There is a drawback in traditional substation area current differential protection, when the scale factor k monotonously changes, the sensitivity of the internal and external fault will vary oppositely. This paper proposes a substation-area backup protection based on phase related current differential which use the multi points redundancy information. The analysis shows that when the scale factor k monotonically increases, sensitivity of the internal and external fault will increase together. As the increasing of ground resistance, sensitivity of fault will increase. On the other hand, when the external faults cause CT saturation, the traditional protection will mis-operate, but the new algorithm can overcome the shortcoming. At last, PSCAD/EMTDC is used to build a 220 kV substation model, simulation results verify the theoretical analysis.