Towers are a crucial part of a transmission line project, carrying overhead lines at a safe height over the ground, right from the generation plant to the load centres. On an average, towers and their foundations account for more than half of the cost involved in the construction of a transmission line. The design, construction and installation of transmission towers have advanced over the years, with the objective of reducing right-of-way (RoW) requirements as well as construction time. Today, India is one of the biggest tower manufacturing hubs in the world, with a manufacturing capacity of over 200,000 million tonnes, catering to both the domestic and international markets.
Since towers bear the weight of overhead lines, they should be sturdy enough to face extreme weather conditions across varied terrain. Various principles of civil, electrical and mechanical engineering are applied in designing towers to ensure seamless transmission over a long term. Designing transmission towers is a complex task and loads under different conditions need to be taken into consideration. Several advanced software solutions are available for designing towers. These include PLS-CADD, PLS-POLE, TOWER, Bocad, AutoCAD and other CAD software.
The voltage and number of conductors as well as the configuration of circuits play a key role in tower design. Besides, ground profile and RoW are other factors that are taken into account while designing towers. The choice of tower foundation depends on the soil type or ground profile of the place where the tower is to be erected. Standard foundation types are available for dry soil, wet soil, rocky soil, sandy soil and submerged type. In earthquake-prone areas, seismicity data is an important consideration for laying the foundation. Some emerging foundation types include augur, precast, grillage and undercut. Meanwhile, the number of transmission towers to be erected per kilometre depends on the topography of the line route.
Transmission companies typically use lattice-type transmission towers, especially for voltages over 100 kV. These towers are self-supporting structures and can be built easily at inaccessible locations as the tower members are lightweight and can be easily transported. The lattice guyed-V transmission towers are also used by transmission companies as they are easy and cheaper to install. However, the main disadvantage of these towers is the high requirement of RoW.
To overcome these shortcomings, advanced tower structures such as monopoles are emerging in the market. Monopoles were first introduced by Power Grid Corporation of India Limited (Powergrid) in 2008-09. They have a sleek body made up of polygonal tubular sections with a tubular cross arm arrangement for fixing the tension or suspension clamps on it. The tubular structure can be a single tubular form or an H-form. Like lattice towers, these can be designed to carry two or more circuits.
Monopoles require one-sixteenth of the space needed by lattice towers, thus significantly reducing the RoW requirement. Since they have a small footprint in terms of space, monopoles can be easily installed on the median of a road or a highway and on the curb of canals. These towers can be installed in thick forest areas in order to mitigate environmental effects/ deforestation to some extent.
As compared to lattice towers, monopole towers offer the benefits of faster installation and improved flexibility. Further, monopoles have a significant wind loading capacity and are reliable even under extreme weather conditions.
Other advanced tower types include delta configuration towers and chainette towers. The former hold electrical conductors in an equilateral triangle and are more compact than conventional lattice towers. Chainette towers are small structures consisting of two small masses supported by guy wires and hanging insulators. These are lightweight, low cost and involve less installation time.
Some special tower types include river crossing towers, railway crossing towers, transposition towers, and bird-friendly towers.
Materials for tower construction
Typically, steel/galvanised steel is used for manufacturing towers. In many countries wooden transmission towers and concrete towers are used for high voltage (HV)/extra high voltage (EHV) transmission. The wooden towers are mainly single pole or H-frame type. Further, research and development efforts are under way to facilitate the use of composite materials such as fibre-reinforced polymers (FRPs) for building towers. FRPs are gaining popularity in the transmission segment on account of their high strength-to-weight ratio and insulation capability, low maintenance, affordability, ultraviolet resistance, rust and rot resistance, and flame-retardant properties. Pultruded FRP towers and cross arms are also being used as replacements for ageing wood poles in remote and extremely humid locations as composites do not absorb moisture and swell up.
The maintenance of transmission systems is crucial to ensure reliable supply of electricity. In the case of steel transmission towers, the primary cause of deterioration is corrosion. It makes the structure weak and more susceptible to damage by strong winds and other extreme weather conditions/natural calamities. This not only results in asset loss for the utility, but also disrupts power supply. Painting the steel lattice towers is a more cost-effective way to mitigate the effects of corrosion than replacing the members or the entire tower. However, for some towers, it is better to replace corroded components such as steel bolts, and individual steel members, so that the need for painting the entire tower can be deferred for some time. Also, the insulator attachment points located at the end of cross arms that provide the fixing point for strain and suspension insulators are subject to corrosion and wear and tear.
In line with international operations and maintenance practices, Powergrid started tower top patrolling in April 2016, after obtaining the requisite clearance from the aviation authorities. The company undertook patrolling of the ± 500 kV Balia-Bhiwadi high voltage direct current (HVDC) line using unmanned aerial vehicles (UAVs). These UAVs are equipped with ultra-high definition video cameras, which can take high resolution and close photographs of towers and its components for the detection of faults/defects. This eliminates the need of tower-top patrolling by climbing the tower.
The way forward
Since RoW is the key challenge facing the transmission segment, utilities are increasingly deploying double- and multi-circuit lines instead of single-circuit lines in order to minimise their RoW requirement. Powergrid, for instance, is focusing on adopting higher voltage levels, specially designed towers and new technologies to gradually increase the power carrying capacity of transmission lines to optimise RoW. For the ±320 kV Pugalur-Trichur HVDC link, the company is deploying overhead lines using pole structures and narrow base towers along with underground cabling to address RoW issues.
In addition, tower designers are designing compact towers that need less space. New tower designs are coming up that use less steel, have fewer sections and bends, and are easy to transport and assemble. Also, insulated cross arms are being used to reduce both the height and width of towers. Also, tower structures that can be installed using helicopters are becoming popular, especially at inaccessible locations. Further, the installation of towers that serve as transmission as well as telecom towers is picking up pace.