This paper introduces the principle of zero sequence differential protection

aiming at the problem of polarity verification of step-up transformer zero sequence differential protection current transformer（TA）

uses the unbalanced current generated by magnetizing inrush current when the transformer is charged

extracts the characteristic quantity of self-produced zero sequence current and neutral point zero sequence current

proposes automatic detection method of TA polarity for zero sequence differential protection of generator transformer unit

and carries out dynamic simulation experiment and the field waveform playback test. The experimental results show that this method can automatically and accurately identify the polarity of neutral point zero sequence current transformer when the no-load step-up transformer is charged

greatly simplify the zero difference polarity verification process

which provides technical support for the wide application of zero sequence differential protection of generator transformer unit.