Over the past few years, tower technologies have undergone significant changes. Owing to factors such as increasing urbanisation, limited space availability and the need to quickly scale up transmission infrastructure, developers are looking at towers that require minimum right-of-way (RoW) approvals, can be installed faster and provide cost benefits. Utilities, therefore, now prefer monopoles or narrow-base lattice towers over conventional lattice-type towers. Moreover, transcos are increasingly deploying emergency restoration system (ERS) towers following natural disasters. There have been advancements in tower foundation designs as well.
Power Line takes a look at current transmission tower designs and foundations, as well as recent advances…
Generally, transmission line towers come in two types: self-supporting (lattice structure/steel pole structure), and guyed. In India, lattice structures are most commonly used by utilities for extra high voltage transmission lines as they are self-supporting, lightweight and easy to transport to inaccessible sites. Meanwhile, lattice guyed V transmission towers are easy and cheaper to install, making them cost effective. However, they entail RoW issues as they require a large area and, therefore, are not preferred in densely populated urban areas or narrow corridors.
In recent years, the use of monopole structures has also been increasing in specific areas due to their much reduced footprint, fewer components and faster erection and commissioning. Monopole towers consist of polygonal tubular sections with a tubular cross-arm arrangement for fixing tension and suspension clamps on them. The structure can be in a single tubular form or an H-form. Monopoles require one-sixteenth the space used by lattice-type towers and, therefore, have fewer RoW requirements. They have a smaller footprint and are available in specific design solutions as per site requirements. They also offer the benefit of faster assembly and installation, as well as flexibility in design modifications. Monopoles have significant wind loading capacity and offer high reliability. They are aesthetically appealing and require less maintenance.
Power Grid Corporation of India Limited (Powergrid) has been installing monopoles since 2008-09 to save space and avoid the felling of trees. Powergrid recently commissioned the ±320 kV Pugalur (Tamil Nadu)-Thrissur (Kerala) voltage source converter-based high voltage direct current system, which has deployed monopoles. Delhi Transco Limited has also installed monopoles in some areas. Sterlite Power has used monopoles for uprating the Mallapuram-Manjeri line from 66 kV single-circuit to 110 kV D/C for the Kerala State Electricity Board. The Gurgaon Palwal Transmission project of India Grid Trust has deployed multicircuit monopole towers for transmission lines to optimise the space challenges related to the project.
Multicircuit towers are designed to carry three, four or even six circuits, which enable them to transfer more power over a particular distance. They are designed with higher factored operating systems. They significantly reduce the aggregate RoW requirement of transmission lines, thereby leading to substantial cost savings. They have been successful in areas such as forests, thickly populated cities, and substation entry and exit corridors.
Multicircuit towers are extensively used for the line-in line-out of 400 kV D/C lines in forest areas and at substation entries. State transmission utility Madhya Pradesh Power Transmission Company Limited has tested a 220 kV multicircuit crossing tower, designed to withstand loads for a 60-degree deviation or dead condition. The use of a multicircuit tower has helped avoid the use of gantry structures for underneath crossings of 220 kV or 132 kV lines, or towers with higher extensions for overhead crossings.
ERS towers are designed to rapidly bypass permanent transmission towers at any voltage in any terrain. Made of box sections that are highly stable structurally, ERS is a lightweight, modular and reusable system that is used as a temporary support structure to restore power immediately after the collapse of transmission line towers during natural calamities or man-made disruptions. While permanent restoration may take several weeks, ERS towers can be erected within a few hours and are suitable for hand, crane and helicopter installation.
Last year, Chennai-based Structural Engineering Research Centre, a constituent laboratory of the Council of Scientific and Industrial Research, developed an indigenous emergency retrieval system technology for quick retrieval of power transmission in the event of a failure of transmission line towers. At present, ERS systems are imported from a few manufacturers across the world at a high cost. This technological development is expected to kick-start ERS manufacturing in India, reducing costs to about 40 per cent of imported systems. The system is compact but provides full functionality on erection. It also provides for suitable configurations for different voltage classes of transmission line systems.
The ERS structure was used by Odisha Power Transmission Corporation Limited to quickly restore power supply in the Fani cyclone-affected areas in May 2019. As of 2019-20, Adani Transmission Limited has two ERS sets in the central part and one ERS set in the western part of the country. More recently, during the 2021 Taukte cyclone, Powergrid used the ERS system to set up 11 towers and also constructed 10 new towers for restoring the 220 kV Timbdi-Dhokadva line, feeding power to the Union Territory of Diu.
Other tower designs
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 require a short installation time. In certain areas, Powergrid has used compact towers such as pole towers, delta configuration towers and narrow-base towers, which require less space for the tower base. Tall towers and multicircuit towers are also being used for conservation of scarce RoW.
Advances in tower foundations
A key component of a tower is a strong and sturdy tower foundation, which helps it withstand strong winds, hurricanes and other adverse weather conditions. A direct embedded foundation, which can be managed with small spans, is suitable at sites where space is very limited, whereas base-plated-type poles, which are easy to assemble and install, can be used for any height and span.
Micropile-based tower foundations comprise piles of diameter smaller than 200 mm. Micropiles are an ideal solution for transmission projects in deserts, mountains and marine environments. Micropiling has been undertaken for monopole transmission towers in Kerala, which were built 60 years back and were upgraded from 66 kV to 132 kV. A transmission project in Jammu also uses micropile towers. Some other tower foundation designs are precast foundations (used during limited construction periods), grillage foundations (used in firm soil areas) and reinforced cement concrete spread (used in a variety of soil conditions). Precast foundations are preferred where the period of construction is limited and grillage foundations are factory-made for use in firm soil where concreting is costly.
Challenges and the way forward
The bottlenecks in the construction of transmission lines with monopole structures include high costs, difficulty in transportation, increase in the number of poles due to a reduction in design span, special design considerations for multicircuit towers and limited manufacturing facilities.
In addition, while the Central Electricity Authority (CEA) gives developers the flexibility to choose their tower design, over 99 per cent of the requests for proposal in India make it mandatory to use lattice structures. Developers outside India (in Canada and the US) have used monopoles liberally to optimise construction time. They are easier to install and can be prefabricated. While standards are in place for lattice towers, and a concept paper has recently been floated by the CEA for insulated cross-arms, there are no specifications for the use of monopole towers, guyed towers and special foundations such as micropiling or grillage. This deters transmission utilities from adopting and benefiting from new technological developments.
Going forward, there is a need to update existing standards and notify new ones to enable transmission utilities to adopt advanced tower technologies in upcoming projects.