Fundamental study on relaxation processes of water tree in aged power cables aimed for diagnosis with spatial resolution

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 polarized with a certain distribution of relaxation time when a DC poling voltage is applied. Although its depolarization process after removing the poling voltage is determined by ambient temperature, applying a ldquodepolarizing voltagerdquo with the opposite polarity can accelerate the 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 mus. As a preliminary study, a coaxial communication cable was aged to form water trees. A DC poling voltage was applied followed by a pulse voltage with opposite polarity. The rising time of the pulse was several hundreds of microseconds. A sharp pulsive current response with 50 mus in width was observed, suggesting that a rapid depolarization took place. No such response was seen when the cable specimen was not aged. We concluded that the technique is quite feasible.