Today, power utilities are focusing on reconductoring and uprating transmission lines, driven by increasing network constraints and demands for high availability and capacity. There is a growing need to augment and strengthen the transmission network in the country, and reconductoring will play a major role for utilities in achieving this goal.
The transmission network across the country is growing continuously, in line with the growth in power demand as well as generation capacity. The per capita consumption of electricity in India has grown from 1,075 kWh in 2015-16 to 1,208 kWh (provisional) in 2019-20, but there is significant scope for increase given that the per capita consumption is still much lower than in the developed countries. On the supply side, the renewable energy growth rate has been very strong too, as the country chases ambitious targets to mitigate climate change. The installed renewable capacity (including large hydro) has increased from 101 GW in 2016-17 to 140 GW in 2020-21. As the country aims to augment non-fossil fuel electric capacity to 500 GW and meet 50 per cent of its energy requirements from renewables by 2030, transmission will play a key role in the energy transition. This also presents a challenge to the transmission utilities, to bill transmission projects quicker than is traditional. Right-of-way (RoW) challenges also make it essential to look for solutions that allow for greater power transfer while saving space. Another driver for reconductoring is the ageing transmission infrastructure in the country. It is estimated that over 192,000 ckt. km of transmission lines (over 66 kV and above) are over 30 years old. So, there is a need for revaluation, reconductoring and strengthening of these aged elements of the transmission network in the country.
Key considerations for reconductoring
The most common approach to increasing capacity in overhead transmission lines has been ampacity increases. To this end, a design change requires analysis of the capacity of the line with its existing conductor choice, taking into consideration various design constraints/criteria including thermal limits. It is possible to quantify any untapped capacity in conductors in terms of ampacity by conducting a detailed analysis of a line. Modern conductors with improved strength-to-weight ratios, higher operating temperature limits and better high temperature sag performance can be deployed after a cost-benefit analysis. If the ampacity-increase approach does not deliver desirable results, a voltage increase can be considered. However, the voltage-increase ap-proach requires suitable voltage options at the line’s ends, or a significant capex on end-equipment in order to move to a new voltage.
There are various policies, regulatory recommendations and transmission planning criteria for reconductoring and uprating of lines. The Central Electricity Authority’s (CEA) Manual on Transmission Planning Criteria (2013) also talks about reconductoring of the existing AC transmission line with higher ampacity conductors. As per the manual, the choice of reconductoring will be based on cost, reliability, RoW requirements, transmission losses, downtime, etc. In February 2019, the CEA also released guidelines for rationalised use of high performance conductors (HPCs). The guidelines state that HPCs are a reasonable and economical option for uprating short lines, which experience occasional high electrical loads because of insignificant increase in electrical losses. For longer lines, reconductoring with HPCs may also be economical, if the frequency and duration of high current loads are less.
For the selection of optimal conductors, various key techno-economic criteria need to be taken into consideration. Since the selection of the right conductor with higher ampacity is a crucial aspect of reconductoring of lines, one must focus on the requirement of ampacity as per new power transfer requirements. Moreover, the forces on existing towers and foundations must not exceed the existing values, that is, the sag must not exceed the existing sag values on the line. Additionally, the maximum operating temperature of the conductor must be taken into consideration. It is also necessary to consider the line losses of various conductors.
Types of conductors
With the advent of new conductors in the industry, a lot of substitutes are available for the traditional aluminium conductor steel reinforced cable (ACSR) conductor, although different solutions may provide different degrees of power capacity enhancement in the transmission corridor. These substitutes include HPCs and high temperature low sag (HTLS) conductors, a subset of HPCs. Utilities are able to obtain a much higher current rating or efficiency on these conductors compared to ACSR, making it possible to carry more current per square mm. These conductors also have the ability to more than double the power carrying capacity on an existing transmission line.
While doing this, the existing tower and foundation can be utilised, making the execution process smooth and fast. RoW-related issues and challenges are minimised due to the absence of additional footprints. If conductors with low sag capacity are used, then there is scope for improving the ground clearances along the corridor, as well as ensuring better safety in areas of operation.
Many transmission utilities have undertaken reconductoring projects in recent years. In July 2021, the Jammu and Kashmir Power Development Department’s (JKPDD) proposal for the upgradation of the Wagoora-Zainakote transmission line was approved by the Administrative Council. It will be carried out by reconductoring the 220 kV DC line. Under the project, JKPDD plans to replace the existing 0.4 ACSR with an HTLS conductor on the 220 kV circuit-II. At an estimated cost of Rs 242.9 million, the project will enable the department to meet the additional load demand, lower T&D losses and ensure reliable power supply to the Srinagar, Baramulla and Ganderbal districts
Earlier, in April 2019, India’s first live-line project in Bengaluru was successfully completed by Sterlite Power. The company undertook the reconductoring of a 66 kV transmission line, connecting to Bengaluru’s Electronic City under live-line conditions. The project uprated the existing line to double its power transfer capacity, and the HPC installed in the lines ensured provision of better clearance from the ground and nearby buildings.
Maharashtra State Electricity Transmission Company Limited (MSETCL) has also undertaken reconductoring projects in the past. MSETCL commissioned the reconductoring of a 132 kV single-circuit transmission line in Nashik city in March 2014 using aluminium conductor composite core conductors. Delhi Transco Limited also carried out HTLS reconductoring works in the 220 kV SOW-Mandola and 220 kV Bawana-Shalimar corridors in Delhi in 2020.
Challenges and the way forward
From an engineering, procurement and construction perspective, the availability of shutdowns on the part of a utility could be a challenge. Secondly, the availability of the existing tower and line profile should be assessed in order to enable optimal conductor choices and validate sag tension calculations. Thirdly, in some cases, the line chosen for reconductoring might be very old. In such cases, there is a need to assess whether the old towers are strong enough to carry out the planned reconductoring exercise.
To conclude, reconductoring effectively increases the capacity of existing transmission lines, increases corridor capacity and plays a pivotal role in strengthening the transmission network. It is important that utilities undertake reconductoring procedures as part of their regular plan, have a series of uprates in their pipeline and compare it to building a new transmission line. They should also consider reconductoring as an operations and maintenance exercise and take it up based on the actual field situation.