Today, power lines are often operated at their limits while utilities continue to face delays in installing new facilities. Thermal limits occur when the maximum allowable conductor sag is reached. Previously, expensive line reconstruction or reconductoring was required to address this problem.

The Electric Power Research Institute (EPRI; Palo Alto, California, U.S.) now proposes field trials of a device that can reduce excessive line sag. The in-line device, SliM (Sagging Line Mitigator), reacts to increasing conductor temperature by decreasing the effective length of the conductor in the span, mitigating the natural thermal expansion experienced by the conductor during high-temperature operation. The device is the result of a three-year R&D program by Material Integrity Solutions Inc. (Berkeley, California), sponsored by the California Energy Commission.

Tests performed in July 2002 at a Pacific Gas & Electric lab in Livermore, California, demonstrate the functionality of the device. Two 500-ft (152-m) spans of 795 kcmil ACSR conductor at 5000 lbs (22,241 newtons) initial tension at 90°F (32°C) temperature were subjected to a current of 1200 amperes. The sag differential between the control span and the span treated with a SLiM device was about 4 ft (1.2 m) at a 210°F (99°C) conductor temperature. These early positive results have provided EPRI with the incentive to seek participants for field tests.

Utilities showing an interest include Arizona Public Service, British Columbia Hydro, National Grid, Excelon and Electricité de France. Additional utilities who wish to join this initiative can contact Dr. Ram Adapa at (650) 855-8988 or via e-mail at