Calculation techniques and results of effects of GIC currents as applied to two large power transformers

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, and loss calculation due to these currents; and temperature rise calculation. These methods are applied to two large shell-form transformers of completely different designs. One of them suffered severe GIC effects and had burnt coils. The other one did not suffer noticeable GIC effects. The methods pinpointed the mechanism of failure and the temperature rise of each strand of each coil. The methods also provide information on the allowable GIC level as a function of loading levels. These methods show that the newer transformer tolerates about six times higher GIC current than the older unit