Repeated failures and water treeing in polyethylene HV cables

The results of investigations of water treeing in samples of cables aged in service and in the laboratory are presented. The service aged samples had a history of two successive failures, with the second failure occurring about one year after the first at close proximity. The results of the investigation show that, due to the damage caused by the first failure, moisture penetrated the cable and as a consequence the second failure was caused by water treeing. This was concluded through a numerical comparison between water trees grown in service-aged and laboratory-aged cable samples of the same type. The laboratory experiment shows that the number of water trees developed in tested cables can be related to the aging conditions through the following formula: log n_v = A × t + B × T + C, where n_v is water tree density (1/mm³), t is aging time, T is temperature (°C) and A, B, C are constant parameters. The numerical estimation of these constants is presented