Pivotal Role

Reconductoring key to strengthening the transmission system

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.

Key drivers

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.

Utility initiatives

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.

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