Towers are critical components of transmission projects as they help carry conductors at a safe height from the ground. Towers and their foundations account for a major proportion of the construction cost of a transmission system. Over the years, the design and technology of the construction and installation of transmission towers has improved vastly and new kinds of towers are being developed with features such as lesser right-of-way (RoW) requirement, resilience to storms and extreme weather events, and compact size in order to reduce costs.
Designing a tower
Since transmission towers bear the weight of overhead lines, the principles of electrical, mechanical and civil engineering are applied in designing towers to ensure their seamless functioning for a long term. While designing a tower, the key factors that need to be considered include minimum ground clearance of the lowest conductor points above the ground level, the length of the insulator string, the minimum clearance to be maintained between the tower and the conductor as well as between conductors, the location of the ground wire with respect to the outermost conductors, the midspan clearance required from considerations of the dynamic behaviour of the conductor, and lightning protection of the power line. High voltage towers have a higher permissible ground clearance and larger vertical spacing between the top and bottom conductors. The choice of tower foundation depends on the soil and standard foundation types are available for wet, dry, rocky, sandy and submerged types of soils. The seismicity data of the area is also an important factor for laying the foundation.
Usually, the material used for tower construction is steel or galvanised steel. In many countries, wooden or concrete towers are used for high voltage and extra high voltage transmission. Research and development efforts are under way to facilitate the use of composite materials such as fibre-reinforced polymers (FRPs) for building towers as they have a high strength-to-weight ratio and insulation capability, are affordable and require low maintenance, from ultraviolet, rust and rot, and have flame-retardant properties. Ageing wood poles in remote and extremely humid locations are being replaced by pultruded FRP towers and crossarms as they do not absorb moisture and swell up.
Challenges in tower designs
When implementing a transmission line project, the main issue is obtaining RoW and acquiring land. As per the guidelines of the Ministry of Power in 2015, compensation at the rate of 85 per cent of the land value (as determined by the district magistrate based on circle rate or stamp act rate of the tower base area) is payable. Also, compensation towards diminution of land value in the width of the RoW corridor due to laying of transmission line and imposing certain restrictions would be decided by the state as per land use (subject to maximum 15 per cent of the maximum circle rate). The width of RoW is an important factor in densely populated areas, areas with high cost of land, forest areas and narrow, constrained geographies with no other alternatives.
Monopoles, compact towers and multi-circuit towers are some of the solutions that have been adopted to overcome RoW issues. For instance, Power Grid Corporation of India Limited 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 high voltage direct current link, the company deployed overhead lines using pole structures and narrow-base towers, along with underground cables to address RoW issues.
Additionally, transmission towers also bear the brunt of extreme and harsh weather conditions such as cyclones, storms, earthquakes, floods and cloudbursts, landslides, avalanches, etc. In order to mitigate the impact of such climatic events, measures such as strengthening of transmission line towers, reducing the span in existing transmission lines, regular monitoring, patrolling and maintenance of transmission lines, reconstruction of damaged infrastructure and anti-corrosive paint coating are important.
In order to develop climate-resilient transmission and distribution infrastructure, there is a need to adopt new design considerations such as a change in design loads – oblique wind on tower body as well as on conductor, special wind zones for coastal zones, use of terrain category I for transmission lines located in coastal areas and change in drag coefficient. Other measures that can be undertaken include modification in the configuration of transmission towers, reducing the span of transmission lines, use of steel poles for lines in coastal areas, use of underground cables and gas-insulated lines, use of pile foundation, increasing galvanising thickness of tower members and epoxy coating for foundation reinforcement, earthing and lightning protection of transmission lines and use of high temperature-low sag conductors.
Types of towers
Nowadays, conventional lattice-type towers are being replaced by monopoles and compact towers owing to RoW issues. Lattice-type transmission towers are self-supporting, lightweight and easy to transport to inaccessible sites. They are cheaper to install and hence cost effective. However, they require a large area and thus are not ideal for narrow areas of densely populated areas.
Monopoles consist of polygonal tubular sections with a tubular crossarm arrangement for fixing tension and suspension clamps on it. The structure can be in a single tubular form or an H-form. Their advantages are that they require about one-sixteenth the space used by lattice-type towers and therefore have fewer RoW requirements. They have the smallest footprint among all tower types, with significant wind-loading capacity, higher reliability under extreme conditions, flexibility in design, good appearance and fast installation with fewer components. Meanwhile, multicircuit towers are designed such that they can carry three to six circuits, enabling them to transfer more power over a particular distance. They have higher factored operating systems and significantly reduce the aggregate RoW requirement of transmission lines.
Of late, utilities are also deploying emergency restoration system (ERS) towers, which are designed for temporary use to rapidly bypass permanent transmission towers at any voltage and in any terrain. ERS towers can be erected in hours and are suitable for hand, crane and helicopter installation methods. During disasters, restoration has to be done within a limited time; otherwise, line outages will impact revenue and end users. Conventional restoration methods, which involve removal of debris, casting of new foundation, erection of tower and stringing work, are time-consuming. Sometimes non-availability of the tower and hardware further adds to delay in the restoration work. However, with the use of ERS, restoration can be carried out in the least possible time, depending on the scale of failure and terrain conditions. ERS towers require lightweight materials such as aluminium alloy for most of the components and they are suited for any voltage level. They also have a modular design with a standard length of 3 metres and require very few components, resulting in reduction of the site erection time.
The Indian power industry has a few unique features and issues such as generation resources being pocketed in a few places, whereas the demand is spread over the entire country. Thus, the transmission network has to be accordingly spread over the length and breadth of the country. To cater to the future accelerated growth in power transfer requirement, the transmission network needs to be strengthened, while utilising minimum RoW. Therefore, it is essential for utilities to adopt modern tower designs and technologies that cater to emerging and future requirements.
(With inputs from a presentation by E.V. Rao, Vice-President, Engineering Services, KEC International Limited)