Investigation of resistance toward water tree growth in XLPE insulated MV cables produced in the Arabian Gulf region

In Saudi Arabia the main electric power utilities are extensively using XLPE cables produced in the Arabian Gulf region. Due to prevailing high temperatures and high sulphate content in the soil coupled with high sub surface water table, these cables have suffered serious breakdowns in a short time span after their installation. This investigation was undertaken to compare the resistance to water treeing in 15 kV, XLPE insulated cables of four regional manufacturers that are producing these cables according to international standards. Cable samples were aged in two stages. At first, the aging was carried out under high electric stress and temperature cycling in the presence of deionized water. After 120 days of this aging, their remaining dielectric strength was determined. These results show poor performance of one type while the others exhibit close values. In the second stage the accelerated aging was carried out in the presence of aqueous ionic solution of CuSO₄ for aging periods that lasted in the range of 500 to 1500 hours. Water tree population and tree lengths were compared in this case. Their number and length distributions were subjected to statistical Weibull and Log-normal models. Water tree density as a function of aging period was found to obey a log-linear relation. On the other hand, water tree population and their length distributions were found to fit better on Log-normal model. These data also show that water tree density in these cables as evaluated in the second stage of aging, comparatively varies significantly and leads to better segregation and selection of the cable that will serve the best in such local environmental conditions.