Investigation of Inrush Currents during a Short Circuit Test on a 440 MVA, 400 kV GSU-Transformer

摘要

Large power transformers in power distribution networksrnare exposed to several kinds of faults underrnservice conditions. One kind of faults are short circuits.rnAccording to the standards, there are twornmethods to demonstrate the ability to withstand therndynamic effects of a short circuit. The first relies onrncalculation and design considerations, the second onrnthe performance during a short circuit test.rnIn many cases it is sufficient for the purchaser to trustrnin the manufacturers long time experience of shortrncircuit capability, based on results and design limitsrnobtained during short circuit tests on winding modelsrnas well as on full size test transformers. A design reviewrnmay help to reinforce the purchasers confidencernin the manufacturers design capabilities. Nevertheless,rnin a few cases, purchaser and manufacturer canrnagree to perform a short circuit test even on largernpower transformers, in addition to short circuit designrnconsiderations.rnIn such a case specific limitations may exist, due, forrnexample, to the testing capability of the high powerrnlaboratory or the type of faults to be applied. For thisrnreason, it is of importance that both, transformerrnmanufacturer and purchaser consider the differentrnoptions and find an agreement on how to perform thernshort circuit test, so that each of them is satisfiedrnwith the proof of the short circuit performance of therntransformer.rnThe intention of this paper is to provide specific informationrnabout Elin′s short circuit test experience,rnwhich was obtained during tests on a 440 MVA, 400rnkV three phase generator step up transformer, performed　and certified by the KEMA High Power　Laboratory in the Netherlands. One specific challengernfor the transformer manufacturer at the pre-setrnshort circuit test method, is the appearance of undesirablerninrush currents and saturation effects, whichrnis investigated in this paper by means of an adequaternnon-linear magnetic circuit model of the transformer.rnThe inrush current effects on core saturation isrnsimulated by the computer model and compared withrnthe experiences at the real short circuit test. Finally,rnthe impact of inrush currents on axial winding forcesrnis discussed and some efforts by the manufacturer asrnwell as by the testing laboratory to reduce inrush currentsrnare described.