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.