Over the years, transmission tower designs have evolved significantly in order to reduce the right-of-way (RoW) requirement, ensure speedy execution of projects and withstand diverse environmental conditions. Utilities are increasingly adopting monopoles and multicircuit transmission towers that have lesser RoW requirement, as against the conventional lattice towers. There have been advancements in tower foundation designs, survey techniques and installation methods to ensure speedy execution and sturdy infrastructure. Besides, utilities are adopting model-based software products to increase productivity and ensure accuracy during the construction layout process.
Lattice-type towers
One of the most widely used transmission tower designs is the lattice type, which is mainly used for over 100 kV levels. These towers are self-supporting structures and can be built easily at inaccessible locations as the tower members are lightweight. The lattice guyed-V transmission towers widely used by transmission companies are easy and cheaper to install. The guyed transmission tower does not use concrete as the construction material and instead uses steel piles for angle towers. Therefore, these towers can be prefabricated and assembled at the desired site. The prefabricated structures are moved to the site and installation is done in the least possible time. The whole structure of the tower needs to be well fitted and bolted before shifting to the installation site. These towers are best suited for regions with extreme weather conditions, particularly snow-covered terrain and forests. One of the disadvantages of lattice towers is high RoW requirement. Lattice towers, especially guyed towers, require a large area and are, therefore, not preferred in dense urban areas or narrow corridors.
Monopoles
The use of monopoles is fast gaining traction. These towers consist of polygonal tubular sections with a tubular crossarm arrangement for fixing tension and suspension clamps on it. The tubular structure can be in a single tubular form or an H-form. Monopoles require one-sixteenth the space needed by lattice-type towers and, therefore, have less RoW requirements. They also offer the benefits of faster installation (as fewer components are required) and greater flexibility in design vis-à-vis lattice towers. Further, monopoles have a significant wind loading capacity and can withstand extreme weather conditions.
For instance, state transmission utility, Madhya Pradesh Power Transmission Company Limited has utilised the existing 132 kV North Zone-Jaitpura line corridor to set up 220 kV multicircuit monopoles. This is the first time monopoles have been used for multivoltage transmission systems in India. The upper panel of the multicircuit monopole was used for the deployment of a 220 kV (double-circuit [D/C]) line, whereas the lower panel was used to deploy a 132 kV D/C line. The move helped cater to the increased load demand of Indore city. Meanwhile, Sterlite Power has used monopoles for uprating the Mallapuram-Manjeri line from the 66 kV single-circuit (S/C) to 110 kV D/C for the Kerala State Electricity Board. The state utility was struggling with a 30-year-old network around which the town grew, leaving no scope for construction of any new line. This 66 kV S/C old line was built to cater to the needs of the population as a solution was required to facilitate the increase in load and improve the quality of power. Sterlite offered its AL59 conductor solution technology, which facilitated higher ampacity to accommodate the peak load demand with reduction in line losses at normal loading conditions. It also used monopoles to install the line, thus reducing its footprint.
Multicircuit tower
Multicircuit towers are designed to carry three, four or even six circuits. Thus, they are able to transfer more power over a particular distance. They are designed with higher factored operating systems. The towers significantly reduce the aggregate RoW requirement of transmission lines. These have been successful in areas such as forests, thickly populated cities, and substation entry and exit corridors.
Other tower designs
Other emerging tower designs include delta or vertical configuration towers, insulated crossarms that help reduce the height and width of the towers, and tower designs with fewer sections and bends to reduce inventory and lead times. Further, tower designs using less steel to make transportation and assembly easy reduce the number of towers required per kilometre. 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.
Survey techniques
Pre-construction surveys help identify the exact and shortest possible route of the transmission line and the number of towers required along the route. Various surveillance techniques aid in selecting the right tower foundations based on the topography and soil type of the area. Conventional surveying techniques including walkover survey, which entails going over to the proposed transmission line route, are both time-consuming and inaccurate. Therefore, utilities are adopting advanced technological tools for tower mapping. These include aerial patrolling solution, geographic information system mapping and light detection and ranging (LiDAR). Tower-top patrolling devices using unmanned aerial vehicles (UAVs)/drones eliminate the need to physically visit the tower site. These UAVs are equipped with gimbal-mounted ultra-HD video cameras that can take close, high resolution photographs and videos. The use of helicopters/drones equipped with gimbal-mounted LiDARthermovision cameras and high resolution video and digital cameras to provide high resolution images of towers, lines and the project site is fast gaining traction. These methods are cost effective and more efficient than the conventional solutions deployed by utilities.
Tower foundation
Another key component of a tower is a strong and sturdy tower foundation, which helps withstand strong winds, hurricanes and other adverse weather conditions. An emerging tower foundation design is micropiling. Micropile-based tower foundations comprise piles with a diameter smaller than 200 mm. Micropiles can be used in a wide range of geotechnical conditions, making them an ideal solution for transmission projects in deserts, mountains and marine environments. Some of the other tower foundation designs are precast foundations (used during limited construction periods), grillage foundation (used in firm soil areas) and reinforced cement concrete spread (used in a variety of soil conditions).
Emergency restoration systems
Emergency restoration systems (ERS) have also gained traction to help limit outages. These are designed to rapidly bypass permanent transmission towers at any voltage in any terrain. ERS can be erected within hours and is suitable for hand, crane and helicopter installation methods. Though designed for temporary use, utilities keep ERS towers in continuous service for over a decade, owing to their robust design. ERS offers a fast, convenient and, above all, safe means for restoring power anywhere. For instance, Odisha’s state power utilities, along with the electrical equipment body, Indian Electrical and Electronics Manufacturers’ Association, were able to quickly restore power supply in the state’s cyclone-affected areas in May 2019. The entire power installation in and around Puri district was devastated due to the cyclone Fani, including 75 towers of 220 kV and 25 towers of 132 kV voltage level. An indigenously developed steel-based ERS tower was used to substitute the wrecked 132 kV towers between Lilo and Puri-Samkuha. It enabled charging between the Puri grid and the Samkuha grid to transfer a bulk load of 70 MW. The tower was restored in five hours using the ERS structure.
Application of ERS is not limited only to calamities that damage transmission line sections. It can also be deployed to reroute an existing line for a brief period to accommodate a construction project or to allow continued power flow, while a line is being upgraded in some way.
Construction practices
For tower construction, the built-up method is widely used. In this method, each tower member is installed in a sequence from top to bottom. Utilities also use the section method for transmission tower installation, wherein major sections of the tower are assembled on the ground and are then erected as units, using a mobile crane or a gin pole. One of the emerging technology solutions for transmission tower erection is helicopters. Sterlite Grid has emerged as a pioneer in the use of aerial technologies. It is the country’s first transmission developer to have deployed an air crane to set up a power transmission line in the mountainous terrain of Jammu & Kashmir as part of the Northern Region Strengthening Scheme XXIX project.
Apart from innovations in design, model-based software products are being used for providing digital architecture. Advanced 3D software provides users with the latest tools for increasing productivity and accuracy during the construction layout process and reduce rework and lost time. It allows users to create layout points of critical field activities. The layout data can then be imported back into the building information model for verification of quality assurance (QA) and quality control (QC). To assist with QA/QC workflows, the user is able to import layout points such as “as-built” or “staked” locations and review the point properties before inserting information for comparison through AutoCAD or Revit. These features provide the user with an efficient and flexible method to create the field points required for layout on a variety of model entities.
Net, net, technological advancements in transmission tower designs, foundations and survey techniques would contribute significantly towards strengthening and expanding the transmission network. This would help in ensuring 24×7 quality power supply by maintaining a congestion-free transmission network and ensuring grid stability.