Laboratory aging of TR-XLPE cables at 500 Hz

Two 15 kV cables insulated with 5.6mm of tree-retardant cross-linked polyethylene (TR-XLPE) have been subjected to wet-electrical aging at 5.9 kV/mm (150 V/mil) and 500 Hz. A daily thermal load cycle is induced, with 8 hours of 50 Hz current and a 16-hour cool in the ambient tank water. The insulation shield temperature in water reaches 45 °C at the end of the heating cycle, similar to the AEIC accelerated water treeing test. The condition of the aged cable has been assessed after 3 months and 6 months of aging by a series of diagnostic tests. Breakdown strength and dissipation factor measurements are similar at the two aging times, indicative of an extremely slow aging mechanism. Water-treeing characterization after 6 months of aging is compared to evaluations after various aging times at 60 Hz as per the accelerated water treeing test protocol. The results of the 500 Hz aging program indicate a much lower degree of aging acceleration for TR-XLPE insulated cables than is predicted by models in the literature for XLPE cables.