Flexible DC distribution network has high reliability and large transmission capacity

whereas the protection system still needs improvement. Therefore

a protection scheme based on waveform similarity of the superimposed current for flexible DC distribution grids is proposed. First

the polarity characteristics of superimposed currents in positive and negative poles under different faults are analyzed. Then

the double-end quantity protection scheme based on the waveform similarity of superimposed currents of double-end negative-positive poles is proposed. Finally

simulation analysis and verification are implemented on the PSCAD/EMTDC. The results show that the proposed scheme can complete fault identification and fault-pole selection simultaneously based on currents

which is easy to be applied. In addition

due to adoption of the similarity of superimposed currents

the scheme is not affected by the distributed capacitance

is robust to fault resistance

has a strong ability of de-noise

and can correctly identify high-resistance faults under heavy load. It is not subject to the blocking of converters as well.