Diagnostic testing of MV XLPE cables with low density of water trees

In this paper, results from diagnostic measurements performed on service aged 12 and 24 kV (MV) XLPE cables equipped with strippable insulation screens are reported. The main purpose of this work has been to examine if condition assessment can be performed on such cables having typically low density of vented water trees. The actual condition of the cables was determined by laboratory AC breakdown tests and water tree examinations performed on the same cables. Secondly, the influence on the diagnostic testing of the joints installed on the cables has been examined. The results show that it is not possible to distinguish the ageing status for cables having breakdown voltages of 5 U. and higher. The evaluation criteria need to be further developed, in order to examine if the condition of cables with breakdown voltages lower than 5 U. can be reliably assessed. Joints with resistive field grading can have very high resistive losses preventing a reliable condition assessment of the cable insulation.