POWERQUALITY BY RICHARD P. BINGHAM
If the root mean square (rms) of the
voltage in a cycle exceeds the upper
threshold—typically 110 percent—it is a
swell. Below 90 percent, it’s a sag until
all three phases go below 10 percent, and
then it’s an interruption. If the rms of the
voltage modulates or varies 1–3 percent
at a rate less than 30 hertz (Hz) and especially around 9 Hz, then that fluctuation
will likely result in light flicker of incandescent and some other types of lighting.
But what about the in-between zones
(the blue regions in Figure 1)? Most
power systems throughout the world
experience it, but in the United States,
many don’t have a label for it. Perhaps
because it doesn’t cause problems like
sags or swells, even though it may be a
precursor to more significant problems.
There is a parameter defined and used
in Europe and around the world that
describes this region as “rapid voltage
change” (RVC). It has appeared in some
international standards for years, but it
wasn’t mathematically defined until the
Norwegian Water Resources and Energy
Directorate did so in 2005. The third edition of the International Electrotechnical
Commission 61000- 4-30 power quality
measurement methods now includes it.
RVC is an abrupt transition between two
steady state voltage levels, sort of like a
step change from the steady state voltage,
usually more than 3 percent and less than
10 percent. The change needs to occur
rapidly; think of it as a “blink.” A slowly
decaying nominal voltage wouldn’t be
considered an RVC event.
The following is the mathematical method for determining if such an
• Compute the arithmetic mean of the
voltage waveforms over 1 second. If all
cycles remain within the RVC threshold,
then steady state has been established.
• Once steady, if an rms voltage cyclic
value crosses the threshold, then an
RVC event has started.
• The RVC event is considered ended
when the steady-state condition over
1 second occurs again. Until then, if
the voltage change is greater than the
previous value, then a new maximum
excursion value is saved.
• Once steady-state is achieved, the
duration is determined by subtracting
the end time from the start time minus
the 1 second delay used for making
sure that things are steady.
• The new steady-state voltage may be
different from the original condition.
In fact, that is quite common, unless
a single load went on and off to cause
the event. This new steady-state value
is subtracted from the original steady
state value to get the change or delta
in steady state voltage.
• These three values (max deviation
voltage, delta steady-state voltage and
duration) are used to characterize an
RVC event. Like with sags and swells,
the values can help determine what
was a probable cause of the event.
Visualized, the event can look like
Figure 2 below, from the 61000- 4-30
standard. Note that, in IEC standards,
U is for voltage, and a dip is the same
as a sag.
The usual suspects that cause an
RVC event are similar to those for sags.
Ohm’s and Kirchhoff’s laws still apply.
A load turning on and drawing a significant amount of current as compared to
the system’s short-circuit current potential will cause a voltage drop across the
combined source impedances, leaving
less voltage for the loads. HVAC units
are common culprits. In a series of articles in ELECTRICAL CONTRACTOR, it was a
5-ton HVAC unit at a residential dwelling that caused the lights to blink every
22 minutes in the summer when outside
temperatures were in the nineties.
In Figure 3, the Phase B voltage (green
plot) is a RVC event while the Phase A
voltage (red plot) would be considered a
sag. If the nominal voltage of A had been
higher, closer to B, then it would also have
been labeled an RVC, not a sag.
Put a Label on It
An introduction to RVC
THE LIGHTS BLINKED FOR AN INSTANT, or a running appliance made a sound
like the motor was slowing down. No digital clocks were blinking, no computers
restarted, no production lines stopped. It wasn’t going on continually like light flicker,
nor was it severe enough of a voltage drop to be classified as a sag. So what was it?
Figure 1: Power quality phenomena
over the voltage range
Figure 2: RVC event depiction per IEC 61000- 4-30 Edition 3, Figure 6
“voltage is-steady-state” logic signal
Changes in “voltage-is-steady-state” logic signal are disabled