Underground Medium-Voltage Cable Rejuvenation—Part I: Laboratory Tests on Cables

This paper, the first of three parts, deals with the penetration depth and concentration of silicone fluid in cable insulation and its duration when injected in real-size, service-aged 28-kV extruded cables and subjected to various simulated service conditions in the laboratory. There were four aging conditions, namely, a cycling conductor temperature of 55°C or 90°C, in air or water, and with or without pressurized soak silicone, and always under service voltage. Samples were taken as a function of time and analyzed for silicone and water content in the insulation, and at the end for water treeing. The results indicate that the silicone content in the insulation changes circumferentially and can reach values between 1 and 4% after 96 thermal cycles, before stabilizing more or less up to 255 cycles. Furthermore, it was found that while water reacts with silicone, it contributes to the silicone retention in the insulation. Also, the study revealed that the number and size of water trees were not affected by the laboratory aging treatment and show no increase due to the presence of silicone.