Transmission & Distribution: State-Of-The Art Management Techniques to Ensure Reliability and Availability of NETC Transmission System

20 Feb 2024

SUMMARY As India is on track to become the world’s third largest economy by 2027, the power requirement has increased manifold. India being a very vast country, in order to meet such increasing demand, transmission lines are to be constructed and maintained in all types of terrain. North East Transmission Company (NETC) has constructed 663 Km of 400 kV Double Circuit Transmission System traversing through difficult hilly terrain covered with dense tropical rain forest with very high rainfall, in the States of Tripura, Meghalaya and Assam in the North Eastern Region (NER) of India and is successfully maintaining this System at an availability of 99.95% and above.

In certain areas, the line corridor passes through the habitat of wild animals, areas of high wind zones & high Isokeraunic level coupled with high soil resistivity and crosses two major rivers of this region. The soil type is predominantly lateritic, alluvial, and loamy and prone to sinking and landslides due to the steep gradient of the terrain. Such type of terrain poses many threats to the security of the Transmission System which demands various cost-effective maintenance measures combined with innovative techniques. This paper describes in detail about various types of challenges being faced and their mitigation measures adopted by NETC to ensure RELIABILITY and AVAILABILITY.

KEYWORDS Tower Footing Resistance (TFR), Random Rubble Masonry (RRM), Emergency Restoration System (ERS), Transmission Line Arrestors (TLA), Punctured Insulator Detection (PID), Normative Annual Transmission System Availability Factor (NATAF), Isokeraunic level, North Eastern Region (NER), Signature analysis, Line Signature Analysis (LSA).

1.0 INTRODUCTION - NETC’S TRANSMISSION LINE CORRIDOR NETC’s 400 KV. Transmission System is presently evacuating power from ONGC Tripura Power Company’s (OTPC) 2x 363.3 MW Palatana Gas Based Combined Cycle Power Project located in the State of Tripura to feed the North Eastern Region of India. This Transmission System is delivering power at 400 KV Silchar, Azara and Bongaigaon Sub Stations in the State of Assam and 400 KV Byrnihat Sub Station in the State of Meghalaya. The transmission line details are as below: i. 400 kV Palatana- Silchar D/c line (247. 39 km) ii. 400 kV (First circuit of D/C) Silchar- Byrnihat line (214.41 km) iii. 400 kV (Second circuit of D/C) Silchar- Azara line (256.41 km) iv. 400 kV (First circuit of D/C) Azara - Bongaigaon line (159 km) v. 400 kV (Second circuit of D/C) ByrnihatBongaigaon line (201 km

2.0 TO MAINTAIN THE TRANSMISSION LINES WITH 100% AVAILABILITY, NETC IS FACING THE FOLLOWING MAJOR CHALLENGES: i. Due to exposure of tower foundations because of landslides, rain cut or erosion of soil by the current of nearby overflowing rivulets during rainy season, the transmission lines become vulnerable. In the North Eastern Part of India, highest rainfall to the tune of about 3 to 5 times the average rainfall in India, occurs. ii. Submergence of towers in low lying areas during rainy season. iii. Tower endangerment during high wind & cyclones and changing course of rivers. iv. Transient fault & safety threat due to fast growth of vegetation including bamboo on hill sides because of the climatic condition of this region. The local tradition of forest burning by tribal inhabitants for Jhum cultivation, below the transmission lines, also causes outage of the transmission lines. v. Outages due to severe lightning in hilly terrain with high soil resistivity. vi. Rapid industrial pollution due to increasing brick kilns also results in failure of transmission line components which leads to negative impact on the life of transmission line. In addition to the above mentioned environmental & climatic challenges, there are many local & social challenges related to theft, severe Right of Way problems etc

3.0 MEASURES ADOPTED TO MITIGATE THESE CHALLENGES A) Protection of endangered Tower Foundations both Temporary and Permanent. During 2018, the State of Tripura experienced unprecedented heavy rainfall which caused very heavy landslides in the entire hilly stretch of Tripura State. Due to such landslides, 8 (Eight) tower locations of NETC’s transmission line became vulnerable. This line being the only high-capacity line for evacuation of power from the 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. Hence, immediate temporary measures were taken to protect these vulnerable towers by providing bamboo palisades in multi steps along with empty cement bags filled with earth in all these locations for providing minimum protection. Sufficient plantation was also done on the eroded sides of the locations to retain the soil. As the measures taken were temporary in nature during that difficult time, Random Rubble Masonry (RRM) walls were subsequently constructed in 8 (Eight) badly affected tower locations before the onset of next monsoon, to provide permanent protection. More plantation work was also done in these locations to increase the stability of the soil and to reduce erosion. The severity of landslides, temporary and permanent arrangement adopted in few locations is exhibited below in Figures 2A, 2B & 2C respectively. B) Stub encasement & additional anti rust painting of tower legs in water logged areas Some sections of the transmission lines remain submerged up to 7 (seven) months in a year in the low-lying areas of the State of Assam causing corrosion and rusting in the stub. For permanent eradication of this problem, the stubs of 300 nos. towers were encased by raising the chimney heights by providing RCC up to various heights (max. up to 1.5 meters) depending upon the submergence water level. Further after encasement in the towers where submergence level was beyond 1.5 meters, the tower legs were painted up to the height of 3.0 meters with silver coating fortified with Zinc & other metallic powders for prevention of rust and corrosion. In addition, such paints were also applied in certain affected towers located near industrial areas as a rust protection measure. C) 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 Fig.- 3A, 1no. pile foundation location and 2 nos. anchor tower locations on open cast foundation 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 10th largest in the world in terms of discharge and known for its character of shifting its course due to continuous erosion. One of the anchor tower locations as exhibited in Fig.- 3B on the right bank of the river was 137 meters away from the river bank edge during construction in 2010. Subsequently, because of continuous erosion of the bank this distance was reduced to 10 meters in the year 2016 necessitating shifting of this location to a safer location on a pile foundation on emergency basis. 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 as exhibited in Fig.-3C. 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 continue the power flow through this line. All tower foundations have been designed with adequate factor of safety. D) Clearance of vegetation at frequent intervals and developing environmental awareness among local people. As the entire Region falls under tropical rain forest zone, the growth of bamboo plants which cover the entire region, is observed to be quickest. Therefore, in addition to the regular patrolling, three cycles of patrolling are done in a year i.e., pre-monsoon, post-monsoon and before high-wind season. Such practices have minimized the outage of T r a n s m i s s i o n 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 Fig.- 4. On many occasions, outage of transmission lines have 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 amongst them. This has considerably reduced the outage of transmission line to a great extent. E) Adoption of chemical earthing in transmission line section passing through terrain with high isokeraunic level coupled with high soil resistivity. The Silchar- Byrnihat section of the NETC Transmission System passing through the state of Meghalaya, encounters severe lightning in the hilly terrain having high isokeraunic level and high soil resistivity [Reference 3]. Due to this, a number of outages were experienced in this section during monsoon period in spite of regular maintenance. For reduction in tripping due to lightning, conventional measures like providing additional earthing of towers with separate earth pit, providing spike on tower top to capture lightning strikes etc were adopted. However, even with above measures number of outages could not be contained. Therefore tower footing resistances (TFR) of this section were measured and these values were found high in many of the tower locations. To minimize the TFR value, sodium bentonite having high moisture characteristics & swelling properties was used as Ground Enhancement Materials as in Fig.-5. The TFR results improved significantly. as exhibited in Fig.-6. The existing counterpoise earthing trenches were excavated and a layer of about 4 cm of sodium bentonite slurry [Reference 4] was applied to the existing counterpoise trenches & the trenches refilled with excavated soil. Further as the life of marconite, another Ground Enhancement Material is comparatively higher therefore the same was also applied in a few towers on experimental basis to to minimize the high TFR value. F) Installation of Transmission Line Arrestors (TLA) in areas of high isokeraunic level with high soil resistivity to control tripping on Earthing fault in inclement weather. Though measures to reduce the tower footing resistance using chemicals like Bentonite slurry & Marconite were taken up, but transient line tripping during inclement weather could not be completely reduced. Therefore, based on experience gained from successful operation of the Transmission Line Arrestors (TLAs) in lightning prone areas for 400 kV, 220kV & 132 kV system in India, NETC has decided to adopt 400kV Transmission Line Arrestors (TLAs) in the system to eliminate the issue of outages permanently. The process for installation of TLAs in NETC System has already started. These Arresters (composed of Zinc Oxide), become conductive when exposed to high voltage and behave as an insulating medium during normal conditions thus protecting the insulators (from flashover) and conductors (from degradation) of the system from the damaging effects of lightning without any outage of the system. When lightning strike occurs at the peak of a tower, part of the lightning current flows through the ground wire to the adjacent towers and the other part of the lightning current flows into the earth through the tower. In case of high tower footing impedance, due to discharge of such high lightning current flowing through the tower, potential difference occurs across the line insulator causing back flashover resulting in outage of the transmission line. Back flashover is most prevalent when tower footing impedance is high. When tower footing impedance is high, on fixing of the TLA as shown in the Fig-7 the surge current flows through the arrester to the adjacent towers, and the current is shattered out of the phase to earth, causing grounding of the surge current and thus ensuring continuous power supply. The Connection diagram of TLA in the transmission line is shown below 

CONNECTION OF TLA IN THE TRANSMISSION LINE G) Replacement of disc insulators with polymer insulators and painting of towers with zinc rich anti corrosive paint in polluted areas. Due to deposition of industrial dust on the insulators, number of flashovers occurred in the towers of a portion of the Silchar- Byrnihat line passing through the industrial zone of Byrnihat town in Meghalaya causing frequent outages of the  line. The pollution combined with humidity was causing flow of leakage current on the surface of insulators and such current with large magnitude flowing for a long period caused degradation of the insulator performance leading to flashovers causing line outages thereby effecting the reliability of the 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 also got severely rusted and corroded causing concern for structure strength. Therefore, as a preventative measure all the towers located in this industrial zone were painted with zinc rich paint combined with anti-pollution paint with necessary primer coat. Further to above, due to the same reason in the towers located in the vicinity of brick kilns NETC replaced all the porcelain disc insulator strings with Polymer insulators. A. Drone based patrolling: Operations and maintenance (O&M) tasks are regular and repetitive in nature and performing these tasks with traditional man power consumes a lot of time. Drones or UAVs capable of reaching risky and difficult-to-reach areas can conduct the visual and thermal inspections and synchronize all the data for further analysis. Therefore, with the advancement in such technology NETC carried out drone inspection of a line section in the Elephant & wild animal corridor for 40 nos. towers and is planning to use drones in maximum line sections in the unapproachable areas, difficult terrain, and for emergency line patrolling. Due to their capability of utilizing different sensors, drones can take high resolution close photographs/videos of tower components and can detect the various defects of the line and with the help of AI powered analytics platform that transforms traditional operation and maintenance (O&M) functions by delivering realtime data-driven insights for providing actionable intelligence to reduce time and costs, can improve safety and make faster decisions in the O&M functions. The UAV is flown in a safe method in order to avoid any kind of damage to humans and the environment. B. App based patrolling: NETC is contemplating the use of smartphonebased application for routine operation and maintenance of the transmission line within a short time. Smartphones pre-loaded with the application, are to be provided to the persons-in-charge including the team members of the transmission line hubs for planning, scheduling and for regular patrolling and inspection. The application is user-friendly for the semi-skilled employees of the teams and has a predefined list of checkpoints and parameters for health assessment of a transmission line and quality patrolling. The software in the application has a consolidated list of tower names or identifiers, along with the latitude and longitude (Northing and Easting) as geographical identifiers which verify the location at the time of inspection. The check-list is pre-programmed in the form of questions and menu driven answers, eliminating the need to record observations and notes in any other form, and to generate reports, photographic evidence for any significant abnormalities, which is sent back to the central repository for an immediate review. The application covers all the relevant scenarios through the check-list before completing the site report and triggering the follow up actions before restarting the process for next monitoring or patrolling site. While the smartphone application captures and collects data in a user-friendly manner, at the head-office the data is monitored, analysed and further action taken upon any abnormal observations. C. Asset Monitoring The asset is being managed by a group of highly experienced personnel with 6(Six) Field Offices in co- ordination with the Corporate Office. As per regular on-line update on weather condition, physical patrolling of the line to clear any vegetations is done on a regular basis. However, during adverse climatic conditions, ground patrolling along with tower top patrolling in the vulnerable and most vulnerable stretches of the transmission system having failure history, is intensified further. Moreover, the practice of three cycles of patrolling is also followed in a year i.e., pre & post monsoon and before high wind season. The defects of non-shutdown nature are attended during regular patrolling and defects of shutdown nature are attended by obtaining shutdown or during opportunity shut down depending upon the severity of the defects. Safety drill is a regular feature before starting any maintenance activity. D. Condition Monitoring For condition monitoring of insulators, hardware, jumper connection etc., Punctured insulator detection (PID) and Thermovision scanning are also taken up at regular intervals. Off Line Fault Locator is used to check the health of the line whenever required. Also, Laser Range Finder is used to measure and ensure various clearances required to maintain the transmission line such as jumper drop clearance, jumper drop clearance from tower body, clearance of Earth wire/OPGW from top phase conductor at null point, ground clearance, power line/ railway line crossing clearance etc. as exhibited in Fig.-8. E. For effective disaster management, Emergency Restoration System (ERS) consisting of 10(Ten) Suspension and 6 (Six) Tension structures alongwith the required accessories is kept in readiness for immediate deployment. F. One of the twin earth wires is under replacement by Optical Ground Wire (OPGW) for better communication and better operation of the grid. G. The entire maintenance activity is suitably aided by different protection mechanisms including disturbance recorders, event loggers, phase measuring units etc. put in place in the connected power sub stations and the grid and well supported by the Grid Operators. H. NETC is exploring the use of Silicon Jackets for the suspension tower insulators in the polluted stretch of the line as Innovative Measure. I. Risk Management NETC is regularly assessing the health of its asset through its risk management mechanism. Further, to manage the life cycle of assets more effectively, to get higher return with the assets and to reduce the total cost of risk, NETC is actively considering certification of ISO 55001. 5.0 AVAILABILITY OF TRANSMISSION LINE CORRIDOR NETC has achieved availability factor of transmission corridor above 99.94 % under severe environmental conditions since last seven years. The details of year wise average availability since last five years is shown in Table 1: Sr. No. Year Average Annual Transmission Availability in % 1) 2018-19 99.97 2) 2019-20 99.86 3) 2020-21 99.95 4) 2021-22 99.93 5) 2022-23 99.98 Table 1: Average Availability Year Wise 5.0 CONCLUSION: NETC has been successful in maintaining average availability of transmission corridor by adopting various innovative and sustainable mitigation measures after identifying the constraints. To reduce the down time, NETC has started drone mapping and contemplating the use of App. Based software for Patrolling, including use of modern equipment & instruments with advance information about cyclones, earthquake & lightning etc. Use of TLA is one of the measures to reduce the lightning related tripping. Safety is treated as non-negotiable in the ethos of NETC and regular safety drills, safety workshops and Tool box talk are integrated in O&M routine. NETC views human resources as the most precious asset of the company and therefore stresses on the high importance of mandatory and advanced training programs for upgradation of skill and knowledge. BIBLIOGRAPHY 1. North Eastern Regional Power Committee (NERPC) website for Transmission line corridor in NER. 2. IEEE-80: Guide for safety in AC Substation Grounding. 3. Improvement of grounding properties by using bentonite, Proceedings: XXIX Conference CIGRE, Paris, 1982 Veledar, M.Timic, Z. Skok, S. and First, Z 4. NETC in house document – System Operating Procedure for Operation & maintenance, Internal Document (not for circulation) 5. Central Electricity Regulatory Commission (Terms and Conditions of Tariff) Regulations, 2019. 6. CBIP O & M Transmission line manual.