Towers

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Introduction

Figure 1. Tower components

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.

Their shape, height and sturdiness (mechanical strength) depend on the stresses to which they are exposed. Towers do not transmit electricity themselves unless lightning strikes the ground wire strung along the top of the structure. This cable is designed to protect conductors by allowing lightning discharges to reach the ground through the tower.

Types

Waist-type tower

Figure 2. Waist-type tower
Figure 3. Waist-type tower

This 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 tower

Figure 4. Double-circuit tower
Figure 5. Double-circuit tower

This small-footprint tower is used for voltages ranging from 110 to 315 kV. Its height ranges from 25 to 60 metres.

Guyed-V tower

Figure 6. Guyed-V tower
Figure 7. Guyed-V tower

This 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 pole

Figure 8. Tublar steel pole
Figure 9. Tublar steel pole

Featuring 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 centres. Measuring between 27 and 45 metres in height, it's suitable for voltages ranging from 110 to 315 kV.

Guyed cross-rope suspension tower

Figure 10. Guyed cross-rope suspension tower
Figure 11. Guyed cross-rope suspension tower

With 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.

Crossings

Figure 12. Crossings
Figure 13. Crossings

An overhead line crossing is the crossing of an obstacle—such as a traffic route, a river, a valley or a strait—by an overhead power line. The style of crossing depends on the local conditions and regulations at the time the power line is constructed. Overhead line crossings can sometimes require extensive construction and can also have operational issues. In such cases, those in charge of construction should consider whether a crossing of the obstacle would be better accomplished by an underground or submarine cable.

Conductors

Conductors transmit electric power. Usually conductors are made of aluminum with a steel core that gives the cable its required strength.

Curiously, these conductors are bare: the air around them provides insulation. Each conductor is stranded, meaning it consists of several wires twisted together. This makes a conductor more flexible and more exposed to air, a feature that helps cool the conductor and therefore increase its conductivity. Electricity meets resistance when passing through hot metal, a phenomenon that transforms part of the electrical energy into thermal energy, a type of energy loss.

A conductor bundle is a series of two, three or four conductors always kept apart by spacers. Spacer dampers separate bundled conductors and control vibrations caused by wind and ice buildup.

Alternating current, which is generated by power stations and transmitted on high-voltage cables, is made up of three parts, or phases. High-voltage towers carry three conductor bundles, one per current phase.

Bundled conductors are used on high-voltage power lines to help reduce energy losses (due to the corona effect1), audible noise and radio interference. As a result, they improve the power transmission process. For example, four small conductors with a 3-cm diameter are just as effective as–and much lighter than–one single conductor with a 46-cm diameter.

Ground wires protect the line against lightning.

Guy wires are anchor cables which ensure the mechanical strength of the support structure.

1The corona effect, intensifies when impurities fall on the wires and when the air is very humid. The crackling sound is loudest when it snows or rains. Electrons moving between the wires and the air cause the corona effect. This back-and-forth movement creates a multitude of tiny electrical discharges, which we perceive as a crackling sound. The higher the voltage, the greater the chance this phenomenon will occur.

Line installation

Step 1: Land clearing The power line right-of-way is cleared of vegetation to allow operation of a i.e. 735-kV line according to the established standards.

Step 2: Temporary access Temporary access roads are used to build the line. A crew sets up the equipment and structures necessary to protect rivers and streams, cultivated lands and other sensitive components.

Step 3: Stockpiling The various tower parts are manufactured and delivered by type. The stockpile managers take care to organize the hundreds of parts in the order in which they'll be used to assemble each tower. In fact, each tower has different characteristics based on angle, topography and soil capacity.

Step 4: Delivery of steel The steel parts needed for the placement of the foundations are delivered by semi-trailer at the edge of the right-of-way and are usually transported to the jobsites by crawler carrier. Then the steel tower parts and hardware are delivered to their sites by the same method.

Step 5: Establishing the foundations A work crew excavates the foundations using bulldozers and hydraulic shovels. Depending on the nature of the soil, the foundation may be made of fill delivered by truck or crawler carrier, or of concrete, which may be delivered or prepared on-site. Once the foundation is in place, the excavation is backfilled.

Step 6: Tower assembly and erection A crew assembles the towers using cranes and bulldozers. The tower is then erected by means of a telescopic crane.

Step 7: Conductor stringing The conductor stringing is done segment by segment. The conductor is paid out from a cable drum at one end of the segment and run through stringing blocks at the tops of the towers. At the other end of the segment are a puller and a take-up reel. Line crews are on hand to ensure that the operation runs smoothly.

Finally, for tower grounding and protection from lightning, "counterpoises" or anchor rods are installed in the ground.

Step 8: Inspection A compliance monitoring plan is prepared for each project according to the specific requirements of the engineering team.

Step 9: Site restoration At the end of the project, work begins on restoring the right-of-way and dismantling temporary access roads. A tour of the site with the landowner ensures that the restoration work meets his expectations. Compensation is also provided for any losses incurred.

Step 10: Preparing land for cultivation Restoring to the initial state of the damaged agricultural field (dependable).

References

[1] http://www.hydroquebec.com/