Characterization of voltage sags in industrial distribution systems

This paper describes the various characteristics of voltage sags experienced by customers within industrial distribution systems; Special emphasis is paid to the influence of the induction motor load on the characterization of voltage sags. During a fault, an induction motor operates as a generator for a short period of time and causes an increase in sag magnitude. Its reacceleration after the fault clearance results in an extended post-fault voltage sag. The influence of the induction motor on the imbalanced sags caused by single line-to-ground faults (SLGF's) and line-to-line faults (LLF's) has been analyzed in detail. For an imbalanced fault, the induction motor current contains only positive- and negative-sequence components, Induction motors create a low impedance path for the negative-sequence voltage due to an imbalanced fault. This causes a small sustained nonzero voltage with large phase-angle jump in the faulted phase and a voltage drop in the nonfaulted phases with a small phase-angle jump. The symmetrical components of the induction motor during the imbalanced sags have been studied. The results show that induction motor behavior is determined by positive- and negative-sequence voltages during the imbalanced sag.