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A report by Christian Johnson and Thirugnanam
Transmission line towers are constructed for power evacuation purpose from generating stations to various load centers. Though they are considered to be the most stable and versatile semi-permanent structure, often they collapse due to failure of foundations, disrupting transfer of large blocks of power affecting the society to a larger extent. A brief review of literature on failure of transmission line tower foundations have been made and Case study involving data collection and visual inspection of transmission line tower foundation apart from diagnosis of transmission line tower stubs and laboratory experimentations have been presented. Based on the literature review and the case study undertaken as a part of the research, three predominant causes leading to failure of transmission line tower foundations have been discussed. Socio – economic impact of failure of transmission line tower foundations have been discussed. Remedial measures found through the research study for preventing failure of transmission line tower foundations have been briefly outlined.
Corrosion, Foundations, Failure, socio- economic, transmission line tower
In India, development of electric power over the years has been phenomenal. The installed capacity has risen from a mere 2301 MW in 1950-51 to 167,278.36 MW as on 31st October 2010. The hydro electric potential in India is estimated to be 84,000 MW at 60 % load factor. Of this about 40% is in the north – eastern region, 38 % in the northern region, 10 % in the southern region, 7% in the eastern region and 5% in the western region. The north eastern region is richly endowed with hydro electric potential. As the demand growth is not large in this region, bulk of the hydro electric potential in this region can be developed and utilized in other regions This would require development of extensive transmission system.
A Brief Review of Literature
Ramakrishna.V and Asthana.A.K (2004) have mentioned that the disposition of the primary resources for electrical power generation in India viz. coal, lignite, and hydro potential are quite uneven. This uneven distribution of generation resources adds to the transmission line tower requirement even in unfavorable and risky locations. Shah et al. (2004) have presented a case study on renovation and modernization of transmission line towers in the Gujarat state, India. Gujarat state has the longest coastal line admeasuring more than 1500 Kilometers. Many transmission lines pass through the coastal areas of south Gujarat and peninsular regions of Saurashtra and deserts of Dutch. This kind of location has a very adverse effect on the life of the transmission line. As some of these lines have lasted for more than 25 years, Gujarat State Electricity Board in the year 2001 had carried out certain renovation works in those lines. Gupta.O.P et al. (2005) have presented a case study on rectification of transmission line tower stubs of 132 kV Aligarh-Hathras transmission line. The phenomenon of excessive corrosion was observed during the last quarter of 1991. A detailed study related to corrosion and taking preventive measures was initiated with a view to prevent failure of transmission line tower and the rectification work was executed for tower no.79 and 87..The investigation revealed that the leakage current (stray current) from power conductors through insulator strings to tower exists in variable magnitude depending on voltage intensity, insulator surface contamination and atmospheric moisture. In addition, due to induction in ground wires from the three phases, resultant induced current flow through the loop formed by ground wire, the two towers at each end of the span and the ground underneath the span .Rokade et al. (2008) have presented a case study on corrosion of transmission line tower foundation of 230 kV lines from Thermal Power Station I to Thermal Power Station II, Neyveli Lignite Corporation Limited, Tamil Nadu.
The transmission activities Tamil Nadu are fully owned by a Government based department called TNEB (Tamil Nadu Electricity Board). The EHV network in Tamil Nadu has around 62,000 steel transmission line towers as on October 2010. Also Tamil Nadu has an EHV network consisting 400 kV- 250 ckm, 230 kV – 6856 ckm, 110 kV – 12765 ckm, 66 kV – 1196 ckm, totaling 21167 ckm. The field study includes collection of data from various field offices of TNEB to identify the most affected transmission line tower foundations, visual inspection of transmission towers in certain selected places to explore the causes for failure of transmission line tower foundations and conducting Half-Cell potential test at the coping portion of certain tower foundations to assess the intensity of corrosion. Two transmission stubs one from inland area (Coimbatore) and the other from coastal area (Chennai) had been removed from the field by deploying JCB and transported to HTI (Hydro Training Institute). Half-Cell potential test, Rebound hammer test, Carbonation test, and Chemical analysis of the stub concrete and soil around the transmission tower foundation have been conducted to draw some useful inferences about the distress phenomenon in the embedded portions of the tower foundations. After the tests, the stub concrete had been broken and the thickness of the stub angle was measured. Soil samples collected around the transmission line tower and the concrete collected from the transmission line tower stub coping area in certain chosen locations in the state of Tamil Nadu have been analyzed for its chemical constituents. Apart from above following experimentations were also made.
Deterioration of Transmission Line Tower Stub Concrete
From the reconnaissance survey conducted, it is identified that factors like improper selection of tower and tower foundation, poor grade of concrete, inferior quality of concrete, insufficient curing of coping / muffing concrete, wrong shape of coping, improper selection of angles, paints, improper alignments, stub levels, and improper tightening of bolts, etc., have directly or indirectly caused deterioration of tower foundations.
Corrosion of Transmission Line Tower stub angle
Owing to highly alkaline nature (pH >12), concrete possesses corrosion protective features and normally provides a non corrosive environment for embedded stub angle of transmission tower foundation. However, defects and cracks in the concrete can allow water and salts to penetrate into the concrete and leads to subsequent corrosion and weakening of the leg. Submergence of stub steel above concrete chimney for some period during rainy season in water acting as salt dissolved electrolyte, the corrosion process is aggravated particularly in the presence of chlorides, sulphates and phosphates. The resultant produces of rust and complex compounds with chloride have a larger volume than the original material. This leads to the formation of local cracks and chip-off, which allows salt to penetrate further into the affected stub where the process of corrosion will be more and more accelerated. A mechanism of pitting or crevice corrosion will initially occur in the presence of aggressive ions such as chlorides. These ions are responsible in the formation of pits on the surface, which accelerates corrosion attack. An important consequence of pitting is that the localized attack may be very severe which may lead to structural catastrophe.
Because of the corrosion phenomenon, ferrous materials are oxidized to ferrous oxide and the volume of the ferrous material increases to a larger extent and sets in strain in the cover concrete leading to formation of cracks. The cracks open, draining the rain water into chimney concrete enhancing the corrosion process resulting finally in spalling of chimney concrete. The process continues further even to the extent of eating away of transmission tower leg starting from the coping level to the bottom level.
Corrosion at the Stub Angle / Coping concrete Interface
Because of different conditions existing at the stub angle / coping concrete interface an electro chemical process sets up at the interfacial zone. As the stub angle interface is exposed to the atmosphere which has a higher concentration of oxygen with respect to the stub angle below the interface, there is a possibility of differential aeration – type of corrosion to occur like a ring of rust around the stub angle interface.The difference in moisture level and salt content at the interfacial zone also aggravates the corrosion process. The symptom of this type corrosion in the interfacial zone is observed widely in all the towers. But the intensity is higher in the coastal zones.
SOCIO – ECONOMIC IMPACT
The basic needs of every human being for survival are considered as food, clothe and shelter. This phenomenon existed for several hundreds of years, till recently. But now the trend has changed as electric power also become one of the basic needs. The electric power plays a very vital role not only in the survival of human being but also for the development of nation. National economy and growth largely depended on the availability of electric power supply. As a basic need and as a raw material, power is necessarily to be made available at competitive price and at regular, uninterrupted, uniform and steady supply. Otherwise, there will be disturbance in the productivity, which will affect the national growth and economy. Since the national growth and development has to be matched with that of the world, the power supply has to be of great concern to get an important role for our nation in the global arena. Besides the above failures in the transmission line tower components amounts to thousands of rupees only in maintenance costs apart from other related expenditures. Many of these failures are corrosion related due to exposure of the system materials to aggressive atmospheric and / or soil environments. These towers are of a variety of designs and were constructed to different specifications. In India while the oldest transmission structure still in service is more than 75 years old, the transmission line towers in service for 40 to 50 years old are experiencing deterioration on various accounts. Hence any improvement in enhancing the durability of transmission line tower foundation can’t be taken lightly. The damages in the existing transmission line tower foundations have been categorized as moderate, severe and very severe damages and remedial measures have been suggested to improve the service life of transmission line towers.
Based on the filed visit, diagnosis of stub specimens and laboratory investigations conducted as a part of the research, the following procedures can be followed to strengthen and to enhance the durability of the transmission line tower foundations.
If the cracks in the coping area are wider and if the crevice corrosion at the interface seems to be developing, and if the reduction in thickness of the stub angle is less than 20 %, and more than 10 %, the damage may be considered to be moderate and stub has to be strengthened as below.
1) The loosened carbonated concrete in the coping area should be completely removed so that the resulting surface would be conducive to achieve high bond strength with over lay concrete.
2) A simple carbonation test using spraying of phenolphthalein may be done for knowing the depth of carbonation and all the carbonated concrete should be scrapped, and the concrete surface should be cleaned by water jet. All the rust and flakes from the corroded stub angle should be removed using emery sheet.
3) Flexi bond ACSR coating (Flexi bond ACSR : water: Cement at the ratio of 1:1:3) to be applied on the stub angle from the lower most point of the groove up to the depth of 2 ft ( 0.60 m) above ground level.
4) Application of overlay concrete out of M20 grade with 20 % Cement replacement by fly Ash and Silpas super chemical admixture and smooth slanting surface should be formed to drain out the water.
5) A groove to be formed in the stub angle /concrete interface and allowed to cure and after setting, Elastober acrylic material to be poured in the groove to form a woring and allowed to set.
6) After the curing of over lay concrete and the setting of Elastober, the concrete surface to be coated with Zycosil Nano penetrant at the ratio 1:10 ( 1 Part of Zycosil and 10 parts of water) to be applied on the concrete horizontal and slanting surface by wet and wet method and at the ratio of 1: 20 (1 part of Zycosil and 20 parts of water ) in the vertical surface up to a depth of 2 ft (0.60 m) below ground level.
If the cracks are extensive in the coping or if the concrete in the stub are deteriorated to a larger extent and if the reduction in the thickness of the stub angle is more than 30 % and less than 50 %, the damages may be considered as severe.
1) All the concrete from the bottom of the corroded portion up to the muffing point should be removed and the surface of the left out stub concrete should be cleaned thoroughly by water jet.
2) All the rust and flakes from the corroded stub angle should be removed using emery sheet.
3) Inner cleats and outer cover plates equal to the thickness of the stub angle for 2 ft ( 0.60 m) length in the corroded length have to be welded by fusion welding
4) Flexi bond ACSR coating (Flexi bond ACSR : water: Cement at the ratio of 1:1:3) to be applied on the stub angle from the lower most point of the groove up to the depth of 3 ft ( 0.90 m) above ground level.
5) Concrete out of M20 grade with 20 % CRM by fly Ash and Silpas super chemical admixture should be used for forming the new stub from the bottom point of the corroded portion and smooth slanting surface should be formed to drain out the water in the muffing area.
6) Groove to be formed in the stub angle /concrete interface and allowed to cure and after setting, Elastober acrylic material to be poured in the groove to form a woring and allowed to set, if the location is dry one. If the tower leg has to stand in a water stagnated area or coastal area, the stagnated, the woring may be introduced.
7) After the curing of concrete and the setting of Elastober, the concrete surface to be coated with Nano Zycosil penetrant at the ratio 1;10 ( 1 Part of Zycosil and 10 parts of water) to be applied on the concrete horizontal and slanting surface by wet and wet method and at the ratio of 1: 20 (1 part of Zycosil and 20 parts of water ) in the vertical surface up to a depth of 2 ft (0.60 m) below ground level.
Very severe damages
If the cracks are extensive in the coping or if the concrete in the stub are deteriorated to a larger extent and if the reduction in the thickness of the stub angle is more than 50 % the damages may be considered as very severe.
1) All the concrete in the corroded portion should be completely removed so that the resulting surface would be conducive to achieve high bond strength with over lay concrete.
2) The corroded angle has to be cut removed by the gas welding after guying the tower.The rust and flakes in the remaining portion of the stub angle should be removed using emery sheet.
3) New stub angle to be welded and splice jointing be made in the corroded portion by fusion welding. Flexi bond ACSR coating (Flexi bond ACSR : water: Cement at the ratio of 1:1:3) to be applied on the stub angle from the lower most point of the groove up to the depth of 3 ft above ground level.
4) Concrete out of M20 grade with 20 % CRM by fly Ash and Silpas super chemical admixture should be used for forming the new stub from the bottom point of the corroded portion and smooth slanting surface should be formed to drain out the water in the muffing area.
5) Groove to be formed in the stub angle /concrete interface and allowed to cure and after setting, Elastober acrylic material to be poured in the groove to form a woring and allowed to set, if the location is dry one. If the tower leg has to stand in a water stagnated area or coastal area, the woring should be formed out of Demech deep grout.
6) After the curing of concrete and the setting of Demech grout , the concrete surface to be coated with Nano Zycosil penetrant at the ratio 1;10 ( 1 Part of Zycosil and 10 parts of water) to be applied on the concrete horizontal and slanting surface by wet and wet method and at the ratio of 1: 20 (1 part of Zycosil and 20 parts of water ) in the vertical surface up to a depth of 2 ft ( 0.90 m) below ground level.
1.Ramakrishna.V and Asthana .A.K (2004), “Towards National Power Transmission Grid in India”, Seminar on “Innovative Techniques for Design, Construction, Maintenance and Renovation of Transmission Lines”, 5-6 February, 2004 New Delhi. pp. I/73 – I /84.
2. Gupta.O.P, Singh.V.B. and Surendra Narain, 2005, “Corrosion of Transmission Line Tower Stubs” Proceedings of Short Term Training Programme on ‘Design, Construction, Operation and Maintenance of Transmission Lines for Civil Engineers’, Organized by Engineering Staff College of India, Hyderabad – Vol-II, 14-25 November, pp.4.1- 4.5
3. Shah .J.B. et al, (2004), “Strategical Renovation and Modernization of EHV Transmission Lines in the State of Gujarat,” Seminar on “Innovative Techniques for Design, Construction, Maintenance and Renovation of Transmission Lines”, 5-6 February, 2004 New Delhi. Pp. 11-23.
4. Rokade.R.P., et.al, 2008 , “Development of Methodology for Refurbishment of Transmission Line Tower Foundation” Journal of Structural Engineering, Vol.35, No.1, April- May, pp.66-72
This research paper by Christian Johnson (Senior Manager at the Hydro Training Institute of Tamil Nadu Electricity Board, Tamil Nadu, India presently doing research at IRTT, Anna University of Technology, Coimbatore) and Thirugnanam (Professor and Head of the Department of Civil Engineering at Institute of Road Transport and Technology (IRTT), Erode) will be of great help to engineers looking for the impact of failure of transmission line tower foundation. We are thankful to Er. Christian Johnson for publishing his paper here on engineeringcivil.com