Limitations of the ruling span method for overhead line conductors at high operating temperatures

Abstract

This report summarises the work by the Task Force to review the accuracy of the ruling span method for conductors operated at high temperatures. The basics of the ruling span approximation method have been examined. The traditional ruling span approach can be used with little or no error for a typical overhead line crossing a rolling terrain to predict sags in suspension spans for conductor operating temperatures in the range of 5@C to 7OOC. Sensitivity studies were performed using conductors "Lapwing" and "Tern" in order to quantify such ruling span assumptions as the effect of the longitudinal swing of suspension and line post insulators on conductor sags at high temperatures, and the effect of the suspension insulator string length on the equalization of conductor tensions in adjacent spans. Significant errors in estimating the sag at conductor temperature above l OOOC may occur if the tension differences are not taken into consideration in line sections consisting of a series of spans of non-equal lengths, It was confirmed that the ruling span method is the most practical way to string conductors in multi-span line sections.

Key Considerations for the Selection of Dynamic Thermal Line Rating Systems

“…Predicted displacements of conductor attachments along the line using the ruling span method can be large; sometimes they exceed 3 m (10 ft) in lines with mixed short and long spans over a normal range of operating temperature. Once taken out of travelers, conductor movements along the line with two or more spans of unequal length are restrained by suspension insulator swings [24] [25]. With care, this enhances the accuracy of predicted high-temperature sags compared to rather poor uncertainties of ±20 C° at 100°C when the ruling span method is used [6] [7][24] [25].…”

The possible impact of weather uncertainty on the Dynamic Thermal Rating of transmission power lines: A Monte Carlo error-based approach

“…to calculate tensions and sags. With this regard, our mechanical model of the line considers not only that the conductor's temperature can vary span by span due to different weather conditions, but also the mechanical interaction between adjacent spans due to the possible rotation of insulator strings [26]; this means considering both vertical and horizontal components of mechanical tension of conductors, which represents a significant improvement in the analysis of multi-span lines with respect to the conventional simplified "rulingspan" technique [30][33] [34]. A fast Newton-Raphson solving technique allows respecting the requirements of real-time operation.…”

“…This is the case, for instance, of a HV line that crosses a wide river: in circumstances like this, the sag of the longest span can reach several tens of meters, sags being proportional to the square of the span's length. Simulating intentionally such a realistic but extreme configuration helps understanding what happens in multi-span power lines when the length of different spans is significantly uneven, a situation that could be hardly captured by the conventional "ruling-span" technique [30][33] [34].…”

Methodology to assess phasor measurement unit in the estimation of dynamic line rating

“…Numerical methods such as finite elements are not commonly used for DLR, because they require specialised software and large computational resources when compared with analytical approximations. As an analytical method, the state change (2) approximates the mechanical tension in an OHL stringing section using the ruling span method [20,21] EA r s m c g 2 24 =…”