Effects of tree mortality on power line security

"Others have reported that instances where trees grow into lines rarely result in power outages. The vast majority of tree-related outages stem from tree failure, particularly if outages during severe weather events are included. Generally, tree–conductor conflicts resulting from tree failure are classified as unpreventable because the trees are located outside the right-of-way. In the emerging competitive environment, utilities will require a means of decreasing so-called unpreventable outages. The primary locations for unpreventable outages are areas where lines run adjacent to or through natural forest tree stands. Tree mortality exposes a power line to a high risk of tree incidents over time. The risk to the line is directly related to the number of trees within striking distance of the line. Conventional clear widths leave a substantial residual tree risk. Hazard tree removal programs do not provide enduring reliability gains. A new mathematical model, the optimal clear width calculator, is used to assess the tree risk over variable clear widths and line heights. The risk ratings in the output line strike probability charts permit quantitative comparisons of construction and maintenance options. The line strike probability chart indicates that there is a point of diminishing return in line security for dollars invested in additional clear width." [Abstract]