An all-fiber optic current transformer measures current in the conductor according to Faraday magneto-optic effect

with a wide measuring range

excellent nonlinearity and no magnetic saturation. Since the magnetic field effect of the current can be superimposed

measurement of differential current from transmission line with sensing optical fiber can be replaced by direct calculation of Faraday magneto-optical effect deflection angle at the level of optical path with optical components

which means differential current of the transmission line can be acquired without complicated time synchronization to constitute differential protection. An optical differential protection model is built with PSCAD

the simulation results showing using the optical differential protection to measure differential current can reflect the variation of actual differential current while optical differential protection using fixed threshold value has a higher sensitivity to fault in the area and definite reliability for one outside. This work is supported by National Natural Science Foundation of China(No. 51477061).