If a dc voltage of 12v is added to the secondary of step down transformer and rating of transformer is 220 input and 12v output what will be the result from primary of transformer?

Adding a DC voltage to the secondary of a transformer will not have an effect on the primary side, as Transformers work on the principle of electromagnetic induction which is based on alternating current. The primary side of the transformer will still operate based on the input AC voltage of 220V. The DC voltage on the secondary side will not be transferred to the primary side.