waross,let me disagree.This is much more complicated.This current-tank earth pad to ground, is from the voltage difference between tank and station earth pad.This varies with load ( leakage flux from windings generates a voltage on tank surface) and position of earthing etc,etc. Let me give actual values for a 1000 MVA 420 KV step up transformer bank.Actual tank earth currents at full load is 10-22 A for two single phase units and 0.6-13.8 A for the third phase.The capacitance current ( LV to core + outer winding to tank) will be only of the order of 0.1 A as estimated.
Some years back, I came across a pretty large rectifier transformer in an aluminum plant where the tank current to earth was the order of 800 A !One out of two earth links to ground was removed to bring down this current to the order of 300 A.