Water treeing and dielectric loss of WTR-XLPE cable insulation

In the investigation, the authors have tested three commercially available water-tree retardant XLPE (WTR) materials and compared some of their properties with conventional XLPE insulation. They have focused on measurements of the initiation and growth of water trees and dielectric loss at power frequency (50 Hz). Water-tree tests were performed on Rogowski-type test objects kept at 20 degrees C and energised at 10 kV/mm for up to nine weeks of aging. Measurements of the dissipation factor tan delta were performed on 12 kV cable samples in the temperature range 20-130 degrees C, up to a maximum voltage of 96 kV_rms. The results show that water trees are also readily initiated in WTR materials, and that their growth rates are approximately as for conventional XLPE. In all the materials tested, introduction of mechanical strain strongly enhanced the initiation and growth of water trees. The WTR-cable samples examined showed different temperature- and voltage-dependent dissipation factors. Typically, the dissipation factor of degassed cable samples at 20 degrees C was about two times higher than that of conventional XLPE, and at 90 degrees C it was six times higher. This difference was found to increase with increasing voltage. Prior to the degassing of the by-products from the curing process, some of the WTR cables showed high dissipation factors even at 20 degrees C, and their values were found to increase nearly proportionally with applied voltage.