Effect of temperature and humidity on electrical breakdown characteristics of low-density polyethylene film for the various stressing modes

The effects of temperature and humidity on the electric strength of low-density polyethylene (LDPE) film for various stressing modes were studied experimentally. As the temperature exceeds the glass-transition temperature of the polymer, the temperature dependence of the impulse electric strength follows the free volume breakdown theory. If the breakdown process is greatly controlled by space charge, the application of free volume breakdown theory is limited. In addition, it is shown that the water molecule acts either as acceptor or as plasticizer, thus exerting a complicated influence on the dielectric breakdown in the polymers. It is also shown that the higher the direct prestressing, the more charges injected in deeper traps as the temperature rises but does not exceed 70°C, because the space charge in deeper traps disappears only partly or slightly; the recovery of the normalized impulse breakdown strength of the dry specimen becomes slow and even vanishes. These findings are confirmed by thermally stimulated current measurements