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 (SLGFs) and line-to-line faults (LLFs) 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.