Basics of the Effect of dc on Power Transformers
When a power transformer is subjected to dc, the result is a unidirectional dc flux in the core. The magnitude of the flux depends on the magnitude of the dc as well as the design of the transformer. The flux density shift adds to the ac flux in one half cycle and subtracts from it in the other half cycle, as shown in Figure 1(a).
The effect of dc on transformer core magnetization: (a) The flux density shift in the core caused by dc and (b) The part-cycle saturation of transformer cores.