Effect of DC tests on induced space charge

Poling-induced space charges in peeled samples of AC-aged, DC-tested cables were studied using the thermal pulse technique. Samples were poled at approximately 0.12 MV/cm for 3.5 h at 70 degrees C using the same polarity as that used for the DC tests. Previous work seemed to indicate that DC testing could have permanent effects which could be detected by the amount of poling-induced charge. An attempt was made to examine this by using three samples with about the same length of AC aging but with different amounts of DC testing. The sample with one DC test and the sample with 23 DC tests were appreciably oxidized, whereas the sample with 11 DC tests was not oxidized. In addition, when the sample was pulsed from the face exposed to the positive poling electrode, a negative space charge was observed. When it was pulsed from the opposite face, a positive space charge was observed. The space charge decay was found to be asymmetric. AT 70 degrees C, the negative space charge decays faster than the positive space charge. At 60 degrees C, the decay was observed to be about the same on both sides. When the poling field is small, both sides have a positive space charge. For larger poling fields, the space charge in the film close to the positive electrode is negative.<
>