The increase of neutral line resistance in current transformer may induce misoperation of line differential protections. On the basis of previous research

this paper further analyzes its mechanism: when neutral line resistance in current transformer is greater than a certain value

if a fault happens

the voltage on neutral line generated by fault phase current will produce false current on non-fault phase by the excitation effect

resulting in the asymmetrical waveform of false current. The root cause of the false current is the accumulation of flux linkage. By changing the parameters of the secondary circuit equation

the effect of different neutral line resistance

non-periodic current

remnant level

and volume of current transformer on the value of false current is analyzed. Finally

the characteristics of this kind of misoperation are concluded. The ratio of second harmonic to first harmonic and the ratio of phase differential current to the three phase maximum current is chosen to be criteria. If both criteria are satisfied

the differential protection shall be locked for a short time to avoid this kind of misoperation. The correctness and effectiveness of the proposed scheme are verified by simulation and recorded diagrams.