A novel Hausdorff distance based restrain criterion for zero-sequence differential protection of converter transformer

Due to its high sensitivity and fast operation rapidity, zero-sequence differential protection (ZSDP) has become the most frequently used main protection for EHV and UHV power transformers. It is generally assumed that extra restrain criterion is unnecessary for ZSDP since it is immune to the magnetizing inrush current in theory. However, it's found that the bias of zero sequence current in converter transformer due to core saturation may lead to the accumulation of magnetic bias within the core of CT, especially during external fault removal period. This may lead to the mal-operation of ZSDP. Based on the Hausdorff graphics recognition algorithm, an extra restrain criterion for ZSDP is proposed in this paper. Verified by simulation and theoretical analysis, the proposed criterion will not influence the performance of traditional ZSDP. However, the proposed criterion can correctly identify the fake differential current and effectively prevent the mal-operation of ZSDP caused by the recovery inrush.