Through the analysis of the recovery surge current generated by external fault reversal of the converter transformer, it is pointed out that the commutating iron core may be saturated under the action of the inrush current, resulting in the biasing of the sensitive neutral line TA and causing its measurement distortion to lead to zero. In this paper, a new scheme of commutation-changing zero-sequence differential protection based on waveform correlation is proposed. By analyzing the characteristics of faults in the converter transformer, out-of-zone faults and zero-sequence currents during external fault removal, the zero-sequence differential protection is used to identify the waveform correlation between the zero-sequence currents on both sides and the zero-sequence current on the neutral line. By reasonably selecting the polarity of the current transformer, the zero-sequence current on both sides of the homodyne protection exhibits a "traversal" characteristic when the fault is outside the zone, and the waveform similarity is high, and the correlation coefficient is positive. When the fault occurs in the area, the similarity of the zero-sequence current waveforms on both sides is small, and the correlation coefficient falls into the negative value area. Through a large number of simulations verified by PSCAD/EMTDC, this criterion can accurately identify faults in and out of the zone, is not affected by the recovery inrush, and has some ability to resist TA saturation. This work is supported by the Key Project of Smart Grid Technologies and Equipment of National Key Research and Development Program of China (No. 2016YFB0900600) and Science and Technology Project of State Grid Corporation of China (No. SGNX0000DKJ S1900460).