Electrical tree and deteriorated region in polyethylene

In polyethylene subjected to high electric fields, a deteriorated region is formed before tree initiation. The chemical analysis of the deteriorated region by FT-IR and Raman spectroscopy showed that it contains excess carbonyl groups (C=O) and double bonds (C=C) compared to normal polyethylene. Scanning electron microscope observation found in the deteriorated region a number of voids with diameter <0.5 μm. The observation by transmission electron microscope revealed that the lamellar structure of polyethylene is destroyed in the deteriorated region. It was found that tree initiation voltages for AC ramp and positive impulse are increased after the formation of the deteriorated region. The change of polyethylene structure is considered to be responsible for the increase of treeing resistance. It is worth noticing that the formation of the deteriorated region has a possibility as a new way for voltage hardening of practical apparatus