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.