Transmission Towers

Designs, construction practices and foundations

Over the years, significant improvements have been made in transmission tower designs. Advanced tower designs have reduced the right-of-way (RoW) requirement, minimised the visual impact, enabled faster execution and provided ease of installation. The new designs have also enabled the expansion of the transmission network to far-flung areas including forests and other difficult terrain. Besides, there have been advancements in tower foundation designs. Going forward, there is a need for utilities to adopt tower designs that can withstand natural disasters such as emergency restoration system (ERS) towers and monopoles with smaller spans in disaster-vulnerable areas.

A look at transmission tower designs and foundations…

Lattice-type towers

Lattice-type transmission towers have been widely used by utilities as they are self-supporting, lightweight and easy to transport to inaccessible sites. Lattice guyed V transmission towers are easy and cheaper to install, and hence 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.

Monopoles

Monopoles are fast gaining traction. These towers 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. Monopoles require one-sixteenth the space used by lattice-type towers and, therefore, have fewer RoW requirements. They have a lower footprint and are available on specific design solutions as per site requirements. They also offer the benefits of faster assembly and installation (as fewer components are required), as well as greater flexibility in design modifications. Further, monopoles have a significant wind loading capacity, and offer high reliability under worst load conditions. They are aesthetically appealing and require less or zero maintenance. For instance, Sterlite Power has used monopoles for uprating the Mallapuram-Manjeri line from 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 the construction of any new line. The old 66 kV S/C line was uprated to cater to the needs of the population and improve the quality of power. Sterlite offered its AL59 conductor solution technology, which facilitated higher ampacity to accommodate the peak load demand and reduce line losses under normal loading conditions. It used monopoles to install the line, thus reducing its footprint.

Multicircuit towers

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.

State transmission utility Madhya Pradesh Power Transmission Company Limited has tested a 220 kV multicircuit crossing tower, designed to withstand load for a 60 degree deviation or dead condition. The tower can be used for crossing the existing 220 kV or 132 kV lines with new 220 kV or 132 kV lines. The use of a multicircuit tower has helped avoid the use of gantry structures for underneath crossing of 220 kV or 132 kV lines, and towers with higher extensions for overhead crossing. The tower has been erected at the crossing point of the line to be crossed. No RoW issues arise as it is erected within the RoW belt of the existing line. This has also led to substantial cost savings.

ERS towers

ERS tower structures are also popular as they are designed to rapidly bypass permanent transmission towers at any voltage in any terrain. ERS towers can be erected in hours and are suitable for hand, crane and helicopter installation methods. Though designed for temporary use, ERS towers are kept in continuous service at utilities for over a decade owing to their robust design. For instance, Odisha Power Transmission Corporation Limited was 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 disrupted due to the cyclone Fani, including 75 towers of 220 kV and 25 towers of the 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.

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.

Construction practices

For tower construction, the build-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.

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. Direct embedded foundations are suitable at sites where space is very limited and can be managed with small spans. This type of foundation can be used for distribution, sub-transmission and transmission poles. The depth of embedment of a pole for the foundation should not be less than one-sixth of the total length of the pole above the ground level. Base-plated-type poles can be used for any height and span. The foundation is easy to assemble and install.

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. Micropiling has been undertaken for monopole transmission towers in Kerala. The transmission line project was built 60 years back, and was upgraded from 66 kV to the 132 kV level. Besides this, a transmission project in Jammu uses micropile towers.

Some of the 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).

The way forward

ERS towers are proven to be disaster resilient, but their production overseas results in higher cost. Thus, more encouragement is needed for domestic capacity building to ensure the easy availability of these towers at lower prices.

While the Central Electricity Authority gives flexibility to developers for choosing the tower design, more than 99 per cent of the requests for proposal (RfPs) in India make it mandatory to use lattice structures.

Developers outside India (in Canada and the US) have used monopoles liberally to optimise the time of construction. These are easier to install and can be prefabricated. While standards are in place for lattice towers, there are no specifications for the use of monopole towers, insulated crossarms, guyed towers and special foundations like micropiling or grillage. This deters transmission utilities from adopting and benefiting from new developments. Thus, there is a need to update the existing standards and specify new standards to enable transmission utilities to adopt these new technologies in the upcoming projects. Going forward, a demand-side push through RfP specification, and the standardisation of design could further accelerate supply.

Nikita Gupta

 

 

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