Though it may seem silly to differentiate these two, it is a helpful clue when
you only have written descriptions of
events. A sag on one phase only would
likely seem to be caused by a single-phase
load. However, knowing that there was a
corresponding RVC event on the other
phase or phases can indicate that there is
a voltage or current unbalance also to be
considered and it is actually a polyphase
load that caused the problem. Ongoing
RVC events that get more severe can
indicate degradation, whether in the electrical distribution system (such as source
impedances getting higher), or the load’s
impedance is drawing more current.
Eventually, it can become big enough
to become a sag. While the sag thresh-
old of 90 percent is a generalization of
a potential problem, setting the thresh-
old to a value meaningful for the ride
through capability of critical loads can
foretell process interruptions or produc-
tion losses in the future if the problem is
It may not just be a load-side-initi-
ated problem. Suppose a photovoltaic
generator is providing a significant por-
tion of the power to a facility. A cloud
transient, such as one caused by a plane
flying between the panels and sun, can
cause a rapid drop in the source voltage,
also qualifying as an RVC. An array of
various distributed generators coming
on and off at different times without
an overall control scheme can have the
RVC is more than just a new label.
Some PQ monitors have the capability to
report it. It’s another data point of what
is going on in the system, often like an
early warning signal, of possible doom in
the future if the source isn’t evaluated.
BINGHAM, a contributing editor for power quality, can be reached at 732.248.4393.
Figure 3: The green plot is an RVC, while the red plot is a sag.