Resonant overvoltages in wind turbine transformers

In this paper, modelling and measurement of resonant overvoltages in wind turbine step-up transformers is presented. A failed 2.7 MVA, 0.690 / 33 kV step-up transformer with the MV side consisting of delta connected layer windings and the LV side consisting of star connected foil windings is presented as a case study. The failed transformer showed substantial damage in the first and second layer of the 33 kV winding with a burn through the HV to LV winding barrier. Two different prototype transformers with the same rating as the failed unit were constructed for the purpose of investigating resonant overvoltages. Both transformers used a stacked core but differed in the winding arrangement where one had split round windings and the other had single round windings. Analytical calculations were done using the Multi-conductor Transmission Line (MTL) model and results were compared with measurements. The calculated results using the MTL model showed a relatively good agreement and followed the trend of the measured results for a frequency range of 1 kHz to 10 MHz. The significance of this work is that it was shown that the MTL model can be used for the determination of the inter-layer stresses and hence can assist during the transformer design stage.