Study On Partial Discharge Mechanism In Laminated Oil-paper Insulation

Transformer bushing is one of the most important components of power transformer.The Oil-immersed paper capacitive bushing is widely used because of its stability,excellent electrical performance,good explosion-proof and flame-retardant performance,small volume,and lightweight.The insulation of oil-immersed paper bushing is mainly insulated by multi-layer paper,but at present most of the oil-paper insulation models are mainly insulated pressboard.Only a few scholars have conducted in-depth experimental research and testing analysis on multi-layer insulation paper.Aiming at the problems existing in the research of oil-immersed paper(or pressboard)under partial discharge,this paper mainly studies and analyzes the whole process from the beginning of the discharge to the breakdown of oil-paper laminated insulation.Firstly,the test sample of oil-immersed paper capacitive bushing of transformer is established,and the test pressurization system is designed,which mainly includes: partial discharge simulation platform,partial discharge detection and acquisition platform;and the structure of pressurized electrode is designed.Secondly,the constant pressure method is used as the pressurization method,the discharge quantity and phase of insulation paper in partial discharge process are measured and counted in real time,and the discharge stage is divided by the detected partial discharge signal.After each discharge process,the area of carbonization trace on the surface of insulation paper is counted and analyzed.Finally,the surface and internal fiber morphology of insulation paper at different discharge stages were observed by digital microscope and scanning electron microscope,and the changes of functional groups were analyzed by Fourier transform infrared spectroscopy,and the changing trend of carbon and oxygen content on the surface of insulation paper was analyzed by X-ray energy dispersive spectrometer.The results show that the area of carbonization trace on the surface decreases layer by layer from high voltage electrode to ground electrode in each stage of partial discharge.With the extension of pressure time,the carbonization trace area of each layer showed an increasing trend.At the initial stage of discharge,the surface of the fiber is arranged neatly and orderly,with only a small number of holes;at the stage of partial discharge development to breakdown,the surface of the carbonized part of the insulation paper is sparse and loose,large pieces of fibers are bonded together and some of the fibers are detached from the fiber body,and a small amount of crystalline solid is attached to the fiber surface.The content of "C-O-C" and-OH of glucose monomer decreases layer by layer as the insulation paper approaches the high-voltage end,which indicates that the accumulation of high-energy electrons in partial discharge will directly cause the breakage and reduction of glucoside and hydrogen bonds,and then lead to damage of fiber macromolecular chains,affecting the surface morphology of insulation paper.In the process of partial discharge damage,high-energy electrons will erode more oxygen in peeled fiber macromolecule.At the same time,it will gradually form a carbonization trend on the surface of insulation paper.As a result,the carbon content of each layer increases with the prolongation of pressure time,and the oxygen content decreases with the prolongation of pressure time.These results provide a reference and research idea for the partial discharge mechanism of multilayer insulation paper.