Photodegradation versus hot-electron impact for electrical tree inception at low electric fields

A mechanism of degradation in polymeric insulation at fields lower than required for hot-electron transport is shown. Cable grade low-density polyethylene was aged under divergent and uniform fields, using needle-plane and dimple-plane geometries, respectively. It is shown that, at points of electric stress enhancement in the polymer, the light emitted during the initiation phase of electrical treeing is not due to hot-electron impact but is due to the recombination of electrons and holes injected into the material. The spectra of the emitted light are in the visible and ultraviolet ranges. The ultraviolet light can photodegrade the polymer and lead to electrical treeing. Long-term aging of polyethylene, under divergent and uniform fields, indicates that treeing does not occur at fields below the light inception level. However, above this level an electrical tree always develops and the time to treeing depends on the voltage applied to the polymer