Analysis of surface damage characteristics through air-gap discharge

Air-gap is one of the most common defects in the manufacturing and transportation of XLPE insulation, which might cause severe damage to power cables. XLPE insulation with air-gap defect would be degradation under long-term electrical stress. In acorrdance with actual defect, three-layer XLPE insulating sheet samples were designed and manufactured. Step-up voltage and pulse current method were carried out in the experiment. Afterwards, partial discharge signals and test samples at different development stages were collected. Then Kernel Pincipal Component Analysis (KPCA) was applied to reduce the characteristic parameters. Four development stages were divided based on the features by clustering: initial stage; developing stage; pre-breakdown stage and breakdown. Scanning electron microscope (SEM) and 3D surface profiler were employed to observe surface morphology and roughness of test samples at different discharge stages. The experimental results show that the results of are consistent with the observed discharge characteristics and trends in the air-gap discharge test. SEM result shows that the surface morphology of the test sample changes dramatically in the different discharge stages. Specifically, electrical tree and “Craters” began to appear on surface at the initial discharge; “3D meshes” and creepage appeared at the developing stage; “fractures” were discovered and molecular structure of insulation changed in the pre-breakdown stage. Roughness of insulation was also examined in this study. Above all, surface damage characteristics of XLPE insulation are strongly related to the development of air-gap discharge. It provides a experimental reference for a further study of the degradation mechanism of XLPE insulation by air-gap discharge.