Setting Standards: Transformer and reactor testing

Transformer and reactor testing

Transformers and reactors are essential assets in a power delivery system. The reliability and availability of these assets plays an important role in ensuring uninterrupted power supply and smooth operation of a power system. The demand for such equipment has been further enhanced by the twin phenomenon of growth in high voltage di­rect current lines and renewable energy-based generation. Hence, it is important to maintain their health through regular testing and maintenance. This not only ensures transmission system efficiency, but also lowers transmission and distribution losses in the system. Testing the eq­uipment prior to installation helps trans­cos expedite the installation and determine the reliability of the equipment in extreme conditions. It also helps them avoid expenses due to maintenance issu­es caused by incompatible equipment.

Key recent developments

In April 2021, the Central Electricity Au­th­o­rity (CEA) outlined the standard specifications and technical parameters for transformers and reactors (66 kV and ab­ove voltage class), highlighting the idea of “one nation, one specification” in the ov­erall interest of the power system. It st­a­n­dardised technical parameters, in­cor­po­rating the best practices and state-of-the-art technology for transformers and reactors.

The notification of standards had been a key demand of the industry and utilities alike, as prior to this there were no uniform standards for design practices and technology specifications of tra­n­sfor­me­rs and reactors. The newly rele­a­sed standards take into account domestic and international requirements by incorporating best design practices, quality control and testing requirements. This is ex­pected to simplify the procurement pr­o­cess, enabling faster delivery, overall ef­fi­ciency, quality and productivity in the entire value chain of transformer procu­rement and operation. Notably, in light of the challenges faced during the Covid-19 crisis, the standards have introduced virtual inspection and testing of transformers and reactors as an alternative to the existing practice of utility representatives being physically present at a manufacturer’s workshop during inspection and testing. Further, proper condition assessment/monitoring and maintenance of a transformer/reactor needs to be ensured for long and trouble-free service.

Tests for transformers and reactors

Type tests, routine tests and special tests are performed at the manufacturing facility itself while pre-commissioning tests such as periodic/condition monitoring tests and emergency tests are carried out at the consumer’s site.

The objective of type tests is to ensure that the transformers and reactor comply with the specified standards. The various kinds of type tests for reactors include temperature rise test, measurement of harmonic content of current, acoustic sound/noise level and knee point voltage measurement of reactor. Meanwhile, type tests for transformers include lightning impulse tests for the neutral terminals, temperature rise tests, measurement of harmonic level in no load current, determination of acoustic sound level and measurement of power taken by fans and liquid pump motors. According to the CEA, the offered transformer/reactor should have been successfully type-tested within the last five years as on the last date for submission of the bid.

Routine tests are conducted to check the operational performance of individual units in a production lot. Such tests in­clude winding resistance tests, tank va­cu­um and pressure tests, tests on on-load tap changers, dielectric tests, insulation power factor tests, induced overvoltage tests with partial discharge measurement and measurement of dissolved gasses in dielectric liquids.

All routine tests are to be carried out on all units, while type tests are to be conducted on one unit. The CEA document mandates testing the transformer, reactor and auxiliary equipment as per the manufacturing quality plan provided by the purchaser. The purchaser is empowered to conduct tests on equipment as well as the raw materials used in the equipment. These specifications also empower the purchaser/consultant ap­po­in­ted by the purchaser to visit the ma­nu­facturing unit in order to inspect the de­sign and manufacturing processes. The new regulations also state that the manufacturer will be penalised if losses during routine tests are in the range of 2 per cent on the maximum specified values with a further possibility of rejection if the values exceed 2 per cent.

The procurer of the transformer and reactor is also allowed to conduct a stage in­spection during the manufacturing st­a­ge. The stage inspection should be fo­c­used on testing the precision and com­pa­ti­bility of the core, winding, core-coil as­se­m­bly and tank. The manufacturer is also man­­d­ated to provide a list of materials along with the requisite documentation ab­out the raw materials used in the production of the transformer and the reactor.

Issues and challenges

While the CEA aims to improve the quality of transformers and reactors provided by manufacturers by standardising the equipment and testing processes, these regulations may result in stifling competition amongst manufacturers for supplying transformers and reactors in the short term. However, over the medium to long term, transmission utilities will accrue massive benefits from these regulations, through the improved efficiency, quality and productivity of tested transformers and reactors, which would be reliable, robust, standardised, low maintenance and interchangeable.