Effects of Charge Injection and Extraction on Tree Initiation in Polyethylene

Initiation of electrical treeing in polyethylene is discussed with the emphasis of a charge injection and extraction process, which is considered to feature the incubation period during which there are no apparently visual change and no detectable partial discharges. Some evidences for the process are shown by electrical, thermal and optical liberation of charge injected from a metal electrode into polyethylene. Effects of metal work functions on tree formation are demonstrated. On the basis of the above find- ings, a field emission and equi-energy theory is proposed for tree initiation and a derived V-t characteristic explains experimental results better than the common power law (t ¿V-n) for lifetime.