The country’s transmission network has been growing steadily in recent years to meet its growing power transfer capacity requirements, as more renewable energy capacity is being added to the grid. Key requirements for the engineering and design of transmission structures, such as towers, to support the new and emerging requirements for utilities, include reducing Right of Way (RoW) requirements, minimising visual impact, enabling faster assembly and installation, and lessening operation and maintenance needs. Construction, operation and maintenance techniques and practices are evolving by the day in order to improve efficiency and asset health. A closer look at the key trends in the construction and erection of transmission towers…
Improving tower designs
A key issue plaguing the transmission segment is RoW. The problem surfaces when laying down transmission towers in difficult terrain and populated regions. RoW is especially a major concern in inhospitable terrain with narrow spaces, densely populated areas, forests and regions where land is expensive.
An approach being adopted to optimise RoW is to focus on improving the efficiency ratio, which is the ratio of the power transmitted to the RoW width. The latter varies based on the voltage of the transmission line to be installed within the corridor. This ratio is highly dependent on tower geometry. Hence, the challenge is to reduce space, which, in turn, improves efficiency. Further, DC transmission lines can be used to make the ratio better. A higher voltage tower in the same space can also improve the ratio. Hence, tower engineering plays a vital role in managing RoW.
There are multiple tower designs that can help optimise and conserve RoW, and transfer bulk power. Double circuit towers and multi-circuit towers are viable options for densely populated areas. Double circuit towers have six conductors, which is a more reliable configuration for areas with greater power requirement and little available space. Multi-circuit towers have multiple circuits on the same tower with the same or different voltage, or even different systems such as AC or DC. These towers can transmit large amounts of power in narrow corridors, but they also require stricter observation of safety norms. Even single circuit towers can help optimise RoW with clever use of tower geometry. For instance, a simple horizontal single circuit 756 kV tower takes up 84 metres of RoW, while a delta tower with the same voltage takes up only 64 metres, as per industry estimates.
If these designs are coupled with extra high voltage transmission lines in 800 kV & 1200 kV ranges, transcos will be able to transfer the maximum power per metre width of available corridor, thus maximising the efficiency ratio. Several power utilities, including Power Grid Corporation of India (Powergrid), Tamil Nadu Transmission Corporation and IL&FS Tamil Nadu Power Company Ltd have opted for double and multi-circuit towers for their latest ongoing projects.
Meanwhile, compact guyed towers are a good fit for transmission over water bodies. A guyed tower is a slender structure comprising truss members, placed on the floors of water bodies. Guyed towers are especially useful for deep and turbulent water bodies, and are a popular choice in the West.
Meanwhile, compact chainette towers consist of masts, which are individually used to support two guys and ropes connected to the top of the two masts. The insulator string and conductor bundles are suspended horizontally. Being narrow based, they are advantageous in steep and angular locations.
Transmission-cum-telecom towers and taller tower designs are helpful for densely populated areas. Winged circuit towers and shorter tower structures such as the triple leg slope tower, which places all bundle conductors at the same level, can be used to protect flora and fauna.
Another design gaining traction is monopole. Being sleek with tubular sections, these towers are extremely useful in RoW-constrained areas where conventional lattice towers cannot be deployed. These are highly preferred in cyclone-prone areas and urban areas with multiple metro and flyover crossings.
There are numerous other technologies that can be incorporated in transmission towers to optimise RoW, such as pilot jumpers, which are particularly helpful for narrow and high-altitude regions. Post insulators can be used for jumpers to reduce cross-arm projections, which, in turn, can decrease the RoW requirement. Higher voltages coupled with DC substations can also help optimise RoW.
Further, existing towers are being upgraded with high thermal rating conductors. High temperature, low sag conductors are being deployed in very narrow transmission corridors. For instance, Powergrid has worked to upgrade the 400 kV Siliguri-Purnea transmission line, which was dealing with RoW constraints.
Detailing and ease of construction are some of the major concerns of transcos with regards to installation. Hence, utilities are increasingly adopting light detection and ranging (LiDAR) technology as well as drones for aerial surveillance and topographic mapping before construction begins.
Helicopters equipped with LiDAR and thermovision cameras are being deployed for aerial patrolling. Tower erection by helicopters and heavy cranes is a new method that has led to an increase in construction efficiency. It is particularly useful in mountainous areas. Pre-construction surveys can be helpful in specifying points for tower lifting by helicopters.
These technologies have eased the process of installation, even in untraversable regions.
A major way to improve the efficiency ratio is to build a strong and durable tower foundation. A well-built tower foundation can also help optimise RoW. Preferences have shifted from the archaic pad and chimney foundations to grillage foundations, which are more useful for ambient soil conditions, and pre-cast foundations, which can survive extreme weather conditions and can even be set up in snow. Raft foundations, which are essentially large plan area foundations, are useful for lightly loaded structures on soft soil conditions. Meanwhile, driven piles or precast piles are prefabricated off-site, can be installed in shallow trenches, and have high load-bearing capacity. They are cost-effective and easier to customise than other options.
More recently, helical and auger pile foundations have been gaining traction. Helical piles can support a variety of structures. They are deeply embedded in the ground and can thus support heavy tower designs. Meanwhile, auger pile foundations are embedded at a specific depth and are built to bear weight on both ends. They are friction-based, and are usually less expensive than helical piles. Both varieties possess greater durability to structural damage, as well as greater terrain adaptability and voltage carrying capacity, than other options.
Digital technology solutions for surveillance, construction, modelling and monitoring hold significant promise for transmission project developers. Software-monitored transmission tower construction is a solution that many transcos can benefit from.
Some of the software that are popular in European markets are now slowly entering the Indian markets. One of them is building information modelling (BIM) software. BIM software enables a collaborative process that allows engineers and developers to plan, design and construct a structure via 3D modelling. It is useful for transcos in tackling the challenges of budget constraints, wastage and planning discrepancies that surface during the construction of transmission towers. BIM can greatly improve the efficiency of a project by leveraging historical data and providing more information in order to fabricate a model. It is also useful for estimation, design, scheduling, project management, and manufacturing and erecting towers.
Digital twin is another upcoming solution. It essentially monitors the health of a transmission tower and lines by tracking all the operational parameters. The twin can provide real-time predictive data on the transmission towers and lines, and enhance operational flexibility and asset risk management. The challenge, however, is the high cost of the technology and the trade-off between capex and opex.
Clearly, transmission utilities are moving towards more practical, cost-effective and durable tower designs. Going forward, digitalisation can be a game changer for transcos, as it has the potential to improve construction technology and asset management, enabling real-time monitoring of assets with predictive analytics. Most importantly, it can address crucial challenge of RoW.