Backscattered electron imaging and energy-dispersive X-ray studies of water-treed polymeric insulation

The backscattered-electron imaging mode in a scanning electron microscope has been used to detect and image water trees in electrically stressed cross-linked polyethylene (XLPE) insulation. The location and distribution of inorganic contaminants, such as Cu or Cl, within the water-treed regions of the insulation were revealed using backscattered-electron imaging. Energy-dispersive X-ray (EDX) spectroscopy and mapping were subsequently used to ascertain the chemical nature of the contaminants. Contaminants were only detected by EDX inside the treed areas delineated by the backscattered-electron images. A careful comparison of backscattered-electron and optical images of thin microtomed XLPE samples did not show any contaminant penetration beyond the visible-treed region. Results from a variety of service-aged and laboratory-stressed samples are presented, as well as a preliminary attempt to extend backscattered-electron imaging to optically opaque ethylene-propylene-rubber insulation