As the extensive application of the microcomputer protection， the secondary load of the current transformer changes from resistance and inductance load to resistance load， and the fault clearing time is shorter， making the remanence of the P class current transformer increase greatly. Too much remanence often leads to the maloperation of the differential current protection for the external fault and the time-delay operation of zoneⅠ of the distance protection. Based on the practical and theoretical analysis， this paper explains the different impacts of the remance and the non-periodic component in the primary current on the current transferring and proposes that PR class current transformers should be used in the 220 kV line protection to eliminate remanence's impacts and to insure the rapid operation of zoneⅠ of the distance protection to clear off the internal fault. For the PR class current transformer， it is only need to consider the transient saturation induced by non-periodic component for the examination of current transformer saturation's impact on protection， so choosing“the transient saturation of the current transformer comes forth at 5 ms after the fault” as the examining standard is reasonable.