The configuration of transmission towers depends on several parameters such as voltage levels, the number and configuration of circuits, the number of conductors in each phase, geographical conditions at the site (whether the terrain is flat or hilly), and, importantly, the right of way (RoW) available for constructing the towers.
The latest trends in tower design include the use of super-high-tensile steel and higher section sizes, optical ground wires with 96 fibres, gas-insulated transmission lines, insulated cross-arms, high temperature low sag conductors that help in covering long spans between river-crossing towers without unnecessarily adding to the tower height, and the use of precast and screw pile foundations. Further, post insulators are being deployed to avoid the swing of conductors in narrow base/ compact towers. This also helps in solving the corridor problem. The use of box sections, bird- and airport-friendly towers, hinge-type post insulators, and transmission-cum-telecom towers are other notable designs being considered. Apart from these usages of modern equipment, new transposition arrangements are also being made to increase the pace of construction.
There are a number of tower foundations that are being used by transmission system developers. One of these is the precast foundation, which is most suitable for transmission projects where the construction time is limited. The dimensions of these foundations are governed by the uplift capacity of cranes. Precast foundations are initially cast in factories under controlled conditions and are thereafter taken to the sites. These foundations are also provided with pretested hooks, which, in turn, facilitate their easy lifting at sites.
Another kind of foundation, which is used in firm soil, is the grillage foundation. This is also a factory-made foundation used in locations where concreting is relatively costly and time-consuming. In contrast, screw pile foundations are most suitable in weak soil conditions.
Monopole foundations too save on erection time. The objective of using such foundations is to minimise the visual impact on the surrounding landscape. With the help of poles, it becomes possible to avoid the gigantic structures usually used by normal towers without a monopole foundation. Monopole foundations are lightweight, compact, visually more aesthetic and involve relatively low costs. They are known for reducing right-of-way requirements.
Amidst growing environmental concerns, bird-friendly towers are largely being used in locations near bird or wildlife sanctuaries. In these towers, all the conductors are placed at a single level. To prevent birds from colliding with the conductors, bird diverting flags are provided on top. To protect wildlife and prevent deforestation, extra-tall towers are used in such locations. Similarly, in airport-friendly towers, conductors are placed at relatively low levels. Low-height towers are also commonly used for power transmission near airports. Guy-type towers are used when there is less time for erection. However, the RoW requirement for such towers is greater and they are unsuccessful in situations where conductors are found impaired.
In the chainette tower structure, insulators are hung by ropes and the structure is further compacted with the help of chains. As a result, the width of the tower as well as the RoW requirement is reduced. The main advantages of chainette towers are that they are light weight as compared to conventional self-supporting towers, and entail less costly transmission lines and largely reduced erection time. However, these structures are only suitable for flat terrain.
Towers can be constructed by using both tube profiles and angular profiles. However, the benefit of using tubes over angular profiles is that the former have more radius of gyration for the closed hollow sections. The buckling capacity of a tube is higher than that of an angular profile having the same area of cross-section.
New developments in construction
Some modern practices that are commonly being used during the construction phase include the use of ready mix concrete for foundations, and large cranes and helicopters for tower erection.
Besides, construction practices take into account specific requirements depending on different terrains. For hilly terrain, for instance, these requirements include ground contour mapping, altitude above the mean sea level, information on valleys, weather conditions, maximum individual span details, minimum and maximum weight spans, horizontal spacing available between two parallel transmission lines, slope stabilisation. Similarly, for snowy terrain, temperature, ice thickness, wind speed, conductor galloping and precast foundations are considered while designing the transmission tower. For desert areas, data regarding sand dunes and information about the stabilisation of foundations to ensure stable erection of towers on sand are required.
Light detection and ranging technology (Lidar) surveying technology is also used to access information about the ground profile of project locations. They help in providing accurate and speedy information. Lidar surveys are most advantageous for inaccessible line routes. However, the cost of this surveying technogy is high in comparision to conventional survey methods and it requires a series of statutory requirements, approvals and permissions from government agencies. The lack of available equipment and expertise further discourage this method of surveying.
RoW is among the most important considerations while undertaking the construction of transmission towers. The RoW depends on the voltage of the line and the height of the structures. It is important in densely populated and forest areas as well as areas with high land costs, non-availability of required conventional width of RoW and narrow, constrained geography with no other alternative, and in areas dedicated to flora and fauna conservation. RoW can be significantly conserved with the help of appropriate strategies, which include bulk power high capacity lines, alternate line and tower configuration involving the use of double-circuit or multicircuit transmission lines, compact structures like delta-type and very high structures, and uprating of existing lines. For instance, pole structures, and insulated cross-arm and tower geometry are often used by operators to reduce RoW requirements.
In sum, the latest trends in transmission and tower designs are directed towards the customisation of tower foundations and structures in accordance with the terrain and other area-specific parameters. This is further being facilitated by upcoming and innovative survey techniques like Lidar, which help operators to quickly access relevant information about ground profiles. In addition, amidst growing concerns for environmental protection, new tower designs and construction practices are becoming more environment friendly. The uptake of modern and technologically sound construction practices is being driven by the need to set up transmission projects in less cost and time.