Experiences and innovations in power transformer short-circuit current withstand testing
Cigré 2016 paper A2-304
The role and rationale of short-circuit testing of power transformer in the quality assurance process is explained, as well as a range of utility considerations to rely on short-circuit testing to verify withstand against the mechanical forces of short-circuit current. Experience is reported on short-circuit testing of large power transformers (≥ 25 MVA) during the past 20 years by DNV GL KEMA Laboratories in the Netherlands. In total, 297 test series were analyzed with 258 different power transformers involved. Statistics show that at first access to standard IEC 60076-5 short-circuit tests, 23% of the tests result in a failure initially. From the 67 test series that did not lead to success, 39 re-tests followed, usually leading to a success. Failure to pass short-circuit tests is mainly by an increase of short-circuit reactance beyond what is accepted in the IEC standard, but several other, directly apparent, failure modes are frequently observed, like oil spill, damage to bushing and leads, etc. and are demonstrated in the paper. A similar analysis was carried out on five years short-circuit testing of 250 distribution transformers (≤ 2.5 MVA). It was observed that around 20% fail to pass the test initially.