ABSTRACT

Transmission line phase conductors and overhead wires are affected by sag that vary the distance of these conductors to the earth. This corresponds to a varying separation distance of current carrying phase conductors producing the magnetic fields to parallel pipelines in shared corridors. This paper explores the effects of phase conductors and shield wires’ sag to evaluate the induced voltages on parallel pipelines. Methodologies to consider the sag of the conductors include catenary curves, straight sloped conductors mimicking the maximum and minimum conductor elevations, and uniform conductor elevations. A sampling of typical voltage levels, spans, and transmission line phasing configurations are evaluated utilizing commercial software.

The paper provides an explanation of the impacts of sag and provides a reference for considering the inclusion of catenary curve compared to simpler approximations of the conductors for a transmission line. Identification of transmission line configurations that increase the significance of the induced voltages on parallel pipelines helps to improve public and personnel safety in shared corridors.

INTRODUCTION

AC interference studies are complex and require complex analysis methods to accurately evaluate these projects. Sag is a variable that affects each of these projects to varying degrees, but many tools are challenged to consider the catenary curve of the phase conductors. Sharing common engineering methodologies for approximating the sag and comparing these to more accurate catenary curves guide engineers where simplification of the sag may result in significant analysis errors, and scenarios where simplification may be acceptable.

TRANSMISSION LINE DESIGN AND GROUND CLEARANCE

A primary design criterion for electric power transmission lines is the selection of conductors and their corresponding tension and sag characteristics. As described in the IEEE(1) NESC1, minimum ground clearances for transmission lines are required for the safe operation of the lines and limiting hazards to possible recipient objects from conductive, inductive, and capacitive effects. The sag of a line is the variation in elevation of conductors from a tower's attachment point across the span. The sag will be the lowest point of the transmission line and is used for determining clearances to meet the NESC. Tension on the line is increased to reduce sagging conductors but is limited by the strength of the conductors and the transmission towers. Longer spans, heavier conductors, lower tension, and other factors contribute to greater sag and variability of the conductors’ elevation. The sag follows a catenary curve that is a hyperbolic function represented in Figure 1.

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