The Prospect of Live-Line Overhead Conductor Replacement

While overhead transmission line conductors are traditionally replaced with the line de-energized, restringing has also been done with the line operating at full voltage and without interruption of load. This was done by providing a separate temporary phase position, supported by auxiliary structures, over the length of the pulling section, removing that phase position after stringing is complete. This procedure, successfully applied at 345 kV, requires a major presence on the right-of-way and, since the new conductor is pulled in close proximity to energized conductors carrying full load current, it also requires measures to mitigate potential hazards associated with voltages and currents coupled to the conductor being pulled. This paper proposes a new concept for full voltage, full load, in-situ re-conductoring; one which eliminates the need for a temporary phase position and likewise eliminates the need to accommodate currents and voltages coupled to the conductor being pulled. In the proposed method other safety issues, such as step-touch-transfer voltages and coupling to other equipment are no more severe that in more traditional live work. Through remote operation of both pulling and tensioning equipment, both operated on insulated platforms, the old conductor is used to pull in the new. Right of way access, other than at pulling terminals, is limited to replacement of conductor clamps with pulling sheaves and vice versa when pulling is complete; thus minimizing intrusion on the right of way. The proposed in situ method is based on a current transfer device (CTD) for conveying current from the conductor on the already restrung section onto the conductor being pulled and from the latter to the yet-to-be pulled section. The CTD makes use of a series of contactor wheels pressed against and collecting current from the moving conductor. Current flows from each contactor wheel to a fixed bus through mercury-based rotating contactors attached to the axle of that whee- - l. Such contactors are widely used in industrial applications. Stringing tension can be controlled within a band much narrower than the range of tensions accommodated by a line in actual service, thus allowing the process to respect minimum line-to-ground clearances. The method implies certain new safety precautions including high speed grounding switches at both pulling and tensioning terminals and sensors to detect broken conductors. If successfully developed, the method may provide a faster, more cost effective and less intrusive method for live conductor replacement.