In order to adapt to the evolving power demand and generation dynamics, there is a need to expand and strengthen the transmission system. This includes constructing new transmission lines and substations, and enhancing existing transmission and transformation capacities through various means. Among these approaches, the reconductoring of lines using high-capacity, high-performance conductors is expected to emerge as a strategic method to both preserve right of way (RoW) and significantly increase transmission capacity.
Reconductoring, defined as the installation of new conductors on existing towers within the same RoW, aims to enhance the thermal capacity of transmission lines. However, in cases where the load-bearing capacity of towers proves insufficient, this process may necessitate the modification or replacement of certain towers. In addition, reconductoring may involve the substitution of terminal bay equipment with higher-rated alternatives that align with the specifications of the newly installed conductors.
The Central Electricity Authority (CEA) recently released a draft paper on reconductoring of transmission lines. The paper identified the key considerations for reconductoring and the challenges associated with it, serving as a guidance for undertaking reconductoring. Highlights of the CEA’s discussion paper…
Key drivers
According to the CEA, the ampacity of the new conductor must align with the updated power transfer demands, taking into account acceptable sag/tension parameters and the maximum permissible conductor temperature. It should be in accordance with the guidelines outlined in the CEA’s Manual on Transmission Planning Criteria for the year 2023.
The new conductors must have a higher continuous operating temperature and higher ampacity in comparison to the current conductors. In addition, the physical design parameters of the new conductors, including sag, swing and diameter, should either match or exceed those of the existing conductors on the transmission lines.
Due to the criticality of sag values in the reconductoring process, particularly for lines with extended spans such as river crossings, wide valley spans, and exposure to high wind and ice loadings, it is imperative to limit the tension forces on existing towers and foundations to their current levels. The short-circuit current rating of the new conductor must equal to or exceed the short-circuit current rating of the existing conductor.
The use of advanced conductors during reconductoring, operated up to their capacity limits, may encounter limitations at the terminals. These limitations may include issues with switches, breakers, protection mechanisms, FACTS (flexible AC transmission system) controllers, and other devices. Further, there is a potential to exceed the existing current rating capacity (kA) of substation buses. Consequently, the need to replace bay equipment at terminal ends to align with the rating of the new conductor arises, requiring simultaneous planning for the upgrade of bay equipment alongside the reconductoring process.
The integrity of towers must be guaranteed by assessing the compatibility of existing foundations and towers, and evaluating mechanical stresses and tension. If there is a requirement for reinforcing any towers, such measures can be taken concurrently with the reconductoring process.
Reconductoring extra high voltage lines on towers that support multiple voltages and circuits may pose challenges. Therefore, it is essential to implement effective strategies, especially when an adjacent circuit on the same tower needs to remain energised.
The growth in load and the need for economical system operation adapted to the load necessitate the adoption of cost-effective approaches to meet transmission capability requirements. In such scenarios, reconductoring using high temperature low sag (HTLS) conductors emerges as a viable option to augment power delivery to the load. HTLS allows for a relatively high current to flow through the conductor without compromising sag parameters. The elevated current rating substantially enhances the thermal capacity of the overhead line, typically doubling its limit. The potential to deploy HTLS conductors, without simultaneous upgrades in towers and insulators, positions them as a promising option to increase the thermal rating of existing transmission paths. This becomes particularly significant in urban areas where obtaining new RoW approvals proves challenging.
Costs and tariffs
According to the draft paper, a crucial aspect in the reconductoring process is determining the tariff mechanism. The choice lies between the regulated tariff mechanism (RTM) and tariff-based competitive bidding (TBCB).
Transmission schemes with a cost lower than Rs 1 billion require approval from the Central Transmission Utility (CTU). Meanwhile, schemes with a cost ranging from Rs 1 billion up to Rs 5 billion are sanctioned by the National Committee on Transmission (NCT). For schemes estimated to cost above Rs 5 billion, the NCT recommends approval to the Ministry of Power. The sanctioned schemes are then implemented through either the RTM or the TBCB route.
RTM is a cost-plus model overseen by Power Grid Corporation of India Limited for project execution. In this system, reconductoring works are assigned to the licensee of the original line. Since the activities associated with reconductoring are typically classified as technical upgrades, these projects are implemented in RTM mode by the owner of the original transmission line.
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However, concerns have been raised by transmission associations and industry stakeholders regarding the RTM approach. They have apprehensions about higher implementation costs and a perceived lack of transparency in the allocation of reconductoring works to the transmission licensee of the original line.
In the TBCB model, private companies have the opportunity to bid, and contracts are granted to the lowest bidders following the build- own- operate-maintain framework.
Further, the standard bidding documents for the procurement of interstate transmission services through TBCB, issued in August 2021, stipulate that the transmission service provider (TSP) is responsible for ensuring the transfer of all project assets, including substation land, RoW and clearances to the CTU or its successors, or an agency determined by the central government after 35 years from the commercial operation date of the project, at zero cost and without any encumbrance or liability. However, if the reconductoring of the transmission line is carried out before the 35th year, a scenario may arise where only the towers have reached the end of their service life and not the conductor. Additionally, the cost of conductors and any associated bay equipment may not have been fully recovered by the time the towers complete 35 years. Disputes may arise regarding tariff sharing, especially when towers are owned by one licensee and the conductor by another.
The way forward
During the useful life of the transmission system, to address technical, commercial and operational challenges in the reconductoring of transmission lines, the CEA recommends assigning the scheme to the existing owner (TSP) through the RTM route. The recovery of reconductoring tariffs should coincide with the recovery schedule of other equipment. If a project has been implemented under the RTM mechanism, its technical upgrading within or after the useful life should be conducted under the RTM mode. The reconductoring of the transmission line and the upgrading of bay equipment should be carried out simultaneously by the respective licensees.
Overall, given that siting and RoW approvals for new lines can significantly extend project execution timelines and in some cases even lead to project cancellations, reconductoring is expected to play a crucial role in the future growth of the transmission network.
