If network-based smart substation takes synchronous values as differential original data, the differential protection performance declines significantly or even goes wrong when the network performance decreases. The principle of phase comparison differential protection is analyzed thoroughly. The fundamental reason of malfunction is found when the current flows out of the busbar in a busbar shorted fault, based on which a differential protection is proposed based on the principle of variable impedance. The new scheme has low synchronization requirement, small amount of data to be transferred, and high CT anti-saturation ability. It can be applied to busbar protection, short-lead line protection, backup protection for the whole substation, etc.