Research On Streamer Characteristics Of Partial Discharge In Vegetable Oil-Paper Insulation Under AC Voltage

Vegetable insulating oil,as an environmentally friendly liquid dielectric,has high ignition point and excellent electrical properties.It has become a good substitute for traditional mineral oil and is gradually used in oil immersed power equipment.The insulation defects of power equipment caused by in process of manufacturing and operation can cause partial discharges in the oil gap or oil-paper interface to damage the insulation system.At present,the research on the discharge development mechanism of partial discharge of vegetable oil-paper insulation at power frequency voltage is still insufficient.And it cannot provide experimental basis and theoretical support for the design,manufacture and maintenance monitoring of vegetable oil power equipment.In order to solve the above problems,this paper carried out partial discharge tests of vegetable oil-paper insulation under power frequency voltage,and studied the streamer characteristics of partial discharges.The main contents include:(1)A partial discharge test platform of vegetable oil-paper insulation under power frequency voltage was built,and the partial discharge process of vegetable oil-paper insulation under severe non-uniform electric field was simulated by needle plate electrode structure.The discharge imaging recording platform was established based on the shadow method.Using the oscilloscope and pulse delay generator to trigger the high-speed camera to record the streamer development process of partial discharge in vegetable oil-paper insulation.The results show that the test platform can effectively collect the current signal and streamer pattern of partial discharge.(2)The partial discharge tests of three vegetable insulating oils(soybean-based FR3 insulating oil,rapeseed-based RDB insulating oil,and palm-based PFAE modified insulating oil)at power frequency voltage were carried out.Through the discharge imaging recording platform,the streamer characteristics of partial discharge was obtained.The results show that the positive and negative half-cycle streamer are different.The positive streamer is filamentous and multi-branch filament-like,and the negative streamer is shrub-like and tree-like.The length of positive streamer is obviously longer than that of negative streamer.These differences are attributed to the different effects of space charge on the space electric field distribution during the development of positive and negative streamer.Although the ionization energy of molecules in vegetable oil is lower than that of mineral oil,the vegetable oil with higher viscosity can delay the electron migration rate and restrict the development of streamer.Therefore,the streamer development speed and stopping length of high viscosity vegetable insulating oil(FR3 and RDB)are lower than that of mineral insulating oil.The streamer development speed of PFAE insulating oil with the lowest viscosity is the fastest,and the streamer stopping length is the longest.After discharge,the residual space charge forms a weakened area of the space electric field under the same voltage polarity,which limits the development of subsequent discharge.The undissipated space charge after the polarity conversion forms an electric field strengthening region to promote the generation and development of discharge.(3)Three kinds of vegetable oil-paper surface partial discharge tests were carried out under power frequency voltage,and the streamer development characteristics and variation rules were obtained.The influence mechanism of surface space charge on streamer development in subsequent surface partial discharge was analyzed,and the cumulative effect of surface partial discharge was studied.The results show that because of the dielectric constant of the pressboard is higher than that of vegetable oil,a large amount of charge in streamer head will generate corresponding polarized charges at the oil-pressboard interface.The electric field formed will attract the charge of streamer head to gather on the surface of paperboard,which makes the charge density of streamer head increase and promote the development of discharge.Therefore,the streamer of partial discharge in vegetable oil-paper insulation develops along the oil paper interface,and the streamer development length,maximum apparent discharge and discharge times under the same voltage are higher than the pure oil gap test results.The streamer development process of the surface partial discharge is affected by the molecular ionization energy and oil viscosity.During continuous application of voltage,after the gas products generated by the surface partial discharge accumulate to a certain extent in the pores on the insulating pressboard surface,a large number of air bubbles overflow from the pressboard surface and the discharge signal changes.And white marks appear on the pressboard surface.