High Voltage Transmission Towers-Structural Issues

India has a vast power transmission network. According to one estimate India has more than 450,000 circuit kilometers of transmission lines at high voltage levels (220 kV and above). High voltage transmission towers support high-voltage power lines that carry electricity over long distances. These towers are necessary because they help to keep power lines from sagging or touching the ground, which prevents outages and keeps the electricity flowing smoothly. They enable the transmission of bulk electric power from generating stations to electrical substations, which is then distributed to electric customers. 

The structural system of high voltage transmission towers depends on the type of transmission line, the voltage level, the terrain, and the environmental conditions. Some of the common types of transmission towers are:

• Suspension tower: This is the most common type of tower that supports a straight section of the line. It has a simple structure with two cross arms that hold the conductors

• Tension tower: This is a stronger type of tower that supports a section of the line where there is a change in direction or elevation. It has a longer cross arm and a heavier base to withstand the tension forces

• Transposition tower: This is a special type of tower that swaps the positions of the conductors to balance the electrical parameters of the line. It has a complex structure with multiple cross arms and insulators

• Terminal tower: This is the last type of tower that connects the transmission line to the substation. It has a rigid structure with a single cross arm and a large insulator string.

  The transmission towers are usually made of structural steel and can be either lattice or tubular. The lattice towers are more common and have a triangular shape with diagonal bracing. The tubular towers are more modern and have a cylindrical shape with smooth surface. The transmission towers are supported on foundations that are designed to resist the loads from the tower, the conductors, the wind, and the soil. The foundations can be either shallow or deep, depending on the soil conditions and the tower height. The foundations are usually made of concrete and reinforced with steel bars.

Corrosion & Bent member

The failures of high voltage transmission towers can be caused by several factors, such as:

• Design & Detailing: Incorrect load estimation, Inadequate foundation design in the absence of reliable soil investigations etc.

• Construction quality: Poor site conditions, improper installation process, inadequate testing and inspection can lead to defects in the tower structure or the cable insulation.

• Environmental impacts: Wind-induced vibration, ice shedding, lightning strikes, corrosion, earthquakes, landslides, floods, and fires can damage the tower structure or the cable insulation

• Operational faults: Overloads, short circuits, switching surges, harmonics, ground faults and insulation breakdown can cause excessive stress or heating in the tower structure or the cable insulation

• Theft of members

  
  

These factors can result in structural failure, conductor failure, insulation failure or tower collapse, which can affect the reliability and safety of the transmission system.

There are several Indian codes that are used in the design and construction of high voltage transmission towers. Some of them are-

• IS:875, IS:800, IS:4091

• Manual on transmission line towers - A guide for the design, fabrication, testing, inspection, and erection of steel towers

• Draft standard technical specification for steel pole structures - A draft document for the design, fabrication, testing and erection of steel pole type transmission towers

• Compendium of tested tower designs for EHV transmission lines - A compilation of the tested tower designs for various voltage levels and configurations by the Central Electricity Authority.

  
  

  

  Unplugged Holes

Altered wind patterns across the Indian sub-continent, which the scientific community, too, has not sufficiently forewarned of, is wreaking havoc on the country’s power, transmission sector, with the towers collapsing at a much faster rate in recent years. While over 90% of tower collapses are believed to be due to high-speed wind, as many as 473 towers failed between 2012 and 2016, compared with 142 in the previous five-year period. Few years back parliament was informed that as many as fifty-two electricity transmission towers collapsed between October 2016 and March 2018 and high-velocity wind was behind the failure of 39 of these towers, which came apart within five years of commissioning. While most of the collapsed towers were owned by the state-run companies, several structures built by private utilities also failed. Of the 615 towers that collapsed across the country since 2007, 263 fell under one specific wind zone (Zone 4).

Missing Bracing

It is quite understandable that upgrading all existing towers based on the probabilities of high-wind events warrants significant due diligence which is a herculean task.  The cost of transmission towers ranges between Rs 10 lakh and Rs 70 lakh. On top of that, laying new lines would cost around Rs 1.5 crore-Rs 3.5 crore per kilometers. Replacing parts of existing towers to strengthen them also warrants long shutdowns of critical power systems. On the face of it, it really looks a challenging task. Transmission lines and towers are required to be designed and constructed according to the specifications (IS-802 and IS-875) prescribed by the Bureau of Indian Standards (BIS). The BIS standards vary across the six different wind zones in the country. Some recent studies conducted by the Structural Engineering Research Centre under the Council of Scientific and Industrial Research have confirmed that climate change has caused wind intensity to change in some parts of the country which has led to the increasing number of towers failures. The problem is complicated because localized high-intensity wind events last for very short durations and are contained in small areas, making it difficult for meteorological equipment to record actual wind speed data for the days when the towers failed. Also, wind movements occurring within ten meters of the general ground level—where wind gusts gain more speed—is nearly impossible to record. According to many experts working in transmission utilities the probability of such occurrences (high-speed wind events) is low, and the tower design will be uneconomical if such situation is considered in the design. But it is very strange and unscientific argument as engineers are supposed to design and construct towers and other structures as per the latest, codes, standards, guidelines, and sound engineering practices. Further it has been noticed and argued by many who engage in such operations that soil investigation at every tower location is not feasible and hence most of the time they adopt the standardized design based on visual data. Soil investigation is done in some critical cases only. Which is again very unscientific way as it might lead to either unsafe foundation or overly provided foundation. In brief it can be concluded that as per current practices and available failure data there is very high probability that design & construction faults are one of the major causes of failure of the transmission towers in high wind zones.

Another major neglected area contributing to the collapse/failures of transmission towers or reduction in service life is poor auditing mechanism & maintenance. This process also becomes difficult as most lines are in difficult terrains or remote areas. As per the pilot audit conducted few years back in some of the transmission lines across India major faults/deficiencies found were as under:

• Missing members (might be theft)

• Sagging/lateral buckling of members 

• Missing bolts & Unplugged holes 

• loose nut & bolts

• Corrosion in members 

• Galvanized coating not up to the mark 

• Top of pedestal/chimney & bottom portion of steel members covered with soil, weeds & grass. In some cases, submerged in wet soil of agricultural field.

• Verticality & Differential settlement issues 

• Joints very poorly made e.g., diagonal member connecting horizontal member much away from joint. 

• Members having excessive holes in a single piece. 

• Inadequate quality of concrete in chimney 

• Non availability of design/as built drawings of tower or foundations 

  
  
  

  

  

Such deficiencies are very serious in nature and have potential of failures or major disruption in services if not detected and rectified in time. Following steps must be taken by the custodians of such utilities:

• All Transmission Towers must be designed & constructed as per latest BIS standards. In case BIS standards are not updated but latest /advanced technological knowledge is available with BIS, SERC, CPRI or with any credible organization/department/institution we must adopt it in the  design & construction of such facilities. Just to highlight new wind zoning map was published by BIS in NBC in the year 2006. There is urgent

  need review the design of all towers based on new wind zones & initiate strengthening of such towers if needed.

• Transmission Tower foundations need to be designed as per provisions of IS: 456‐2000. Using IS: 5613 alone may not be sufficient at least for concrete & foundation specifications. 

•  Vetting of Tower & foundation designs by reputed institutions or qualified & competent consultants must be made mandatory before awarding the work. Ensuring correct design is first step towards safety. 

• Chimney top must be at least 600 mm above the existing ground level to prevent tower members from getting corroded. At present it is 200 mm which highly inadequate. 

• Regular patrolling of TL at predefined interval @ 4‐6 months depending upon regions & damage perception must be carried out by all utilities. They need to use latest methods & remote sensing; GPS based technologies for this purpose. 

• Transmission lines may be categorized in 2‐3 categories based on their importance Third Party comprehensive Audit must be compulsorily done @ 5 years & all TL which are older than 10 years & others if located at critical locations like highway/railway crossings, river/canal crossings, near inhabited areas & in severe wind zones must be audited compulsorily by third 

  party @3 years. This is over & above regular O& M as mentioned in para above.

  
  

  

  Polluting Chimney near Tower

The O&M and Third-party audit shall at least cover the following: 

1. Visual inspection [with high resolution binoculars] of towers regarding missing members, sagging, lateral buckling, bending, twisting & correctness of joints of members. 

2. Missing bolts, extra & unplugged holes. 

3. Height of chimney, weeds, grass, soil & water submergence. 

4. Quality of concrete of chimney/foundation [ RH/Core/UPV tests may be required at some locations] 

5. Dimensional accuracy of Tower like base width, diagonal length etc. 

6. Checking of member sizes with respect to design drawings. 

7. Corrosion of members 

8. Galvanized coating 

9. Differential settlement 

10. X & Y direction verticality of towers 

11. Quality of material used & workmanship during any repair rectification. Usually, such works are

    often unsupervised. Other observations regarding stability of slopes, change in river course,

    and any construction in the vicinity of tower, exposure of towers to aggressive environment etcshould be recorded and reported accurately. 

    
    

All stakeholders of power sector need to take such steps on priority basis. Since this sector is very crucial for our economy. Transmission network enables the efficient transfer of electricity across vast distances, connecting power generation sources with consumption centers. This infrastructure supports the industrial sector by providing reliable power supply, which is essential for manufacturing and production. Additionally, high voltage transmission networks help in balancing supply and demand, reducing power losses, and improving grid stability. By facilitating the integration of renewable energy sources, such as solar and wind, these networks also contribute to India's sustainable energy goals, driving economic progress and enhancing the quality of life for millions.

[MANOJ MITTAL, NOIDA, December 3, 2024]

NOTE: This writeup is based on author's firsthand experience in auditing of towers, field visits and involvement in policy matters. Many facts are based on internet sources. There might be some variations in them, but it will not change the points raised in the writeup. Views expressed are of author only and they do not reflect the views of any organization. Purpose of this article is educative and creating awareness about safety issues.

© This blog post is the intellectual property of MANOJ MITTAL. Unauthorized use or reproduction is prohibited.