Study On Low Temperature Composite Insulation Damage Characteristics Of High Temperature Superconducting Cable Under Air Gap Discharge

The stress of superconducting cable insulation material in the production process and the influence of mechanical force during synthetic wrapping of superconducting cable will cause slight damage to the insulation structure and leave small gaps in the insulation material.Under the action of external large electric field,the local electric field distortion occurs at the air gap defect,which will damage the superconducting insulation.In this paper,the air gap discharge test platform of high temperature superconducting composite insulation is built in the high voltage laboratory to simulate the air gap fault defects of superconducting cable in the real operating environment.The pulse current method is used to collect the discharge signal of the discharge model from the initial discharge to before breakdown.In this paper,the air gap discharge process of defect model is divided into initial discharge stage,development stage,explosion stage and pre breakdown stage by using hierarchical clustering method.The insulation damage of the surface and middle layers of polypropylene laminate（PPLP）,a superconducting insulating layer material at different stages under air gap discharge,was determined.The roughness,surface morphology,functional group change,dielectric loss,surface resistivity and other phased characteristics of the insulating layer were observed.The main conclusions are as follows:（1）At different discharge stages,the kraft paper layer on the surface of superconducting cable insulation material PPLP has two obvious change characteristics:first,the texture expands and breaks,and the whole tends to fragment;Secondly,there are many voids,the number and size of which increase with the discharge stage,and close to the connected state before breakdown.The surface resistivity decreased by 76.48%from the original sample to the pre breakdown stage.The dielectric loss increases from0.05048 in the original state to 0.15521 before breakdown.The cleavage of cellulose molecules on the surface of cable insulation material PPLP is mainly the breaking of C-O-C bond on the main chain,the formation of a monomer molecule with glycosidic side chain and a four carbon molecular fragment,and the formation of glucose monomer structure.The structure of glucose monomer and the dimer product are further cleaved to form smaller two carbon molecular fragments and small molecular substances containing hydroxyl groups.（2）In different discharge development stages,the polypropylene layer of the insulating material PPLP intermediate layer has the characteristics of small particles,"pits","cracks"and"dense convex morphology"on the smooth surface;The two-dimensional and three-dimensional indexes corresponding to the observation results of scanning electron microscope showed phased changes.The surface resistivity of the material decreases by 86.16%from the original state to the stage before pre breakdown at50 Hz.The dielectric loss increases from 0.178395 in the original state to 0.197845before breakdown.In the spectrum after breakdown,four high-intensity absorption peaks appear at the wave number of 770 cm^-1,1150 cm^-1,1300 cm^-1,1700 cm^-1.The molecular groups of polypropylene in the interlayer of insulation material PPLP were mainly broken by the C-C bond and C-H bond on the main chain,and the original structure was destroyed.