Thermally stimulated currents and space charge studies on field-aged extruded cable material

In the perspective of gaining more knowledge on extruded cable field aging diagnosis, complementary techniques were investigated: thermally stimulated currents (TSC) and space charge measurements, the latter being performed by the thermal step (TS) method. Measurements were taken on 28 kV extruded cable samples of cross-linked polyethylene (XLPE). Samples were peeled-off from three cables; one unaged and two field-aged. Both techniques show differences between field-aged and unaged cable material. Results obtained by TS show that aged material can store more space charges that lead to greater intensity of the electrical field in some sites in the polymer matrix and eventually initiate electrical trees leading to breakdown. Comparison with TSC results show that the origin of space charge formation cannot be attributed uniquely to traps formed by carbonyl groups from polymer oxidation.