Sag and tension calculations for overhead transmission lines at high temperatures-modified ruling span method

The ruling span concept is widely used to calculate sags and tensions for new overhead transmission lines and for upgrading existing lines. It provides satisfactory results for a level line with relatively uniform spans at any temperature, or for any span length of a level line at low temperature. The ruling span concept may result in an unacceptable error if it is used to calculate sags and tensions in a line segment with significantly unequal spans at high temperature. This paper presents a method to calculate sags and tensions of multi-span line segments at different temperatures based on the rotational stiffness of suspension insulator strings. A simple equation, based on the parabolic approximation, is derived to calculate changes in the span lengths and conductor sags and tensions. The method follows the ruling span concept but relaxes the fundamental assumption of the ruling span method. The accuracy of the method is compared with the more complex, nonlinear, finite element method.