anchor pylon
A pylon that forms the endpoint of a high-power transmission line and uses horizontal insulators, necessary when interfacing with other modes of power transmission and, due to the inflexibility of the conductors, when significantly altering the direction of the pylon chain.
anchor pylon: the heavy-duty turning point in a transmission line
An anchor pylon is a reinforced steel tower built to withstand the mechanical forces that arise when a high-voltage transmission line changes direction or terminates. Unlike a suspension pylon, which carries the weight of conductors hanging from it, an anchor pylon must resist the lateral and longitudinal pull exerted by the tensioned conductors themselves. This force can reach hundreds of tonnes, especially on lines carrying 110 kV to 765 kV.
Anchor pylons are necessary at three types of locations: at the end of a transmission line, where the full tension of all conductors is anchored to the tower foundation; at major direction changes (typically 30 degrees or more), where the change in conductor path generates unbalanced horizontal forces; and occasionally at mid-span positions on extremely long lines to prevent excessive sag. The pylon must be substantially heavier and deeper-founded than suspension towers used elsewhere in the same line.
Design and construction
The horizontal insulator arrangement is the defining feature. Rather than supporting insulators vertically as a suspension pylon does, an anchor pylon uses insulators oriented horizontally or nearly so, attached to crossarms that project perpendicular to the line direction. This geometry allows the pylons to be anchored directly to a concrete foundation capable of resisting the enormous tension loads. The base dimensions can exceed 12 metres on either side for high-voltage lines, and foundation depths of 3 to 4 metres are common in stable soil.
Conductors are dead-ended at an anchor pylon using compression or wedge-type connectors rather than being hung from clamps. Each phase conductor, and often the earth wire, must be individually anchored. The pylons themselves are typically lattice structures of hot-rolled steel angles bolted together, galvanized to resist corrosion. Guying may be employed on anchor pylons in exposed terrain to increase stability further.
The term 'anchor' reflects the nautical analogy: the pylon functions as an anchor point in the line, holding the conductors fast against the enormous mechanical tension, much as a ship's anchor holds it against wind and current. This name distinguishes the structure clearly from suspension pylons and intermediate pylons, which play passive or transition roles. In some European electrical standards, these structures are called terminal towers or dead-end towers, but anchor pylon remains the most common North American term.