The space charge decay of LDPE and HDPE under different temperatures

Polyethylene is a material that is widely used as insulation for power cables. In this paper, the effect of temperature on the space charge decay of LDPE and HDPE is described. After voltage application of 5 kV (50 kV/mm) at 25°C negative space charge was mainly found in the bulk of HDPE samples, on the other hand positive charge was retained in LDPE samples. When the applied voltage of 18 kV (180 kV/mm) was removed negative space charge was mainly found in the bulk region of the LDPE samples, whereas positive space charge was predominantly observed in HDPE samples. In this paper attention is focused on the space charge profile and dynamics at the higher field (180 kV/mm) as this is near the dielectric breakdown field of polyethylene. The experiments were carried out at four different temperatures (25, 40, 60, and 90°C). The space charge decay time in LDPE was found to be shorter than that in HDPE after removal of the 180 kV/mm field. It is suggested that trapped positive charge was not de-trapped easily in HDPE. The space charge decay time was found to decrease with increases in either temperature or magnitude of the anode (LDPE) and cathode (HDPE) field measured just before voltage was removed.