Transmission systems are critical for India’s renewable energy plans. Without proper transmission systems in place, renewable energy cannot be transported from generation points to demand centres. Transmission lines are critical for ensuring renewable energy integration into the grid and facilitating its conveyance to power distribution utilities and commercial and industrial consumers. Recently, several projects have been announced in order to accelerate the development of transmission lines to connect key renewable energy zones with demand centres. Further, there is a significant focus on improving technology, reducing faults, and enhancing reliability.
High voltage transmission lines are being developed across the country using technologies such as flexible alternating current transmission systems and high voltage direct current (HVDC) to enhance the stability of the grid system. These technologies help in reducing transmission losses, optimising power flow, and increasing transmission capacity.
In addition, there have been improvements in the design of cables and conductors to increase their efficiency. Dry extruded cables are now replacing wet paper-insulated ones, and covered cables such as cross-linked polyethylene, high density polyethylene, aerial bunched cables and spacer cable systems have become popular.
Further, electron beam or e-beam cables, which can be used in high temperature applications such as solar space, are also gaining traction. Similarly, gas-insulated transmission lines are witnessing increasing uptake, taking up less space while delivering the same power transmission capacity as conventional overhead lines.
In the conductor space, there is a growing focus on higher temperature ratings and current carrying capacities, and thus, in recent years, advanced conductor technologies such as high temperature low sag conductors, superconductors and extra high voltage (EHV) lines are being increasingly preferred to conventional conductors. Further, aluminium conductors such as aluminium conductor alloy reinforced, aluminium conductor steel reinforced and aluminium conductor carbon fibre reinforced are now replacing copper conductors.
Transmission line deployment
One of the major issues that continue to plague renewable energy development in the country is the unavailability of adequate transmission capacity. Renewable energy projects have shorter gestation periods compared to transmission systems, and thus, many projects face delays in securing grid connectivity. Although measures have been taken to accelerate the development of transmission towers, lines and substations, a significant backlog still exists.
In 2022, the Central Electricity Authority (CEA) released a report titled “Transmission System for Integration of over 500 GW RE Capacity by 2030”, outlining plans for the development of transmission systems to accommodate about 537 GW of renewable energy capacity in major renewable energy potential zones. The transmission schemes in this study took energy storage into consideration, estimating that about 50,890 ckt. km of additional transmission line capacity is required in the inter state transmission system (ISTS) to integrate additional wind and solar capacity by 2030. Further, several HVDC transmission corridors have been planned for the evacuation of power from large renewable energy potential zones.
The last few months of 2023 witnessed significant activity in the transmission space as the country rushed to meet its 2030 renewable energy targets. For instance, Adani Energy Solutions Limited announced that it had received a letter of intent to acquire Halvad Transmission Limited from PFC Consulting Limited. The Halvad transmission line will facilitate the evacuation of 7 GW of renewable energy by connecting Khavda and Halvad, both located in Gujarat. The transmission project will span a distance of 301 km (656 ckt. km) and involve the line-in line-out of the Lakadia-Ahmedabad 765 kV DC line at Halvad and the establishment of a 765 kV Halvad switching station, equipped with two 330 MVAr bus reactors.
Further, in December 2023, NTPC Renewable Energy Limited issued a request for proposal for land and an EHV transmission package for 800 MW ISTS-connected solar projects coming up in Maharashtra. Around the same time, Tata Power Limited received a letter of intent for Bikaner-III Neemrana-II Transmission Limited, which aims to facilitate the evacuation of 7.7 GW of clean energy from the Bikaner complex in Rajasthan. The project involves creating a transmission corridor spanning approximately 340 km, connecting the Bikaner-III pooling station to the Neemrana-II substation.
Further, in November 2023, Sterlite Power Transmission Limited was awarded the contract for the transmission project in Rajasthan’s renewable energy zone, Phase IV (Bikaner complex). The project aims to facilitate the transmission of approximately 8,000 MW of renewable energy from Bikaner to load centres in Rajasthan, Haryana and Uttar Pradesh. It includes building a 6,000 MVA, 765/400 kV substation at Neemrana, and constructing two 400 kV transmission lines covering approximately 250 km. These lines will link Neemrana to the current Kotputli substation and also connect the existing Gurugram-Sohna line with both the Gurugram and Sohna substations.
In the same month, ReNew announced the commissioning of its first ISTS project, which will help facilitate the transmission of 1,500 MW of clean energy in the Koppal area of Karnataka. The Koppal Transmission Scheme encompasses the construction of a new 400/220 kV substation in Koppal, along with a 276 ckt. km 400 kV D/C quad moose transmission line and the extension of 400 kV gas-insulated substation bays at the new Power Grid Corporation of India Limited Narendra substation. Thus, there has been a notable increase in the deployment of transmission lines.
Issues and solutions
In addition to the delays in commissioning transmission lines and related systems to support renewable energy projects, there are other challenges concerning clean energy integration into the grid. Due to grid constraints, there have been several instances of generation curtailment, resulting in losses for renewable power projects. This is particularly evident during the high-wind monsoon season when wind power generators frequently face grid curtailment issues.
Further, there remains the issue regarding the urgent need for ample and robust transmission capacity that can offtake large volumes of infirm renewable power. Both solar and wind power have erratic generation patterns, and this can impact grid operations. Transmission system faults can occur within renewable power projects or externally, and issues of overvoltage during line or reactor switching can also arise. These grid events can create ripples, leading to grid failures and power supply losses, thereby threatening grid security.
Grid India’s recent report on “Events Involving Transmission Grid Connected Wind and Solar Power Plant” investigates 31 such grid events. According to the study, these events, associated with the frequent loss of renewable energy generation over the period January 2022 to May 2023 in ISTS-connected renewable energy pockets, resulted in generation losses ranging from 1,000 MW to 7,000 MW in individual instances. Further, non-compliance during low voltage ride-through and/or high voltage ride-through events led to a power loss of 6-7 GW in Rajasthan’s renewable energy complexes. An in-depth analysis of these events reveals various causes, such as undesirable behaviour of inverter-based resources, low short-circuit ratio in certain pockets, improper coordination of protection settings between renewable energy plants and the evacuating transmission system, non-compliance with the CEA’s technical standards and inverter controller interactions.
To prevent the occurrence of such events in the future, forward-looking technical standards and grid codes need to be developed to keep pace with new technologies in inverter-based resources, battery energy storage systems and hydrogen electrolysers. Further, constant vigilance and monitoring are essential to avoid incidents that could cause significant harm to both property and life. There is a need for increased stakeholder consultations and the transfer of technologies and ideas to prepare the Indian grid for the significant influx of renewables.
Going forward, adequate planning and streamlined implementation of transmission lines and associated systems are vital for the continued deployment of renewables in India. Continuous technology upgradation and the adoption of state-of-the-art quality products are crucial, as these systems are built to last for years and are key assets for the overall energy security of the country.
