The fault components of the current and the voltage have been widely used in relay protection for several decades. Generally， it can only be obtained within two fundamental cycles following the occurrence of the fault. In order to obtain the long-term fault variation component， calculation of the pre-fault and post-fault variation is proposed. The characteristics of differential protection with the scheme proposed are investigated. The conclusions can be drawn that the differential protection can operate correctly when the fault occurred out of the protection zone although its sensitivity may decrease to a certain extent.