FAST ASSESSMENT METHODS FOR VOLTAGE SAGS IN DISTRIBUTION-SYSTEMS

Based on a simple voltage divider model, a relation is derived between the load sensitivity to voltage sags (expressed as a critical voltage ) and the exposed area (expressed as a critical distance). The critica l distance increases about Linearly with voltage level. The relation f ound is remarkably similar to relations found in power quality surveys . An equivalent expression is found for the critical distance as a fun ction of the critical phase-angle jump. Realizing that faults downstre am of a transformer do not significantly influence the expected number of sags, it is possible to estimate the number of sags at a specific load location. The method is extended to some nonradial systems: simpl e subtransmission loops, local generation, feeding from two substation s, and operating with a normally open breaker.