Transmission tower testing: Focus on technical accuracy and quality excellence

The transmission line length in India grew at a compound annual growth rate of around 4.2 per cent between 2016-17 and 2022-23 to stand at 472,345 ckt. km (220 kV and above) as of May 2022, resulting in increased demand for transmission infrastructure and equipm­e­nt including towers. The transmission segment is highly dependent on the he­al­th of transmission towers, which can be cer­tified by tower design testing. Tra­n­s­mi­­ssion tower tests assess the tower’s qu­a­lity, longevity and installation pro­ced­u­re, helping transmission businesses mitigate right of way and land acquisiti­on challe­n­ges. Further­more, tower testing st­ations assist transcos in testing prototy­p­es and towers of various specifi­catio­ns, allowing them to construct econo­mi­cally feasible transmission towers that me­et the tender’s specific technical criteria.

Tower testing methods

To test a tower design for durability and strength, a dedicated tower testing station (TTS) is created. A typical TTS should consist of a test bed to withstand bending of tower and torsional moments and shears; permanent anchors to take transverse, longitudinal and vertical pulls applied on test towers; arrangements for applying any combinations of load tests at various intensities; and load and defection measuring devices and apparatus, among other things. The test set-up is made to conform to design specifications and verify the adequacy of the main components of the structure and their connections to withstand the static design loads specified for that particular structure as an individual entity under controlled conditions.

A tower is tested for a number of conditions, which are usually six to eight individual cases of scenarios such as high speed wind and ice. The towers to be tested are erected on a rigid foundation and then wire ropes are attached to the loading point required. The loading conditions may be applied by either “dead” weights using scale pans, winches or hy­d­raulic rams. The control equipment ga­uges the electronic equipment’s capability of taking all loads, with constant data recording facilities.

For new designs, proof tests are often ordered. In a typical proof test, the test bed is set up to test for design conditions, and hence, only static loads are applied. A proof test is conducted on a full-size prototype structure or another tower of similar design. This test can verify the ca­pacity of members and their connections to withstand design loads specified for the structure, under controlled conditions. Proof tests record information on su­pport behaviour under load, fit-up verification, actions on the structure in deflected positions and adequacy of connections. The test provides a close app­roximation as to how a tower with certain design will react under dynamic loads. It also shows whether the foundation str­ength is adequate or not. Usually, prototype towers are tested to the extent of destruction in order to assess their durability in the face of extreme events.

Tower testing in India

In India, the Central Power Research Ins­titute (CPRI) provides tower testing facilities to public sector utilities. The CPRI established a TTS in 1976, which was ac­credited by the National Accre­ditation Board for Testing and Calibration Labo­ratories, Government of India. This stati­on is open for commercial tests as well as for research and development (R&D)-ori­ented tests and more than 900 towers had been successfully tested within a span of 40 years as per Indian and international standards. Along with the head office located at Bengaluru, the CPRI has seven state-of-the-art testing facilities, at Bhopal, Hyderabad, Nagpur, Noida, Kolkata and Guwahati. The test bed has permanent footings to withstand bending, torsional moments and shears. The permanent anchor structures are of adequate capacity to take transverse, longitudinal and vertical loads to be applied on to the test towers. Many private players that are engaged in tower designing and transmission and distribution in the engineering procurement and construction space have state-of-the-art TTSs and R&D centres near their production facilities. These facilities are capable of testing up to 800 kV of double circuit towers and 1,200 kV of single circuit towers, with 30×30 metres of base width and 85 me­tres of height, and are able to test for voltages ranging from 220 kV to 1,200 kV.

Future outlook

With the digitalisation of transmission lines and substations, a TTS needs to create scope for testing digital communication simultaneously as to how fast these systems can transmit data to control centres and how fast the system reacts to real-time issues in the line. Many private players have begun offering distributed test manager (DTM) testing with tower testing. The DTM is a Windows application that can simulate supervisory control and data acquisition communications in the line and perform automated tests to confirm system or individual device behaviour. Conducting network load testing, which can include testing a single device or an entire system, can be challenging to set up in a lab.

Going forward, with the expansion of the transmission infrastructure to support the increasing renewable energy capacity, the demand for transmission tower testing is expected to continue growing. Besides, there will be greater focus on technical accuracy and quality excellence in tower testing.