Assessment of the number of voltage sags experienced by a large industrial customer

Circuit theory models are used to estimate the number of voltage sags in the supply to a paper mill. For every voltage level, the radius of the exposed area (the critical distance) gives the expected number of sags. As the main sensitive load is formed by large adjustable-speed drives, a classification of sags into four types is used. The conclusion of the paper is that the equipment should first be made resilient against sags due to single-phase faults at 400 kV. These faults cause the majority of sags. Sags due to three-phase faults are more severe (especially due to faults at 11 kV), but are less common.