Discrete Fréchet distance algorithm based criterion of transformer differential protection with the immunity to saturation of current transformer

•Fréchet distance algorithm is applied to transformer differential protection.•Phase similarity difference between internal and external fault currents is utilized.•The criterion can discriminate internal and external fault.•The criterion is provided with the immunity to CT saturations.•The criterion is applicable for transformer ultra-saturation.•The proposed criterion has superior performance under abnormal sampling points.•The effectiveness is validated with simulation scenarios and experimental cases.