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Techniques for calculating overheating effects of GIC (geomagnetically induced currents) on large shell-form transformers are described. The techniques include: the method of calculating the exciting current and its harmonic content under DC injection in the transformer neutral; three-dimensional magnetic flux calculation; circulating current calculation, loss calculation due to these currents; and temperature rise calculation. These methods are applied to two large shell-form transformers of different design. One of them suffered severe GIC effects and had burnt coils. The other one did not suffer noticeable GIC effects. These methods also show that the more recent transformer (the sound unit) tolerates about six times higher GIC current than the older unit (the first unit).>
Girgis et al. (1992) studied this question.