North East Transmission Company Limited (NETCL) has constructed & is operating the 662.8 Km, 400 kV Double Circuit inter-state transmission corridor through difficult hilly & dense forest terrain of the States of Tripura, Meghalaya and Assam in the North Eastern Region (NER) of India. The transmission line crosses two major rivers namely Brahmaputra & Barak. The mighty Brahmaputra River with its source in Tibet, flows down through the steep Southern Himalayas with great speed and flows with huge discharge & speed during monsoon season from its numerous tributaries causing erosion, flash floods and siltation leading to frequent changes of its course.
Background
North East Transmission Company Limited limited(NETCL) is a Joint Venture Company of POWERGRID, OTPC and Six states of North Eastern region. The main objective of the company is to evacuate 726.6 MW power from Gas Based Combined Cycle Power Project (GBCCPP) of OTPC at Palatana in Tripura. NETC is operating 400kV D/C Palatana-Silchar-Bongaigaon transmission line of 662.8kM traversing through the states of Tripura, Meghalaya and Assam. One circuit of 400kV Silchar–Bongaigaon double circuit Interstate line is looped-in looped-out at Byrnihat substation in Meghalaya and another circuit is looped-in looped-out at Azara substation in Assam. The task for construction transmission line corridor was very challenging due to heavy rainfall. In addition to the above mentioned environmental & climatic challenges, there are many local & social challenges related to insurgency, theft, right of way, various approvals from government, cyclones etc. faced during the construction of river crossing towers and so on.
River Crossing Tower Constructional Challenge
Construction of River Crossing Towers: River crossing involved in NETC Transmission line, is most important link of the line. The NETC line has undergone 4 river crossings namely: Brahmaputra (son of Brahma), Champamati, Barak, Lukha river that is selected judiciously and carefully by NETC.
Construction of 2 well foundations (1023 & 1024) in Brahmaputra River: The construction of 2 Deep Circular well foundation (Caissons) on the mighty Brahmaputra river is one of the historical milestones achieved by NETC in Indian Tower Engineering. The mighty Brahmaputra river flows down from Himalaya to Bay of Bengal, with its source in Tibet. The Brahmaputra river is one of the largest rivers of India. The river crossing span is 2.56 km. The weight of the tower is 272MT and its height is 138 metre. The myriad challenges that were faced during the construction & erection of the tower have been discussed hereafter.
For well foundations, soil & geotechnical survey was carried out in a very challenging condition. NETC has adopted all the standard practices, which are required for the river crossing tower design as well as Erection & Commissioning (E&C) of tower with special T&P. River crossing tower requires solar-based Obstruction Lights as per GRID Code. The pipe type earthing is installed in the river for well foundations. The special conductor AACSR size (Al54/3.53,Steel 7/3.53) & AACSR Earth wire(16/2.86mm +19/2.48mm) were used in river crossing towers. River crossing span stringing, testing and commissioning were done successfully.
Construction of well foundations underwent the following stages:
- Laying the Well Curb & Cutting Edge.
- Alignment Control.
- Construction of Well Curb.
- Well Steining.
- Well Sinking, Bottom plug Well Cap, Fender, Wall formation, Island of 22 metre, Chauffer dam.
- Technical details of well foundation
- Pile foundations on Brahmaputra river
One pile foundation constructed on the bank of Brahmaputra with the 1500mm diameter R.C.C. vertical bored pile with approximate depth of 50 meter with Pile cap, Erection of tower and stringing and commissioning.
Pile foundation in Champabati River Crossing with the 1200 mm diameter R.C.C. vertical bored pile with 50 metre depth pile cap.
- NETC Tower in Barak River Crossing in Assam, which also has a large catchment area prone to flood and erosion with crossing span of 712 metre. The height of the tower is 122 metre & weight is 210 MT.
- NETC Tower in Lukha River crossing: Lukha River is located in the southern part of East Jaintia Hills, Meghalaya. Its main tributaries originate from the catchments of Saipung Reserve Forest. Along its route, it also receives water from Lunar River (Wah Lunar) and small streams draining from the Narpuh Reserve Forest and the undulating hills.
All the river crossing towers are constructed with all the safety precautions. All the towers are being monitored by NETC periodically from construction time and NETCL average availability is 99.95 %.
Operational challenges faced due to change of river course by Brahmaputra
Adoption of pile foundations for towers under threat due to Brahmaputra River changing its course in O&M phase: In operation stage, due to heavy rainfall & flood in Assam, one of the anchor tower location (1026) on the right bank of the river was 137 metres away from the river bank edge during construction in 2010.
Subsequently, because of continuous erosion of the bank, the distance reduced to 10 metres in the year 2016, which was 137 metres during 2010. During the year 2016, this Anchor Tower location also got submerged by Brahmaputra river water. Considering the fact that any further shifting of bank line or formation of any whirlpool in this area may cause a problem to the stability of the Anchor Tower foundation, this location was shifted to a safer location with pile foundation as per details indicated in the previous Table. Prior to shifting, to protect this location from the continuous river bank erosion, Bamboo Palisading work, Bank protection had to be provided at the river bank face as a temporary protection measure.
For smooth and accident-free work of pile foundation, one of the circuits, which was directly above the new pile foundation under construction had to be shifted by using an Emergency Restoration System (ERS) tower. Subsequently, on completion of the pile foundation the entire work of tower erection and final stringing was carried out by keeping one of the circuits continuously charged as per requirement – so as to keep the power flow through this line intact. All tower foundations have been designed with adequate factor of safety.
In addition to regular on-line update on weather condition, rain, physical patrolling of the river crossing tower continued. The patrolling through water boat and regular defect rectification were carried out by the O&M team.
Asset monitoring of River Crossing Towers
In addition to regular patrolling & defect rectification of river crossing towers, NETC has been monitoring the line through advanced Condition-based Maintenance Technique. In future NETC is exploring the idea of Installation of TLA in river crossing spans or vulnerable locations. It is one of the best options to avoid the tripping due to lightning in river crossing span. NETC is also exploring the idea for use of drones for patrolling, Artificial Intelligence (AI) & Machine Learning (ML), advance sensors technology in near future.
Conclusion
Power transmission lines constitute essential infrastructures of the the society and these are very vital elements of the power system. The transmission lines pass through varying environment. It is concluded that NETC has successfully constructed the 2 Well & 2 pile foundations (towers) on Brahmaputra River Crossing spans ,after facing myriads of the challenges. The Brahmaputra River is the largest river in the Indian sub-continent with very high velocity and 10th largest in the world in terms of discharge and known for its character of shifting its course. Various operational challenges are also faced by NETC due to change of river course. Mitigation measures are taken thereon, for example: shifting of raft foundation to pile foundation on tower Loc.1026 on Brahmaputra river.
Harshal Radheshaym Malewar is a graduate in Electrical Engineering from the Govt. College of Engineering Chandrapur, Maharashtra. He also possesses an MBA degree in Power & Operation Management. He has diversified experience of more than 14 years in the Power Transmission & Distribution with various power sectors’ companies – such as RECTPCL, MSETCL, Sunflag Steel & Power, Jyoti Structures Ltd. and North East Transmission Company Ltd.
