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 vulnerable area (expressed as a critical distance). The critical distance increases roughly linear with voltage level. The relation found is similar to relations found in power quality surveys. An equivalent expression is found for the critical distance as a function of the critical phase-angle jump. Realizing that faults downstream 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: simple subtransmission loops; local generation; feeding from two substations; and operating with a normally open breaker