Climatic, environmental and fire behaviour class verification on dry-type transformers; KEMA laboratories testing

Transformers belong to the crucial components in distribution and transmission networks. Reliable use for many years needs to be guaranteed. Besides quality assurance of the manufacturer type testing is the most common way to prove this reliability under normal and extreme conditions. In addition, for dry-type transformers, special tests to prove climatic (C), environmental (E) and fire behaviour (F) class are defined in the international standard IEC 60076-11. End users, e.g. utilities and wind turbine manufacturers, are demanding proof of the claimed qualities of dry-type transformers increasingly. The independent KEMA laboratories, owned by DNV-GL, gained large testing experience for C and E class verification. In 2013 also verification for F class was added to KEMA laboratories portfolio, where now experience has been built-up. For C class verification, a thermal shock test is to be performed. The most common failure is the appearance of cracks, found during visual inspection after the test. The most important part for E2 class verification is the performance of a condensation test, where the transformer is energized while salt condensation is to be present on the surfaces of the object. Failures are flashovers causing voltage breakdowns and serious tracking degrading the transformers dielectric qualities. As part of the F class verification, a complete assembled phase of a transformer is set on fire in a test chamber equipped with air inlet and chimney. Temperatures in the chimney above the allowed limits will lead to unsuccessful results. In the last five years, many (> 40) dry-type distribution transformers (power range up to 3,5 MVA) were tested for verification of one or more of these classes. Often (> 50%) the requirements as per IEC 60076-11 and/or clients specifications could not be met. In some cases design changes and retests were required to meet the requirements, in other cases test programs were stopped due to failure of the object. This paper will explain the importance of the above mentioned tests. Also explanation will be given on the testing methods and procedures required for reliable and reproducible tests, results and conclusions. Climatic, environmental and fire behaviour tests are important special tests proving utilities and other end users of dry-type transformers the quality of the product.