Water-tree Degradation Characteristics in Aged Extruded Three-layer (E-E type) 6.6kV XLPE Cables

One of the main causes of degradation of extruded three-layer (E-E type) 6.6kV cross-linked polyethylene (XLPE) cable is water trees in the insulator. Approximately 40 years have passed since its introduction, and it is important to clarify the degradation characteristics of its insulation performance for the long-term operation and to establish the degradation judgment criteria in degradation diagnosis methods through the investigation on the aged and decommissioned cables. Here, it is expected that water trees causing the degradation of the insulation performance of aged XLPE cables can be confirmed by conducting the pre-breakdown discharge detection test. In this study, we investigated the insulation performance of a number of aged E-E type 6.6kV XLPE cables using the pre-breakdown discharge detection test and AC breakdown test, and evaluated the relationship between the operating period and the insulation performance. We also applied the degradation diagnosis methods such as the DC leakage current method and the AC superposition method to the E-E type 6.6kV XLPE cable with bridging water trees, and found that the DC leakage current and the AC superposition current were significantly larger than the degradation judgment criteria.