The correct answer is: A. 1.5 to 2.5 A/mm2
Current density is the amount of current flowing through a conductor per unit cross-sectional area. It is a measure of how much current is being forced through a conductor, and it is an important factor in determining the conductor’s resistance and temperature rise.
For large power transformers, the current density is typically limited to 1.5 to 2.5 A/mm2. This is because higher current densities can cause the conductor to overheat, which can lead to insulation failure and transformer damage.
The other options are incorrect because they are outside of the safe operating range for large power transformers. Option B (3.5 to 4.5 A/mm2) is too high, and option C (4.0 to 5.0 A/mm2) is too close to the upper limit. Option D (5.4 to 6.2 A/mm2) is definitely too high, and it would likely cause the transformer to fail.
In conclusion, the correct answer is A. 1.5 to 2.5 A/mm2.