaps were obtained,and the dynamic process of the streamer was recorded.The results show that the breakdown voltage of natural ester is significantly lower than that of mineral oil under long oil gap.Natural ester is more inclined to accelerated breakdown under positive lightning impulse voltage.During the pre-breakdown process of natural ester,the streamer experienced the process of initiation,development and dissipation,and the existence of secondary reverse streamer was also found.The streamer pattern was analyzed,stopping length,streamer area and fractal dimension were extracted as characteristic quantities.At the same time,the partial discharge streamer and its corresponding current under the AC voltage were compared and analyzed.(3)The effect of high temperature on AC breakdown voltage of natural ester was tested.The samples before and after breakdown were tested by gas chromatography.The molecular dynamics simulation of triglyceride system under thermal and electrical stress was realized by ReaxFF reaction field.The results show that the breakdown voltage increases first and then decreases with the increase of temperature.Under the nonuniform electric field,the influence of temperature on the breakdown voltage is relatively small.After the breakdown,the polyunsaturated fatty acid represented by linoleic acid was greatly reduced,and the fatty acid saturation was significantly increased.Both the increase in temperature and the application of an electric field can accelerate the rate of decomposition of the triglyceride.The variation of decomposition products with time under breakdown electric field was obtained,and the reaction mechanism of triglyceride simulation system under thermal and electrical stress was revealed.