The Influence of Pre-conditioning on Space Charge Formation in LDPE

The effect of thermal conditioning, at reduced pressure, on space charge accumulation in planar LDPE samples under DC stress has been investigated. The samples were 1.8 mm thick, including two 0.1 mm thick semicon electrodes, and they were conditioned prior to voltage application by being held at 80oC for 48 hrs in short-circuit at rotary pump pressure. Some were then cooled to 20oC, over a 6 hr period, still under rotary pump pressure and in short circuit, while others were cooled to 20oC in less than 5 minutes, in the laboratory air (still in short circuit). DC fields of 20 kV/mm were then applied at room temperature, and space charge accumulation was monitored as a function of time, using the pulsed-electro-acoustic (PEA) method (Denmark) and the laser-induced-pressure-pulse (LIPP) method (Australia). The space charge density was calibrated from measurements at fields up to 5 kV/mm, where charge injection from the electrodes or field-assisted thermal ionisation of impurities would not be expected.