Diagnosis and Extending Life of Aged XLPE Cables in Distribution System

XLPE cable is widely used in urban power grid and the field of long-distance transmission of submarine due to its superior electrical, mechanical and heat-resistant performance. Decades of operation experience shows that aging of cable insulation is the main cause of cable blackout in addition to improper installation and external force damage. Water tree is an important ageing form for cross-linked Polyethylene (XLPE) cables in distribution grids, which is one of the main inducements leading to the decline of cable insulation level and service life. The cables insulation level and breakdown voltage drop significantly with the appearance of water tree. When the over-voltage in the system invades, the aging cable of water tree will easily lead to the electric tree or even the direct breakdown of the insulation, which will cause the blackout accident. In order to solve the water tree aging problems of large-scale service-aged cables in urban power grids, this paper reviews the water tree diagnosis methods and its rejuvenation technology for XLPE cables. Firstly, various diagnostic methods are discussed for water tree aging cables based on previous research achievements to evaluate the insulation level of the XLPE cables and the water tree aging degree. The diagnostic methods mainly include micro-characterization, breakdown voltage tests, non-destructive online or off-line diagnosis. Through the comparison and analysis of the various diagnosis methods characteristics, it shows that the future diagnosis technology of water tree aged cables should be developed in the direction of online detection, non-destructive diagnosis and directly evaluating water tree aging degree. Then insulation rejuvenation technology is proposed to extend lives of water tree aged XLPE cables and promote insulation performances. The paper introduced the principle of water tree restoration; Finally, the successful on-site application cases are briefly introduced for the water tree diagnosis and rejuvenation technology.