Do ABB have any, very approx. idea of level of magnetic field/flux just above a power transformer cover and around the transformer? Range 40-150 MVA, 70-145 kV
To provide you with an accurate answer, we have approached the International Electrotechnical Commission (IEC).
The IEC conclusion is that the only leakage flux outside a power transformer equipped with a magnetic steel tank is coming from the bushing leads on the outside of the cover (or tank).
The general attitude within IEC, is that this is not an issue for transformer people – it is an issue for people dealing with the equipment outside a transformer. We can well understand this attitude, because of the penetration depth at 50 Hz is 1-2 mm and the thickness of a steel tank is 8-10 mm.
This is slightly different for the large generator step-up units with non-magnetic inserts, particularly in the cover. For non-magnetic steel, the penetration depth at 50 Hz is about 50 mm and through these parts a lot of flux from the LV-leads will come to the outside.
Large and medium power transformers
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