# Probabilistic Derivation of Overstress for Overhead Distribution In-Line Structures

Stillman R.H. (1994) Probabilistic Derivation of Overstress for Overhead Distribution In-Line Structures. IEEE Transactions on Reliability, 43 3: 366-374. doi:10.1109/24.326427

Author Stillman R.H. Probabilistic Derivation of Overstress for Overhead Distribution In-Line Structures IEEE Transactions on Reliability 1558-1721 1994-01-01 Article (original research) 10.1109/24.326427 43 3 366 374 9 eng 2208 Electrical and Electronic Engineering2213 Safety, Risk, Reliability and Quality1704 Computer Graphics and Computer-Aided Design1708 Hardware and Architecture1712 Software In 1991, the International Electrotechnical Commission Technical Report 826, “Loading and Strength of Overhead Transmission Lines”. The report proposes probabilistic methods in the design of lines operating above 45 kV. This paper shows how probability techniques can be applied to low & medium voltage distribution and high voltage sub-transmission lines which use self-supporting single-pole structures. Line regulations and codes of practice have generally used deterministic multiplying factors in the design of the strength-loads applicable to overhead distribution and sub-transmission line structures. Such an approach, based on a worst case scenario, applies a continuously sustained maximized stress over some indefinite design life. This is unrealistic and a waste of resources. In contrast, the prob- 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 utili- 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. calculatedoverturning momentPole structurepressureresisting momentriskstructural overstresstime-to-failure distributionwind distribution C1 Provisional Code Unknown

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