These are all interesting challenges
that can often be overcome but only if the
data presented is valid. Despite advances
in PQ monitors that try to make the connection, configuration and setup of the
instruments less prone to human error,
there are still many times when the data
cannot be used to achieve a certain goal.
Just a few simple checks can take some
of the human error out of the equation.
First, does the monitor have the
capability to do what you want it to and
capture the type of data you are interested in? Reading the specifications on
the instrument brochures can be a daunting task. Try looking for these few things.
Is the voltage and current range compatible with your electrical system and
equipment? Is the sampling rate at least
twice the highest harmonic frequency
you are interested in? Is the storage
capacity large enough for the time that
you want to monitor and the amount of
data you think you will collect?
That last one is tricky because it is
difficult to predict all variables. Since
less-experienced users tend to save a lot
of waveforms and nearly every parameter on the menu, storage capacities in
gigabytes is probably a safe bet.
Next, connect the instrument’s volt-
age leads and the proper rated current
probe on the correct conductor pointing
in the right direction. Despite claims to
“fix incorrectly connected instrument
data,” that is generally only possible if
every voltage and current waveform is
saved over the entire time you are inter-
ested in seeing what is happening. You
can’t post correct a delta connection
on a wye circuit with phase A probe on
phase B conductor pointing toward the
source instead of the load, and so on. It
is unlikely that you would have captured
a neutral-to-ground voltage swell, since
the line-to-line voltage of the delta con-
nections wouldn’t see that. Determining
power parameters and the directivity
of sags and swells (upstream or down-
stream) requires the proper phase
relationships, which can’t be recreated
without waveforms. In addition, you
have to know which is the correct phase
relationship after you have left the site.
Going back to the current probes or
current transformers (CTs), are they
properly sized for the current levels
expected, and are the range selection
switches (if applicable) and scale factors
set properly? Putting a 30-ampere (A)
CT on a circuit with fault currents over
100A likely will result in clipped waveforms and incorrect maximum values.
Conversely, a 1,000A CT looking for less
than 1A of fifth harmonic current out
of 5A rms total current is likely going
to have a lot of noise in the data. Most
C Ts have a 3x overcurrent range and can
read down to 10 percent of full scale, but
check the specs to be sure.
Also, if you suspect there might be a
DC component in the current (or voltage), you should have a probe that can
pass DC (such as a Hall-effect probe for
current), and ensure the instrument’s
measuring inputs aren’t AC-coupled.
If the problem occurs frequently and
you plan on monitoring only for a day or
two, you can set the periodic capture or
journal rate to one minute or less, pro-
vided the storage memory is adequate.
When doing a month-long benchmark
survey, there is rarely a reason for set-
ting that rate to less than 10 minutes,
especially when the instrument records
the minimum/maximum/average values
over that interval.
Similarly, setting the waveform
capture more than 30 cycles pre- and
post-event is usually not necessary, except
when doing inrush measurements of large
motors, transformer energizations, or the
startup or switching of generators.
Finally, if you are a new user or an
infrequent user of the instrument, it’s
a good idea to leave the threshold settings for triggering event capture in the
default or preprogrammed setups from
the manufacturer. These values are often
based on typical limits used by many
customers and referenced to standards,
where +/– 10 percent from nominal is the
sag/swell limits. Customers have set the
instantaneous peak triggers to 170V, not
realizing that a 120V rms undistorted
signal has a peak value of that, which
can result in a continuous recording of
nothing useful while the memory fills up.
Another favorite memory filler is to set
current limits within normal operating
ranges of the loads.
Of course, there is the fall back of
reading the user’s guide or operator’s
manual. But short of that, follow the
above steps each time, and the data you
record (and I will analyze) will likely
have information that you (or I) need to
resolve the problem.
Doing It Right the First Time
Removing human error (as much as possible)
I OFTEN RECEIVE DATA FILES RECORDED from power quality monitors
to analyze the reason a piece of equipment experienced an event. This analysis
is followed by determining the cause and who is to blame. Others want to know
if their supply quality is acceptable or “normal” (whatever that means) for their
unspecified equipment or if they should call their utility to improve it.
BINGHAM, a contributing editor for power quality, can be reached at 732.248.4393. SH