How good are electrical diagnostic tests to predict water trees in HV cables - An evaluation using optical microscopy

DC Current, DC Conductivity and Partial Discharge testa were performed as diagnostic tools to indicate the presence of water trees in 15 kV. 35 kV and 69 kV underground field-aged high voltage cable samples from the Eastern Province of Saudi Arabia. The insulation micro-sections of these cable samples were then observed under an optical microscope for visual detection of water trees. The DC Current and the DC Conductivity tests showed insulation deterioration in the 15 kV, 35 kV and the 69 kV cable samples, with the possibility of existence of water trees in them. Optical microscopy confirmed the presence of water trees in the 35 kV and 69 kV cable samples, but not in the 15 kV sample. The Partial Discharge test carried out on one 35 kV cable sample of sufficient length also showed insulation deterioration. However, optical microscopy could not detect any water tree in this cable sample.