The effect of winding design on transformer frequency response with application on offshore wind farm energization

The application of transformers in offshore wind farms requires more consideration in the sense of reliability and protection than onshore ones due to the higher cost of accessibility and repair. Switching of offshore wind farms and lightning strikes are the main two transient phenomena which can lead to harmful damages to Wind Turbine Transformer (WTT) as well as other equipment. Thus, the frequency response of WTTs should be analyzed especially for 10 kHz<;f<;10 MHz. To obtain the necessary protection methods for WTTs, the measured frequency response of a special 500 kVA transformer with layer, disc and pancake windings are assessed and compared in this paper. The results show that all winding designs have a resonance frequency around 800 kHz with disc windings showing the lowest transferred overvoltages. The layer winding, also has a resonance at 1.6 MHz with an even higher transferred overvoltages. The disc winding have pronounced resonance frequencies also around 100 kHz and this results in higher internal overvoltages.