A New Checking Method of CT Polarity for Transformer Zero-sequence Differential Protection

Zero-sequence differential protection is one of the foremost protections for EHV and UHV power transformers due to the advantages of high sensitivity and fast operation speed. The precondition of its correct operation is the correct CT polarity, but the checking of its CT polarity requires large zero-sequence current. However, there is almost no zero-sequence current during normal operation of transformer, which makes it difficult to carry out CT polarity checking of zero-sequence differential protection. To solve this problem, a new checking method of CT polarity for zero-sequence differential protection is proposed in this paper. First, calculate the similarity (S and F) between the self-produced zero-mode inrush current (3I
0p
) and the current of neutral point (3I
0n
). Second, compare the similarity (S and F) with the setting values determined in this paper. Third, assume different error types of CT polarity and reverse the corresponding CT current. Fourth, recalculate the similarity (S and F) in different error types and get the the maximum value of S and the corresponding F (S
max
and F
m
). Finally, repeat step 2 and the error type corresponding to the S
max
and F
m
can be identified. This method is verified by digital simulation in this paper. The results show that this method can accurately identify all error types of CT polarity under different working conditions.