New Heights

Technology trends in transmission towers

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.

Tower installation

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.

Tower foundation

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 solutions

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.

Conclusion

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.

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