1. School of Information and Electrical Engineering, China University of Mining and Technology, Xuzhou 221008, China; 2. State Grid Huaian Power Supply Company, Huaian 223002, China

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.