DC bias is an abnormal phenomenon which may occur in the operation of a power transformer, which means the transformer winding is invaded by DC due to some reason and DC magnetic potential or dc flux occur in the transformer core and the result in series of electromagnetic effects. DC bias may cause adverse effect for the safety operation of the equipment of the transformer substation and power grid. Therefore, there will be the engineering application value to study the principle of the dc bias, the effect of the electrical equipment of substation and the way of inhibiting the generation of dc bias.