Repeating the demagnetization function more than once does theoretically not result in a
much better demagnetization of the transformer because the procedure starts with a
maximal magnetisation to get a defined initial condition of the core.
Please verify that the measured "Winding Resistance" "R" matches the real winding
resistance with good accuracy (e.g. 1%-2%). Otherwise the demagnetization will eventually
not be performed reliable.
Power transformers with closed delta windings (>100MVA, YNd or YNynd)
Transformers with low voltage delta windings create a circulating current which can
influence the demagnetisation. This is because of the fact that the magnetic flux is not
distributed equally on all limbs within reasonable time.
Wherever possible, we recommend to open a Delta Winding, to improve Demagnetization
performance.
It could also be helpful to repeat the demagnetisation on all three phases.
However – at power transformers with closed delta windings, a demagnetisation without a
real 3 phase high-voltage power source is difficult and mobile devices reach their limits.
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