Impulse Testing on a Power Transformer

Try this site:

http://www.haefely.com/pdf/scientific/e1-73.pdf

I believe the overshoots and undershoots of an inductor as in the case of a transformer will give an indication of how good a particular inductor will react when presented with an input signal with a very high frequency content, as in a step response function or when a coil is subjected to an impulse of current. The transformer's response to this step function will produce either a dampened (shortened ringing) sinusoidal response or will have a normal sinusoidal decay. This non-destructive test and its resulting waveforms will give an indication on the quality of the transformer windings. Indications of partial resistance or inter-winding shorts and high winding resistances of the coil under test will be indicated..