Voltage sag has become one of the most concerned power quality problems. A method of tracing voltage source based on the sequence incremental power current direction is proposed. First the positive reference direction of a monitoring point is determined by its current and voltage mutual inductor corresponding terminals. If the sequence incremental power current of the monitoring point is positive

the symmetric three-phase voltage sag source is in the same direction as the positive reference direction

namely

in the downstream of the monitoring point

otherwise in the upstream. When the sequence incremental power current of the monitoring point is positive

the asymmetry sag source is opposite to the positive reference direction

namely

in the upstream of the monitoring point

otherwise in the downstream of the monitoring point. It is proved by both simulation and experiment that this voltage sag source tracing method can accurately locate the voltage sag source in a radial distribution power system regardless of fault types. Simple in calculation and asking for only a small amount of computation

this method can be realized by single-chip

or embedded in the relaying for accomplishment of voltage sag source tracing.