DC Offset

DC offset is created during a fault but it is not "true" DC. The offset-ness decays with time. True DC offset is time invariant. The only reason I bring this up is that true DC cannot pass through transformers. True DC offset cannot be pass through your current transformers since the induced voltages in a transformer require changing flux. True DC doesn't effect d(flux)/dT and can't induce a voltages. "DC offset" is really a low frequency component and since it changes with time, it will create decaying flux in the a transformer core and the decaying offset-ness will pass through the transformer.

It is often said that the asymmetrical component can be fully offset, making I(assymetrical) = 2 * I(symmetrical) but this isn't accurate. The point at which the fault has to happen to produce the most dc offset is at or near the voltage maximum (positive or negative). Since the current will lag the voltage by near 90 degrees in any system with a large X/R ratio, the peak will occur 1/2 cycle after the fault and not be fully offset. The DC component is a maximum after the fault and decays exponentially with time constant ,R/X.