Every high voltage transmission line is supported by a transmission tower. Tower structures are different from basic power poles. They support multiple high voltage lines as opposed to the single poles that handle local delivery. Towers are important for the transport of large quantities of electricity over a long distance. Transmission towers combine with substations and pole lines to form the electrical grid. The types of towers in use are often relative to the weight and strain they support and the direction or angle of their power lines.
Electrical power line routes are designed based on geography and the available paths according to easements, placement abilities and the ultimate destination. Surveyors work to maintain straight lines along the route as much as possible, but angle towers are needed each time the line takes a directional change. The angle means the tower must support the directional change. Anchors are added to the tower to counteract the pressure placed against the angle. The angle or tension-style tower is more heavy duty than a regular suspension tower because it must support a greater load.
Suspension power line towers make up the majority of the structure types on a high voltage line. The towers run the straight line routes where the angle deviation is less than 5 degrees. Like all transmission lines, the suspension towers have conductors attached to the lines. Numerous structure variations exist on suspension towers, and engineers must determine if extra arms and supports are required for the line. They also base the tower size and supports on the line type and load placed on the tower.
The transposition structure is designed to support extra weight on a long distance line. They are most common in the three-phase line system and are often used on long distance spans where the sag and weight in the center of the span places a heavy load on the structure. Like all towers, a transposition tower has a peak, a cage and a body for support. The cross arms are extended to hold the conductors and lines with spacing that prevents contact. The overall design concept is simple, but the actual building and execution process is complex and difficult.
Other custom tower types are built when circumstances require a major angle change or additional support requirements based on environmental factors. Heavy wind, porous soils, freezing rain and other factors influence tower construction decisions.