Meeting the increasing power demand requires expansion and strengthening of the power transmission and distribution network. As of October 2021, the total transmission line length stood at 450,552 ckt. km (220 kV and above), having grown at a CAGR of nearly 5 per cent between 2016-17 and 2019-20. The demand for transmission towers is growing at a similar pace, in turn creating demand for more tower testing stations.
It is commonly observed that the installation of transmission towers is delayed due to causes such as right-of-way (RoW), and land acquisition issues, internal issues such as contractors’ unreliability and faultiness, and substandard component quality. These problems force the installing agency to rework the design or method of installation and specifications of transmission towers. Hence, testing the towers in advance before their on-field deployment helps in controlling and preventing the occurrence of residual asset-specific risk. This risk can be managed by rigourously testing these transmission towers prior to their on-field deployment. This helps to thoroughly evaluate the strength and functionality of these towers in advance and expedites their deployment by eliminating the need to rework or reinstall them.
In addition, testing transmission towers is crucial to determine their reliability in adverse weather conditions and ensure public safety and compliance with various standards/regulations. Several tower manufacturers and accredited government labs have set up dedicated transmission tower testing facilities to conduct tests for commercial, and research and development purposes. Some of the common testing techniques are full-scale prototype testing for towers and tower components while simulating various variables and conditions, real-time strain gage monitoring of member loads and reliability tests.
Testing methods and transmission tower testing stations
Tower testing is performed for several reasons. In a traditional proof test where design conditions are verified, only static loads are applied, and the restraints at the load points are the same as the design model. If a proof test is ordered, it should be done on a full-size prototype structure or another tower of similar design, before it is fabricated in quantity. This kind of test will verify the adequacy of towers and their connections to withstand design loads specified for that structure. Proof tests provide information on support behaviour under load, fit-up verification, actions on the structure in deflected positions, adequacy of connections, and other benefits. The test cannot completely confirm how the tower will react under dynamic loads, and whether the foundations are inadequate.
A testing station enables design and validation of innovative configurations without any limitations on size, shape and magnitude of loading, which is currently not possible in the industry due to restriction of available testing facilities. Timely validation of tower design will help in conforming to foundation design and procurement of steel, thereby reducing the project gestation period greatly.
A testing station consists of components such as the test bed, remote control loading mechanisms, precision measurement instruments, a control room, and a tower crane for vertical erection of towers. Test beds are areas where towers are erected and tested. Tower test beds are categorised on the basis of their loading capacities. It has a maximum uplift capacity of 1,250 tonnes and maximum overturning movement (torsional movement) of 70,000 turns per metre.
Precision measurement instruments are used to test the transmission tower’s capacity to withstand bending of tower and torsional movements and shears. These instruments consist of transverse masts, vertical masts and longitudinal masts to test the load-bearing capacity of transmission towers. These auxiliary structures are classified by their height and generally most of these structures are of 90 metres as far as vertical, longitudinal and transverse masts are concerned.
Tower cranes are used to erect transmission towers as well as auxiliary structures. These cranes have a height of 100 metres in general so that they can load masts, which have a height of 90 metres. They can lift up to 16 tonnes and have different loading capacities. They can add up to 52 auxiliary load points in the transverse and longitudinal directions and around 60 auxiliary load points in the vertical direction.
The remote control loading mechanisms utilise electrically operated winches controlled by SCADA software and ot-her load measuring devices. These components aid in measuring the effect of stress tests in real time. From the control room, a maximum load of 125 tonnes can be remotely exerted at 52 points of the tower. These points can stress test the transmission tower’s reliability in transverse, longitudinal and vertical directions. The control room also includes a viewer’s gallery. The engineers test a tower and its consistency by measuring its display load, that is, the practical load-bearing capacity as well as its design load, that is, the maximum potential load-bearing capacity of the tower.
Precision measuring instruments play a vital role in collecting data on the transmission tower’s data collection capacities. Therefore, these instruments are themselves tested and calibrated with utmost precision prior to the testing of the transmission tower to ensure maximum accuracy. The calibration is done against an externally calibrated universal testing machine.
Test facilities in India
There are less than a dozen tower testing stations in India and hence tower manufacturing companies with forward integrated presence in both transmission tower erection and transmission tower testing are more capable of timely realisation of their transmission deliverables than their counterparts.
Notably, the Central Power Research Institute set up the first transmission tower testing station in 1976. This station is still open for commercial tests as well as R&D tests.
Among the initiatives undertaken by private players, Larsen & Toubro established one of the largest transmission line research and testing stations in 2009, at a cost of Rs 250 million, with imposing steel loading structures for three directional load applications and an expansive concrete test pad. It also includes an Advanced Human Machine Interface, which will help in ensuring reliability of the transmission network by validating design through full-scale proto testing of towers up to 1,200 kV and 95 metres in height.
KEC International is another private major which has developed significant tower testing stations. It has four such stations – three in India (Maharashtra, Rajasthan and Madhya Pradesh), and one in Brazil. They are capable of testing all types of towers – lattice towers, guyed towers, tubular and monopoles across the 400 kV, 765 kV, 800 kV and 1,200 kV levels. Also, Kalpataru Power Transmission Limited has developed a tower testing and R&D station, equipped with sophisticated and state-of-the-art equipment. It has the capacity of testing double circuit towers up to 800 kV and single circuit towers up to 1,200 kV, with 30×30 metres base width and 85 metres height. In March 2020, Skipper Limited commissioned a transmission testing station in West Bengal.
The facility has an annual testing capacity of 1,800 mt of transmission and distribution towers and poles. This testing station and R&D centre has been recognised by the Department of Scientific and Industrial Research, Government of India.
The way forward
Overall, transmission tower tests gauge quality, endurance and the fit-up method of the tower, allowing transmission companies to avoid or minimise RoW and land acquisition issues. In addition, tower testing stations help transcos test prototypes and towers of different specifications, aiding them in developing economically viable transmission towers in conformance to the specific technical requirements of the tender.
Therefore, demand for transmission tower testing, which greatly reduces cost and time overruns, is set to grow in keeping with the rapid growth in the transmission sector and its ever-increasing need for technical precision and qualitative superiority. This outlook is further improved when we account for the fact that in India there are relatively few testing stations in comparison to actual tower installations. In the medium term, the number of testing stations are expected to grow rapidly.