In India power utilities are under tremendous pressure to meet the growing demand for power as the country is on a firm footing on the development trajectory to become one of the largest economies of the world. To meet the increasing energy demand coupled with the fact that the load centers being far away from generating stations, transmission lines are to be constructed and maintained in all types of terrain. The task of maintaining these transmission lines becomes very arduous and difficult, if the terrain is hilly with steep gradients covered by dense tropical rainforest bordering river valleys. Such type of terrain poses many threats to the security of the Transmission system, which entails various cost-effective and innovative techniques of maintenance protocol.
North East Transmission Company (NETC) is entrusted with maintenance of its 662.8 Km, 400 kV Double Circuit transmission corridor through similar difficult hilly & dense forest terrain of the States of Tripura, Meghalaya and Assam in the North Eastern Region (NER) of India. Some of the forest terrains are also inhabited by wild animals mainly Elephants & Leopards. The transmission line crosses two major rivers namely Brahmaputra and 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. Similarly, the Barak river also causes flood and erosion. The terrain consists of various types of soil viz., laterite, alluvial, loamy etc., and is prone to sinking and landslides due to steep gradient and soil characteristics. The transmission lines traverses through terrain with the high rainfall, wind zones of V & VI and high Isokeraunic level. Increasing threat from insurgency adds up to the multiplicity of problems.
This article strives to describe in detail various types of challenges being faced and their mitigation measures adopted by NETC.
NETC’s transmission line corridor
NETC’s transmission line corridor is evacuating almost one third of the power demand of the North Eastern Region of India along with exporting power to a neighboring country. A map showing transmission line corridor in NER [Reference 1] is exhibited in Fig-1.
This 400 kV transmission line corridor has been envisaged to evacuate power from ONGC Tripura Power Company’s (OTPC) Palatana Gas Based Combined Cycle Power Project of 2x 363.3 MW in Tripura state. NETC has constructed 662.8 km of 400 kV Palatana- Silchar- Bongaigaon Double circuit line as detailed below
- 400 kV Palatana- Silchar D/c line (247. 39 km)
- 400 kV (First circuit of D/C) Silchar- Byrnihat line (214.41 km)
- 400 kV (Second circuit of D/C) Silchar- Azara line (256.41 km)
- 400 kV (First circuit of D/C) Azara – Bongaigaon line (159 km)
- 400 kV (Second circuit of D/C) Byrnihat-Bongaigaon line (201 km)
Challenges
The task of maintaining the transmission line corridor with 100% availability is a prime concern. NETC is facing the following major challenges:
Vulnerability of line due to exposure of tower foundations because of land slide & rains. The transmission line is traversing through Assam and Meghalaya having highest rainfall to the tune of about 3 to 5 times the average rainfall in India. The average annual rainfall in India is 300- 650 millimetres.
- Submergence of towers in low lying areas
- Tower endangerment during high wind & cyclones and changing course of rivers
- Outages due to severe lightning in hilly terrains with high soil resistivity
- Transient fault & safety threat due to fast growth of vegetation and bamboo trees on hillsidesdue to the climatic condition of NER, and local tradition of subsequent burning of these under transmission lines
- Impact on the life of transmission line and outages due to rapid industrial pollution.
In addition to the above mentioned environmental & climatic challenges, there are many local & social challenges related to insurgency, theft, Right of Way etc.
Measures adopted to mitigate these challenges
Temporary and permanent wall protection
In the year 2018, the State of Tripura experienced unprecedented heavy monsoon with incessant rains in the month of June. The intensity of rain was so severe that within a time span of 7 – 10 days the free flowing rain water caused very heavy landslides in the entire hilly stretch of Tripura State resulting in number of road blockades in the Inter-State National Highway including some arterial roads.
Due to the above landslide, 15 tower locations were badly affected and 8 tower locations became vulnerable. This line being the only high-capacity line for evacuation of power from a 726.6 MW Generation Plant, any break down like tower collapse etc., would have caused outage of the plant resulting in severe power crisis in the North Eastern Region. This emphasized immediate temporary measure to protect these vulnerable towers by providing bamboo palisade in multi steps along with empty cement bags filled with earth were taken in all these locations for providing minimum protection. Sufficient plantation was also done in the eroded sides of the locations to retain the soil.
However, as the measures taken were temporary in nature during that difficult time, Random Rubble Masonry (RRM) walls were subsequently constructed in 8 badly affected towers’ locations before the onset of next monsoon to provide permanent protection. More plantation work is also being continued in these locations to increase the stability of the soil and to reduce erosion.
Fig: 3A to C: The severity of landslides, temporary and permanent arrangement adopted in few locations…
Stub encasement & additional anti rust painting of tower legs in water logged areas
Some sections of the transmission lines (i.e. around 300 towers) remain submerged up to 7 months in a year in the low lying area in the State of Assam causing corrosion and rusting in the stub. For permanent eradication of this problem, the stubs of these towers were encased by raising the chimney heights by providing RCC up to various heights (max. up to 1.5 metres) depending upon the submergence water level of the towers. Further after encasement, in the towers where submergence level was beyond 1.5 metres, tower legs were painted up to the height of 3.0 metres with silverish coating fortified with Zinc & other metallic powder (MOSIL ZS – 44 & MOSIL RCH – 100) for prevention of rust and corrosion.
Adoption of pile foundations for towers under threat due to river changing its course
The Byrnihat / Azara – Bongaigaon section of the line is crossing the Brahmaputra River in the state of Assam. The crossing involves 2 nos. well foundation locations as exhibited in Figure 4A, 1 no. pile foundation location and 2 nos. anchor tower locations on open cast foundation having depth of 3 metres with total crossing distance of 2.564 km. The Brahmaputra River is the largest river in the Indian sub-continent with very high velocity and the 10th largest in the world in terms of discharge and known for its character of shifting its course.
One of the anchor tower locations 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 as exhibited in Figure 4B. During the year 2016, this Anchor Tower location also got submerged under the 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 below in Table 2. Prior to shifting, to protect this location from the continuous river bank erosion, Bamboo Palisading work had to be provided at the river bank face as a temporary protection measure as exhibited in Figure 4C.
For smooth and accident-free work of pile foundation, one of the circuits that was directly above the new pile foundation under construction had to be shifted by using an Emergency Restoration System (ERS) tower as exhibited in Figure 4D. 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.
Adoption of chemical earthing along with provision of Transmission Line Arrestors (TLA) in high isokeraunic level coupled with high soil resistivity
The Silchar- Byrnihat section in the state of Meghalaya, encounters severe lightning in the hilly terrain having high soil resistivity [Reference 3]. Due to which number of outages were experienced in this section during monsoon period in spite of regular maintenance of the section. The main cause of tripping was attributed to the lightning during and on set of monsoons. On measurement of Tower Footing Resistance (TFR) of the section, value of TFR was found high in 129 tower locations.
To minimize the TFR value, sodium bentonite having high moisture characteristics & swelling properties was used as Ground Enhancement Materials. The TFR results improved significantly as exhibited in Figure 5. The existing counterpoise earthing trenches were excavated, a layer of about 4 cm of sodium bentonite slurry [Reference 4] applied to the existing counterpoise trenches & the trenches refilled with excavated soil.
As the life of marconite, another Ground Enhancement Material is comparatively higher, NETC is in process to use it on experimental basis. Further, based on experience gained from successful operation of the Transmission Line Arrestors (TLAs) in lightning prone areas for 220kV & 132 kV system in India, use of 400 kV TLAs is considered to be the best option to eliminate the issue of outages permanently. NETC is exploring adoption of 400kV Transmission Line Arrestors (TLA) in near future though these are costlier and its successful operational experience are yet to be established in Indian grid.
Clearance of vegetation at frequent intervals and developing environmental awareness among local people
The entire North Eastern part of India falls under tropical rain forest zone and the trees and plants grow very fast in these areas. The quickest such growth is observed with Bamboo, which covers the entire region. Therefore, along with a schedule of ground patrolling with three cycles in a year i.e., pre-monsoon, post-monsoon and before high-wind season being followed for clearing the vegetation, the line sections traversing through the forest areas are being patrolled regularly. Such practices have minimized the outage of Transmission Lines due to vegetation faults.
In NER, the tribal forest dwellers follow the age-old practice of burning the forest cover for cultivation purpose known as Jhum Cultivation as exhibited in Figure 6. On many occasions, outage of transmission lines had been observed due to such burning. Therefore, to stop such practices the forest dwellers have been educated in collaboration with the local administration, to develop environmental awareness among them. This has considerably reduced the outage of transmission line to a great extent.
Replacement of disc insulators with polymer insulators and painting of towers with zinc rich anti-corrosive paint in polluted area.
Some towers in Silchar- Byrnihat section are located in the industrial zone of Byrnihat town in Meghalaya, causing flash over due to deposition of industrial dust and subsequent outage of the line. The pollution combined with humidity reduces the surface resistance of porcelain disc insulators, which lead to flow of leakage current on the surface. Leakage current with large magnitude flowing on the surface for long period causes degradation of the insulator performance leading to flashovers ultimately thereby effecting the reliability of power supply system.
NETC replaced all porcelain disc insulator strings with polymer insulators due to the fact that hydrophobicity properties of polymer insulators are best suited in polluted environment.
Due to such heavy pollution in the area, the tower members including the leg members also get severely rusted and corroded causing concern for structure strength. Therefore, Zinc rich paint and anti-pollution paint with necessary primer coat have been provided as preventative measures.
Asset monitoring
In addition to regular on-line update on weather condition, physical patrolling of the line to clear any bamboos & vegetation, three cycles of patrolling are done in a year i.e., pre & post monsoon and before high wind season [Reference 5]. Due to climatic and geographical condition of the area and data analysis done so far, NETC has its priorities on the asset, as per following:
- Foundations- In lands slide area, river bank foundations, low lying areas
- Towers – In theft prone area
- Insulators – In lightning prone area / polluted / industrial / mining area
- Hardwares – In lightning prone area/ high wind / normal
- Conductors – In river crossings spans/ other areas
- Earthwire – In river crossings/ lightning prone areas/ high
- Earthing of towers- In lightning prone areas
Due to above, NETC has strengthened the foundations, provided missing members in towers, replaced porcelain disc & composite insulators with polymer insulators, replaced damaged clamps & jumpers, replaced broken strand conductors & earth wire and strengthened earthing conductors. For condition monitoring of insulators, hardware, jumper connection etc., Punctured Insulator Detection (PID), thermo vision scanning and night patrolling is also taken up. To identify & rectify the fault as per indication of fault locators, tower top emergency patrolling is done on regular basis. The identified vulnerable location, leaning trees on uphill side, land slide areas, mining & blasting prone areas are monitored through monthly inspection. For on line monitoring, data analysis and inspection, NETC is exploring use of drone in near future.
…To be continued
Satyajit Ganguly (Main Author) is the Managing Director of NETC (North East Transmission Company Ltd.).
Co- authors: Ashwani Kumar Srivastava, D. K. Sarma, Robin Kumar Sarkar and Harshal Radheshyam Malewar; are also from NETC.
