According to the general inrush restraint theory

it is hard for the current differential relay of transformer to overcome the mal-operation when the transformer is energized. Based on an in-depth analysis of the relationship between the secondary harmonic restraint theory and wave symmetry theory

it is pointed out that the reliability of the protection is influenced by the type of phase compensation. When the phase is compensated for at the star side in the conventional way

the decrease of two-phase inrush may result in the reduction of the secondary harmonic in the differential current. If the wave of differential current seems to be symmetrical

the above two inrush restraint theories may operate incorrectly. When the phase compensation is done on the delta side

it will be hard to confirm the ratio of the secondary harmonic restraint.