Influence of Structural Change by Water-tree Degradation on Electrical-tree Inception Voltage

To improve accuracy of degradation diagnosis for CV power cables, it is important to clarify the mechanism of inception and propagation of electrical-tree from water-tree degradation. In our previous work, ac-electrical-tree inception voltage was measured at 303 K for the samples with water tree formed mainly at 363 K. As a result, the samples with water tree, in spite of the existence of water tree degradation, showed higher ac-electrical-tree inception voltage than those without water tree. This was attributed to electrical field relaxation. To study the effect of change in material structure itself, it is necessary to reduce the field relaxation effect. In the present work, therefore, we used the samples with dried water tree and applied impulse voltages, and discussed the effect of structural change due to water tree degradation. As a result, the impulse-tree inception voltage in the samples with dried water tree was lower than those with water tree but was slightly higher than those without water tree. In the sample with dried water tree, decrease in water molecule density which is considered to restrain the electron may lead to the lower impulse-tree inception voltage. However, the remaining degradation products and structural change may lead to the slightly higher impulse-tree inception voltage than those without water tree. The effect of prolonged ac voltage application prior to impulse voltage was also studied to discuss the effect of water tree degradation on long term deterioration of cable performance.