Investigation of electric drive system responses to lightning related voltage sags

Lightning strokes on utility transmission lines may cause flashovers between line to line or line to ground, which would consequently result in voltage sags at the lower AC voltage buses. A voltage sag at the AC bus of electric drives would cause a voltage drop at the drive DC bus voltage which may have an adverse impact on electric drive systems. Both AC and DC drives are sensitive to the DC bus voltage drop. Electric drive tripping, which would cause paper machine shutdown, is a frequent occurrence during lightning seasons. This paper presents the result of an investigation of the drive responses to the utility lightning strokes for a paper mill which has both AC and DC drives. Specifically, the following topics are discussed: (1) voltage sag phenomena related to the fault caused by lightning strokes on the utility transmission systems; (2) electric drive system responses to the voltage sags; a computer model on a three-phase system was developed to investigate the problems; and (3) possible system and drive modifications for riding through voltage sags. The impacts of the modifications have also been evaluated to assure proper paper machine operations.