Minimising Failures: Testing requirements for transformers and reactors in India

The exponential growth of India’s power transmission network has led to the creation of one of the largest single synchronous grids in the world. Going forward, significant generation capacity addition, including large-scale integration of renewable energy sources, ex­pansion of the electricity market and cross-border power exchange will nece­ssitate the expansion and reinforcement of the associated transmission and distribution network.

In such a scenario, the major assets of a power system, such as transformers and reactors, will play a crucial role not only in terms of investment but also in ensuring the availability and reliability of the system. The growing energy demand will increase the need for both transformation capacity and reactive compensation. However, transformers often ex­perience premature failure due to factors such as poor quality of raw materials, workmanship and manufacturing techniques, normal and abnormal stre­s­ses on the system during operation (su­ch as frequent system faults, overloading, unexpected continuous operating voltage and overvoltage stresses), and poor maintenance practices.

Long repair times are a significant concern in cases of transformer/reactor failure. The restoration of transformers/rea­c­­tors takes approximately three to six mo­nths after major repairs, depending on the type of repair. Similarly, procuring a new transformer/reactor can take six to ten months, depending on the voltage cl­ass. Hence, the testing of such critical eq­uipment is essential for utilities.

Transformer testing

To assess the performance of an electrical power transformer, a series of testing procedures have to be undertaken. Transformer manufacturers conduct two primary types of transformer testing – type tests and routine tests. These tests are carried out at production sites or are conducted under real-time conditions on site.

Type tests are generally conducted on any equipment that adheres to relevant national or international standards. These tests serve to validate the equipment’s design and demonstrate its ability to meet functional requirements and ensure reliable performance throu­gh­out its service life. These tests are also referred to as “proof tests” or “design validation tests”. In type testing, a variety of tests are conducted, including winding resistance tests, transformer ratio tests, transformer vector group tests, measurement of impedance voltage or short circuit impedance (principal tap), load loss (also called the short circuit test), measurement of load loss, open circuit tests, measurement of insulation resistance, dielectric tests of transformers, performance evaluation under higher temperatures, tests on on-load tap changers (OLTC) and vacuum tests on tanks and radiators.

Routine tests of transformers are carried out on every manufactured unit to assess the operational performance of individual units in a production lot. These tests are similar to type tests, with the exception of temperature tests, and they include oil pressure tests on transformers to detect any leakages past joints and gaskets.

Pre-commissioning tests, periodic condition monitoring tests and emergency tests are carried out on site. These in­clude the withstand voltage tests, indu­c­ed ov­er­voltage tests, measurement of zero sequence impedances on three-phase transformers, measurement of neutral currents, measurement of the harmonics of no-load currents, magnetic balance tests on three-phase transformers, testing for permissible flux density and over-fluxing, determination of sound level, determination of capacitances of windings-to-earth and bet­we­en windings, measurement of the dissipation factor of the insulation system capacitance, and lightning impulse tes­ts. These tests may be conducted in ad­dition to the tests that fall under the type test and routine test categories.

Reactor testing

Reactors are made of inductive materials to limit the reacting currents that ha­ve the potential to damage power tra­ns­formers during the transmission or dis­tri­bution in the substation. Various ty­p­es of reactors are used in electrical po­wer systems, such as shunt reactors, se­ries reactors, damping reactors, tuning reactors and arc suspension reactors. Of these, shunt reactors are the most commonly used. The tests for reactors are similar to those of transformers, divided bet­ween on-site and factory unit tests. Reactor testing involves multiple tests such as measurement of reactance, loss, zero-sequence reactance on three-pha­se reactors, mutual reactance on three-phase reactors, power consumption by fan and oil pumps, impedance of continuous current, inductance, Q factor, current at all adjustments, and linearity of reactance; turns ratio error tes­ts; and determination of the secondary loop time constant.

Regulatory framework

In April 2021, the Central Electricity Au­tho­rity (CEA) issued a notification revising the parameters and specifications for transformers and reactors, as well as the procedures for testing them. The revision primarily focused on changes made to the requirement and validity of the dyna­mic short circuit test. The new draft mandates that the transformer design must undergo a short circuit withstand capability test in accordance with IS 2026 Part-5 within the past five years, aligning with the requirements outlined in the CEA (Tech­nical Standards for Cons­truc­tion of Electrical Plants and Electric Lin­es) Regu­lations. The relevant test repo­rt/cer­ti­fi­cate will be enclosed along with the bid. The design review of the offered transfo­r­mer should be carried out based on the des­ign of a reference transformer that has already undergone short circuit tests, eliminating the need for repeating the short circuit tests. The manufacturers are re­quired to perform compulsory type tests.

This amendment, along with other specifications, simplifies the procurement process and reduces delivery time, ultimately facilitating the early completion of projects. Design standardisation hel­ps eliminate the need for frequent de­sign reviews and promotes the interch­angeability of various transformers and reactors. Furthermore, the standardisation of fittings and accessories reduces the requirement for inventories.

Since the dynamic short circuit tested design is currently unavailable for autotransformers in the 765 kV and 500 MVA, 400 kV voltage classes, additional time will be required for testing these transfor­mers. Therefore, the requirement of the dynamic short circuit (DSC) withstand test for these transformers will apply to projects with bid invitation dates after 31 August 2023. For these transformers, a theoretical evaluation of their ability to withstand the dynamic effects of short circuits shall be conducted.

Manufacturers can also seek assistance from NABL-certified labs to carry out the tests specified by the CEA, while they up­grade their testing equipment and establish timelines for clients. There are 12 power and distribution transformer testing labs in India that offer these testing spaces.

As per the CEA’s 2022 general guidelines on type tests, if the short circuit tests on a subject transformer are conducted in ac­cordance with relevant standards (IS/IEC), and it successfully passes the SC tests and other type tests as per relevant standards, the utility shall not reject the transformer for supply against the contract.

Additionally, there is no need for the repetition of the short circuit test on the transformer if there has been a change in the make and type of bushings and/or the make of the OLTC, provided that the bushings and OLTC of the supplied ma­ke have the same or better ratings and have undergone successful type testing as per relevant IS/IEC standards.


With India’s technological advancements and the power sector preparing itself for this transformation, testing facilities equipped with state-of-the-art equipment are expected to witness increased demand for the testing of critical equipment such as transformers and reactors, circuit breaker testing, protection testing and battery testing. This surge in demand is anticipated as both PSUs and private players in the transmission and distribution segments expand their network. Fur­ther­more, with technological advance­me­nts and thorough testing of equipme­nt like transformers and reactors, coupl­ed with the introduction of newer technologies, proper testing practices can pave the way for the establishment of a robust grid and technologically advanced power system.