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Maintenance
Maintenance and
Energy Savings
Feeder maintenance practices can reduce energy waste.
You can find several energy-saving opportunities, starting with your
distribution transformers.
Wherever you have liquid-filled transformers, take advantage of the
various tests they permit. What about dry-type distribution
transformers? Contrary to popular opinion, dry-type distribution
transformers can (and should) be maintained. Unfortunately, they
typically receive no maintenance. The inevitable results are higher
costs and shorter times between failures.
True, you can't employ all of the predictive maintenance tests with
dry-types that you can with liquid-filled transformers. But there's
still plenty you can do to reduce energy waste while improving
reliability.
Thermal analysis will reveal defective connections, leaking
insulation, and other defects, if conducted by a qualified person who
knows what to look for. The investment typically pays for itself
several
times over.
One of the easiest tests to perform, especially with the newer test
equipment, is the insulation resistance test. This can warn you of
potential winding faults and other issues before they result in
failure.
A catastrophic failure may cause more than just loss of power. It can
cause loss of life plus massive property damage.
An insulation resistance testing program is most effective if you
trend results over time. Insulation deterioration is normal, so as you
plot the results you will gradually see a gently sloping downward
trend.
When this trend suddenly "breaks" -- that is, the gentle slope starts
to nosedive -- you have evidence of impending failure. Schedule a
replacement before failure occurs.
Vented units accumulate dust, which is a thermal insulator. You can
remove most dust with a vacuum cleaner, though more robust cleaning
methods may be required. Caution: De-energize transformers
before
cleaning. For one thing, the static charge on a plastic vacuum cleaner
nozzle is a potential fault path. For another, blindly reaching into an
energized transformer is likely to prove suicidal.
Finally, be diligent about maintaining clearances around
transformers
so they have proper ventilation.
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Repair
Electrical
Troubleshooting Quiz
This is an actual case history. Over a period of a few
weeks, several loads on the same feeder dropped out inexplicably and
the
breaker for this feeder tripped several times. The drawings showed only
75% load. What is the first step in solving this problem?
The answer to this question appears at the end of this
newsletter.
PF Capacitor
Replacement
Power factor (PF) correction at the service allows you
to avoid a PF surcharge from the utility. If service PF capacitors
fail,
you'll probably incur a PF surcharge until they've been replaced.
However, exact replacements may be incorrect. Existing capacitors
may
be sized for a different inductive load than what you have now. Since
the original installation, it may be true that:
- Inductive loads have been added, making the capacitors
undersized.
- Inductive loads have been removed, making the capacitors
oversized.
If PF is now "borderline," a minor change can result in a PF surcharge.
System inductance may have changed significantly since the original
installation, even if you haven't added or removed motors. PF-corrected
motor drives replaced with uncorrected drives put motors back into the
PF picture. Lighting design changes can also change the total inductive
load.
PF correction at the service lowers your electric bill. But it
doesn't address PF-related losses within your facility. To do
that, correct PF at individual inductive loads.
It's more effective and less costly to address PF correction via a
planned program, rather than reacting to a failure. The two steps
are:
- Calculate the PF correction for your largest inductive loads
(consult your drive manufacturer for motor PF correction).
- Calculate the service PF correction for the remaining inductive
component.
If you've already PF-corrected some loads, ensure your service PF
correction capacitors are properly sized for the remaining inductance.
Too much PF correction might not trigger a surcharge, but either
direction from unity results in more energy consumption.
Finally, don't buy new PF correction capacitors without taking a
close look at lid designs. Choose a unit that will permit replacement
of
single capacitors with the least amount of hassle.
Operation
NEC at the
Facility
Junction boxes and other enclosures often have unused
openings. These need to be properly plugged [408.7]. Duct tape doesn't
count. Caps made specifically for that purpose are inexpensive and easy
to apply, making it a snap (literally) to maintain the integrity of
your
enclosures.
NFPA 70E
The latest revision of this standard strongly
discourages "working hot." However, there are some things you just
can't
do with the power off. Voltage measurements, power analysis, and
thermal
imaging all require the equipment to be energized. You can probably
think of many more tasks that must be done with the power on.
One impetus for the "don't work hot" slant of the latest NFPA 70E is
the persistent assumption that circuit breakers provide personal
protection. But a circuit breaker or fuse might never trip on a
low-grade fault. A breaker isn't designed to protect people. It's
designed to protect conductors from melting due to excessive
current.
If you work around energized equipment, use the correct personal
protective equipment (PPE) and follow the relevant procedures for shock
and blast protection.
Show & Events
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Enter by July 10, 2007. Visit the Generac Power Systems virtual
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go to www.roadamerica.com.
Don't Miss These FREE Live Sessions
Scheduled for June 14th in
the
EC&M e-Tradeshow
- "Major Energy Savings Through Lighting Management" presented
by Patrick Kelly, Encelium, at 10:00 a.m. Eastern and Pacific time
- "Mitigating Harmonics in Industrial Environments" presented
by John Houdek, Allied Industrial Marketing, at 10:00 a.m. Eastern and
Pacific time
Before and after the conference sessions, visit the many exhibitors in
this virtual tradeshow and take a look at the On-Demand Theater, where
you can view past online webcasts 24 hours a day, 7 days a week, 365
days a year.
Click here
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Quiz Answers
Answer to
Electrical Troubleshooting Quiz
The first step is to take the drawings in hand and
physically walk down this feeder. That's what this engineer did,
thinking he'd find the "missing load" that was "left off" of the
drawings. This "missing load" wasn't on the drawings, because it was an
artificial load.
The 480V feeder fed a stepdown transformer that supplied 120V to
several panels. The transformer and panels were in an unventilated
equipment room (roughly 8 feet x 12 feet). When he opened the door, the
heat about knocked him over. It was worse for the equipment.
Cardboard boxes stacked neatly against transformer vent openings
blocked airflow across the windings. Boxes stacked on top of the
transformer provided significant thermal insulation. The big surprise
was that those boxes hadn't yet ignited.
The engineer decided the transformer was operating at saturation
because it hadn't been derated for high-temperature operation. He
removed the rogue materials from the room and had upper and lower vents
installed in the door. The problems never occurred again.
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