Correlation between electrical signals, tree sizes and breakdown voltages of water treed model cables

Correlation between electrical signals, tree sizes, and breakdown voltages was investigated for XLPE (cross-linked polyethylene) water-treed model cables. After electric aging in the water bath the current-voltage characteristics of the cable specimens were examined, and the breakdown strengths were measured using a slowly ramped voltage (1 kV_rms/min, active length of 0.5 m). The insulation was then cut into slices, stained with methylene blue, and examined microscopically using a semiautomatic image analyzer. It is concluded that polarization and depolarization currents are a more sensitive indicator for water trees in model cables than loss tangent measurements at 50 Hz. The electrical signals correlate to the actual water tree content and the breakdown strength measured afterward. Measuring from time to time the polarization and depolarization currents and the loss tangents of cable insulation stressed in a long term AC test is a possible way to monitor `continuously´ the generation and growth of water trees. The advantage of this method lies in the fact that only one specimen is used for the different load times, so there is no individual scatter