A voltage sag study in an industry with adjustable speed drives

This article presents voltage sag measurements and their analysis, which were performed in two industries for a period of 17 months. Voltage sags are caused by faults in the utility's transmission system, since both industries are fed by a 115 kV line, but from different circuits. Even faults in 230 or 400 kV lines are felt by the industries' entrance substation as voltage sags. These events cause interruptions in important continuous processes because the adjustable speed drives (ASDs) involved are sensitive to voltage variations. It is shown that, with the measurements made, ASDs are more sensitive to voltage sags than data processing equipment, according to the tolerance curve shown in C51. Also identified were ASDs that tripped more frequently with voltage sags. This allowed the selection of problem areas in both industrial plants, requiring a more detailed analysis in order to select some type of power line conditioning equipment.