Short circuit verification for a 570 MVA, 420 kV single-phase GSU-transformer by SC-withstand tests on a mock-up unit

Abstract

Large generator step-up transformers in power generation plants are exposed to several kinds of stresses under service conditions. One kind of stress is caused by forces at short circuits, which can even endanger and affect the entire reliability of a power plant.

According to the international standards, there are three methods to demonstrate the ability to withstand the dynamic effects of a short circuit. The first one relies only on calculation and design checks with the manufacturer’s design rules for short circuits. The second one observes the performance during a conventional full-scale short circuit test and the last one involves a design comparison to a similar reference transformer which has successfully passed a short circuit test.

EDF and Siemens decided to evaluate the short circuit capability of a 570 MVA single-phase GSU-transformer by comparison to a similar reference mock-up transformer designed, manufactured and short circuit tested for this purpose. The reference transformer was designed to undergo equal or similar mechanical stresses as in the real full-scale unit during a short circuit. In this paper the most significant stress types in windings are analysed, described and evaluated. Similarity criteria and constraints regarding radial and axial winding stresses are discussed in detail.

The short circuit tests on the mock-up transformer were performed in two test sequences. The first one was a successfully performed “customer acceptance test” where nominal short circuit currents and stresses were applied to demonstrate the short circuit stresses in the real unit. The second test series was a “destructive test”. Currents shots were increased step by step until the unit experienced a notable change in the winding reactance. The purpose of the second test was to identify the critical mechanical stresses which caused permanent winding deformations and loss of stability on the mechanical structure.

Results from a visual out–of tank active part inspection and some failure modes detected after winding dismantling are presented in this paper. Frequency Response Analysis (FRA) measurements performed before and after the short circuit test series show high correlations to detected mechanical movements.

A not well-known failure mode of helical windings, the so called “spiralling effect”, where compressive forces tighten up the winding turns towards the inner support, are analysed. Corrective measures to prevent such effects in transformer windings are suggested.

The paper presents an experimental as well as a theoretical design evaluation and calculation approach to verify the short circuit withstand capability of large power transformers unable to be tested in high-power laboratories.