Spatial dependence of electric fields due to space charges in films of organic dielectrics used for insulation of power cables

The electric fields due to space charge within film samples of several commercial polyolefins used as medium-voltage power cable insulation were measured. The films were charged at 60°C with DC voltages applied for several hours. For dry charged films, vacuum-deposited gold or aluminum provided the electrodes. For wet charged films, one or both surfaces of the film were bare and in contact with an aqueous 0.05 molar solution of NaCl. The fields were measured using the thermal pulse (TP) method. To use this method with the wet charged films, colloidal graphite electrodes were painted onto the films after they were charged. The results show that thermoplastic polyethylene (PE) charges negatively for both wet and dry charging. Cross-linked polyethylene (XLPE) charges negatively when dry charged, but small amounts of either positive or negative charge were found after wet charging. Water-tree retardant cross-linked polyethylene (TRXLPE) charged positively for both dry and wet charging. A cross-linked (ethylene-propylene-diene) terpolymer (EPDM) formulation charges negatively when dry charged and positively when wet charged. The EPDM formulation was pyroelectric; this pyroelectricity was absent when the clay filler was removed from the formulation