Transmission towers support the high-voltage conductors of overhead power lines, from the generating station switchyard right up to the source substations and satellite substations located near populated areas. Types of Towers : Waist Type TowerThis is the most common type of transmission tower. It's used for voltages ranging from 110 to 735 kV. Because they're easily assembled, these towers are suitable for power lines that cross very uneven terrain. Double Circuit TowerThis small-footprint tower is used for voltages ranging from 110 to 315 kV. Its height ranges from 25 to 60 metres.Guyed-V TowerThis tower is designed for voltages ranging from 230 to 735 kV. It's used mainly for power lines leaving the La Grande and Manic-Outardes hydroelectric complexes. The guyed-V tower is more economical than the double-circuit and waist-type towers.Tublar Steel PoleFeaturing a streamlined, aesthetic shape, this structure is less massive than other towers, allowing it to blend easily into the environment. For this reason, it's being used more and more in urban centers.Guyed Cross Rope Suspension TowerWith its simple design, this tower is easy to assemble. It's used on some sections of power lines leaving the La Grande complex and supports 735-kV conductors. This type of structure requires less galvanized steel than the guyed-V tower, making it lighter and less costly.CrossingsHydro-Quebec's most prominent towers are used when overhead power lines must cross large water bodies, such as the Rivi-re Saguenay or the St. Lawrence near Carignan.However, there is another way to reach the opposite shore: underneath the river. As a matter of fact, Hydro-Quebec operates a rather unique underwater power line designed for this purpose. In 1990, the company built the world's first underwater river crossing for a 450,000-volt direct-current power line. The tunnel leaves the north shore near Grondines and surfaces on the south shore close to Lotbini-re substation.