抄録
Water trees in insulator of XLPE cable may considerably reduce the dielectric breakdown voltage, thus being an important lifetime-governing factor. AC superposition method we have investigated is a new technique for hot-line diagnosis of 6.6kV XLPE cables, and the diagnostic apparatus using this technique is now widely used in the field of distribution line. In order to study an application of the AC superposition method to degradation diagnosis of 22/33kV class XLPE cables, we measured deterioration signals of the cables with water trees by modified above diagnostic apparatus. The deterioration signals, hereafter called “AC superposition current”, were generated by an amplitude modulation effect due to non-linear resistance of water tree. Moreover, we evaluated the relationship between the AC superposition current and the AC breakdown voltage. It is difficult to judge whether the correlation of them is present or not because of uneven distribution of the measured data. However, the AC superposition current tends to increase linearly with a decrease in the residual thickness of the insulator. For example, the AC superposition current was about 40nA when the residual thickness decreased to 3mm. Thus, we consider that the AC superposition method is available for degradation diagnosis of 22/33kV class XLPE cables.
