Estimation of reduction factors for combined wind and ice loading of power transmission lines using multivariate scenario sampling

Abstract

In this paper, a multivariate scenario sampling method was used for estimation of reduction factors for loading of structures in power transmission lines in the combined wind and ice load case. The values of wind speed and ice thickness that must be used in combined load cases are obtained through multiplying the reduction factors by the design extreme (reference) values of each variable. The reduction factors were estimated according to the criteria of IEC 60826 standard for the mean recurrence intervals (MRI) of 50, 150, and 500 years. The reduction factors have been established through bivariate (ice and concurrent wind) hazard curves that extracted via two approaches; one based on a statistical analysis of the wind force applied to an ice-covered conductor (force-based approach) and the other based on a joint probability distribution of wind speed and ice thickness (joint probability-based approaches). In order to extract the hazard curves, an annual scenario sampling approach was utilized based on historical data recorded in 12 selected meteorological stations located in the cold regions of western Iran. Due to the lack of historical data, the thickness of ice formed around the conductor of transmission lines has been evaluated via simulations of ice accretion using a numerical model for cylindrical growth of wet snow sleeves. The results obtained for most of the meteorological stations show that the combined wind and ice reduction factors presented in IEC 60826 are conservative. It was also shown that two main reduction factors, (which specify the reduced value of a variable that must be combined with the extreme value of another) can be calculated merely by the force-based approach, and are not sensitive to MRI. In the same way, the MRI effect on hazard curves constructed by joint probability-based approaches is not significant, except for the relatively smaller ice thickness.