The American "sag" and the British
"dip" are both names for a decrease in voltage to between 10 and 90% of
nominal voltage for one-half cycle to one minute Sags account for
the vast majority of power problems experienced by end users. They can
be generated both internally and externally from an end users facility. External
causes of sags primarily come from the utility transmission and
distribution network. Sags coming from the utility have a variety of
cause including lightning, animal and human activity, and normal and
abnormal utility equipment operation. Sags generated on the
transmission or distribution system can travel hundreds of miles
thereby affecting thousands of customers during a single event. Sometimes
externally caused sags can be generated by other customers nearby. The
starting of large electrical loads or switching off shunt capacitor
banks can generate a sag large enough to affect a local area. If the
end user is already subject to chronic undervoltage, then even a
relatively small amplitude sag can have detrimental effects. Sags
caused internally to an end user's facility are typically generated by
the starting of large electrical loads such as motors or magnets. The
large inrush of current required to starts these types of loads
depresses the voltage level available to other equipment that share the
same electrical system. As with externally caused sags, ones generated
internally will be magnified by chronic undervoltage. |