Settling the creep correction debate
The Institute of Electrical and Electronics Engineers (IEEE) has published a guide called IEEE Standard 524, Guide for the Installation of Overhead Transmission Line Conductors. Of particular note in the guide is a section on correcting conductor creep. Existing guidance, published in 2016, suggests that line crews should consult with the conductor manufacturer to determine if short-term creep correction factors are needed if they leave a conductor in the rope blocks for more than 24 hours. . Additionally, the IEEE 524-2016 standard states that conductors and/or cord blocks could be damaged if the conductor is left in the blocks for more than 72 hours.
The overhead conductor installation process is carefully prescribed to ensure that the as-built transmission lines match the engineering design. Rope blocks — sometimes called travelers – are pulleys that are temporarily suspended from the crosspieces of transmission towers to allow a pull line, and then the conductor itself, to be pulled through, from tower to tower. After threading the conductor through the blocks, crews sometimes have to leave it overnight or over the weekend before moving on to the next phase of installation. This pause is when conductor creep is most likely to occur.
In the next phase, called sag, crews tension the conductor to the specified sag before moving it from the string blocks to the permanent hanger clamps and dead end terminations. If the responsible engineer suspects that the stringing process may have caused undesirable conductor creep, the conductor manufacturer should be contacted to see if the sag tension needs to be adjusted to meet design specifications. This is called creep correction. “Current guidelines direct utilities to consult with the conductor manufacturer on the amount of creep correction needed, if any, after only 24 hours in the blocks,” said Drew Pearson, transmission engineer at Southwire. “Because the need for creep correction depends on installation practices, the IEEE has left guidance on corrective action to the conductor manufacturer.”
Conflicting models in the industry
The confusion arose when utilities asked conductor manufacturers about creep correction. Some manufacturers have recommended creep correction in response to short installation times. However, Southwire generally did not recommend creep correction, as evidenced by their predictive model using actual stress-strain test results.
In 2018, Southwire performed lab tests to determine the accuracy of its predictive model. The company compared stress-strain test results on two conductor samples from the same production spool. Prior to testing, one of the samples was tensioned at 15% of Rated Breaking Strength (RBS) for six days. As noted in Southwire’s article, “Creep Correction for Bare Overhead Conductors”, stress-strain test results showed no difference in performance between the two samples, validating the accuracy of the current model.
The tests confirmed what Southwire had told its utility customers. “Under normal conditions and moderate voltage, short delays in the installation process have no effect on line design,” Pearson said.
Fix the problem for good
Southwire presented its test results and recommendations at the IEEE Overhead Lines Subcommittee meeting in Portland, Oregon in August 2018. In the presentation, Southwire recommended considering creep correction only when the string tension exceeded 10% RBS or 50% sag tension.
Southwire provides creep correction on a case-by-case basis only if a conductor is exposed to voltages in excess of these values for more than one hour. “It’s understandable that utilities would be suspicious when one manufacturer told them a creep fix was needed, and another said the opposite,” Pearson said. “Recent discussions have shown that conductor manufacturers are now on the same page with their creep correction guidance, and the next revision of the IEEE 524 standard may now include more useful language than all conductor manufacturers can support.”