Power transformer windings and transient voltage damage

The distributed inductances and capacitance of each turn to its neighbors, to other winding layers, to the LV winding, and to ground form a complex transmission line-like system. The result is that the transient voltage distribution across the HV winding is always nonuniform, with a shape that's highly dependent upon physical construction of the transformer and the risetime, shape, and duration of the incoming transient.

With a grounded neutral, incoming transients tend to cause highest turn-turn stress to appear across turns nearest the HV end of the winding. However, it's possible to develop similar voltage stresses on the neutral end of the winding under certain (chopped wave) transient conditions in systems that use a grounded neutral. In a system with an ungrounded neutral (i.e., star-star configuration) similar incoming transients can generate very high transient voltages on the neutral end of the winding. In either case, these conditions can occur when a nearby lightning strike create a high voltage impulse shortly followed by a bushing flashover.

An excellent review of the effects of lightning impulse, chopped wave, and oscillatory transients on power transformer winding voltage stress can be found in the newer editions of the J&P Transformer Book and in the technical literature.