HOTTER RULING SPANS? WATCH THE SAG
An IEEE task group is looking at higher temperatures in overhead conductors.
- Published in Southwire's T&D Update newsletter in April 1998
- Reprint permission granted
How hot can you run it? In the era of deregulated power transmission, many utilities are carrying more power on existing lines by increasing operating temperatures. Sometimes these temperatures are higher than the original design assumptions.
"Most existing transmission lines were designed using the 'ruling span' method devised in the 1920s," says Ridley Thrash, chief engineer for overhead conductors. "The ruling span is still the best design tool available, but its limitations become important when you increase conductor temperatures."
An IEEE task force on "Bare Conductor Sag at High Temperature" is investigating the operation of conductors at high temperatures. They're focusing primarily on existing lines, and their first step is an investigation of the accuracy of the ruling span design method.
What Is a Ruling Span?
The ruling span is a model span with a rate of slack equal to the average rate of slack of an entire line section. The ruling span model lets you predict conductor sag changes as mechanical loads and temperatures vary.
"The ruling span formula shows reasonable accuracy within the typical operating temperature range of overhead conductors, from about 50'C to 750C," says Thrash. "But as ACSR operating temperatures climb toward 100'C, sag in individual spans may react unexpectedly, compared to what you think the ruling span might do. The main source of errors is the limit on insulator movement in line sections with unequal spans."
Part of the problem is that short spans and long spans react differently to changes in temperature. In general, short spans are more sensitive to temperature changes than long spans. As operating temperatures increase, a suspension point between a short span and a long span will move away from the short span and toward the long one to equalize tensions.
The ruling span model assumes that insulators will swing far enough to completely equalize the tension between spans. To a certain extent they will, but the weight and length of the insulator strings limit movement. And some insulators such as post-type insulators-just don't allow much movement.
"When we limit conductor movement at high temperatures-where we have greatest conductor elongation — we introduce errors, some times substantial errors, in the predicted sags of the line sections," Thrash says. The worry is that these errors may create clearance problems.
Multi-Span Tools May Help
To predict sag-temperature behavior accurately when you operate a line at higher temperatures than the original design assumptions, you may need to analyze multiple spans for sag errors. The advent of computers now allows detailed, span-by-span analysis that simply wasn't practical before.
The IEEE task force will be looking at ways to improve on the accuracy of single-ruling-span calculations. Alternatives include multiple-span analysis methods.
"The task group will be publishing the first of several papers discussing high temperature overhead conductor operation within the next few months," says Thrash. "This is a complex topic. The complete study will take several years, but the results will lead to safer and more efficient overhead conductor operations." |
