Application Note

2 Fluke Corporation Basic power quality methodology and common culprits
It’s true, however, that the
sites that need the most help are
the ones least likely to have a
good record of what’s going on in
their system. Many a consultant
has earned his fee by upgrading
the documentation handed him
with what actually exists on-site.
So the simple rule is, at this point
in the investigation, do the best
you can to get good documenta-
tion, but don’t count on it being
available.
2. Do a walk around of the site
Sometimes a visual inspection
will offer immediate clues:
•
A transformer that’s much too
hot
Lightning
Can be extremely destructive if
proper surge protection is not
installed. It also causes sags and
undervoltages on the utility line if
far away. If close by, it causes
swells and overvoltages. But in
the final analysis, lightning is an
act of nature and not in the same
category as the damage man
does to himself.
Utility automatic breaker
reclosure
Causes short duration sags/out-
ages, but better than the
alternative, a longer-term outage.
Utility capacitor switching
Causes a high-energy voltage
disturbance (looks like an oscil-
lating transient riding on the
wave). If the cap bank is near the
facility, this transient can propa-
gate all through the building.
Commercial high rises
w
ithout enough distribution
transformers
Trying to cut corners in the
w
rong plac
es; running 20
8 V
feeder up twenty stories is not
the road to PQ.
Gen-sets not sized for
harmonic loads
Excessive voltage distortion
affects electronic control circuits.
If SCR converter loads are pres-
ent, notching can affect
frequency control circuits.
Applying PF correction
capacitors without considering
the effects of harmonics
Harmonics and caps don’t mix.
Those bulging capacitors are cry-
ing for help.
Inrush currents from high
torque motor loads started
across-the-line
Causes voltage sags if the load is
too large or the source imped-
ance too great. Staggered motor
starts can help.
Undersized neutrals at
panelboard
In the era of the 3rd harmonic,
neutrals can easily carry as much
current or more current than the
A Lineup of Power Quality Culprits
From utility source to receptacle
Isolated ground rod can cause ground loops. Common problem with CNC machine tool installations.
Load Current
Neutral Return
Current Error
Panel
Line Line
Neutral Neutral
Ground
Ground
Separately
Derived
System
N - G
Bond
Earth Ground
“Isolated” Ground,
Ground Rod, Cold Water Pipe, Etc.
3. Interview affected personnel
and keep an incident log
Interview the people operating
the affected equipment. You will
get a description of the problem
and often turn up unexpected
clues. It’s also good practice to
keep a record of when problems
happen and what the symptoms
are. This is most important for
problems that are intermittent.
The goal is to find some pattern
that helps correlate the occur-
rence of the problem in the
“victim load” to a simultaneous
event elsewhere. Logically, this
trouble-logging is the responsi-
bility of the operator closest to
the affected equipment.
•
Wiring or connections discol-
ored from heat
•
Receptacles with extension
strips daisy-chained to exten-
sion strips
•
Signal wiring running in the
same trays as power cables
•
Extra neutral-ground bonds in
sub-panels.
•
Grounding conductors con-
nected to pipes that end in
mid-air.
At a minimum, you will get a
sense of how the facility is wired
and what the typical loads are.
phase conductor. Keeping them
undersized leads to overheated
lugs, potential fire hazards and
high N-G voltage.
Running power and signal
cables together
Think of the signal cable as a
single-wire transformer second-
ary and the power cable as the
primary. The opportunities for
coupling are endless.
Loose conduit connections and
lack of green w
ire grounding
conductor
Causes open or high impedance
ground circuit. Not good for PQ or
safety.