THE DIRECT-CURRENT PRE-STRESS EFFECT ON ELECTRICAL BREAKDOWN IN OXIDIZED POLYETHYLENE

The effects of direct current pre-stress on impulse electric strength Fb Of oxidized polyethylene were investigated. For the sample oxidized for 50 h (heavy oxidation), Fb decreased with pre-stress field with b oth the same and opposite polarity to the subsequent impulse. These re sults were interpreted in terms of either deep penetration of home spa ce charge injected from an electrode or thermal breakdown due to the l arge current increase caused by heavy oxidation. For oxidation time 25 h; F-b increased and decreased with field aiding and opposing pre-str ess, respectively. The space-charge profile was also observed by the l aser induced pressure pulse method. The relation between the pre-stres s effect and the space charge distribution can be quantitatively expla ined in terms of the electric field distortion due to the effect of ho me space charge formed during the pre-stress. The effects of pre-stres s time and rest time following application of pre-stress on Fb were al so investigated.