Effect of DC testing water tree deteriorated cable and a preliminary evaluation of VLF, as alternate

According to the experience of some power utilities, the application of industry recommended high voltage DC field tests on 5-35 kV extruded dielectric cables, containing water trees, sometimes causes further deterioration of the insulation. Tests conducted on laboratory aged 15 kV ethylene propylene rubber (EP) and 15 and 28 kV crosslinked polyethylene (XLPE) insulated cables indicate that DC proof tests in accordance with AEIC specifications and IEEE test guides without flashover do not appear to cause further deterioration. Depending on the degree of cable aging and the level of test voltage, when flashovers take place, damage may be inflicted on XLPE cables. No damage was observed on aged EP cable at the same test levels. Tests conducted on new XLPE and EP cables indicate that damage to the insulation structure can be detected using VLF (0.1 Hz) voltage at approximately one-third the DC voltage level. Field tests conducted on severely tree deteriorated 15 kV polyethylene (PE) cable using the AEIC recommended DC voltage level of about five times operating voltage level caused cable failure; VLF voltage levels up to two times operating voltage did not. VLF voltage appears to be a suitable alternative to DC voltage for field proof testing