Fundamental study on location of water tree degradation in power cables

Water tree is a degradation mode of power cable with polymeric insulation. A water tree is composed of small droplets filled with water. As the conductivity in water tree is very high, it leads to dielectric breakdown when it grows up. As inside the water tree is filled with trap sites, it is poled with a certain distribution of relaxation time, when a poling voltage is applied. Although its depolarization process after removing the poling voltage is determined by ambient temperature, applying a “depolarizing voltage” with the opposite polarity can accelerate the relaxation process. If a short pulse propagating through the cable is employed as a depolarization voltage, we may locate the water tree by looking at the time-resolved pulse response. This would lead to a diagnosing method with spatial resolution. In order to retain 100 m of spatial resolution, the response should be as sharp as 1 μs. Based on the feasibility study with preliminary experiment, we constructed a measurement system for cable with a long length. In order to upgrade sensitivity and resolution, we introduced several techniques like noise canceling and digital signal processing. The degraded point in a 400 m mimic cable line was successively located.