Probabilistic derivation of overstress for overhead distribution in-line structures

Abstract

This paper shows how probability techniques can be applied to low and medium voltage distribution and high voltage sub-transmission lines which use self-supporting single-pole structures. The probabilistic concept uses overlapping distributions in which the randomized stress induced by wind pressure is matched to the resisting strength of a pole structure. In this way a risk load is evaluated which optimizes structural strength and enhances the economic utility of the asset. Specific to the work is the inclusion of the degeneration of pole strength with age. This is important in distribution systems where wood is the most common construction material. The modeling uses Monte Carlo simulation to establish a failure risk of a line structure within a design return period and a life. Input to the model involves the static load imposed by line conductors and their ancillaries, random gust wind pressures (modeled by a Gumbel distribution), and a 3-parameter Weibull distribution to describe the dispersion of strength and degradation of the material. The pole overturning (wind) moment is compared to the degrading resisting (strength) bending-moment over daily or monthly intervals related to a designated life time. The work, for a large electric utility, analyzes treated hardwood and steel-reinforced concrete poles for new works, with emphasis on urban and semi-urban area construction. In the context of an urban and semi-rural environment, cost reductions in the order of 10% to 15% can be achieved.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>