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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
mendations listed herein, it is good practice to de-
Table 7-l. Transformer inspection and maintenance checklist
velop a habit of visual inspection whenever a
General inspection items
transformer area is visited. In this way leaks,
Load current
Weekly or monthly
cracked insulators, loose connections, and similar
Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Weekly or monthly
problems may be noticed before serious problems
Temperature . . . . . . . . . . . . . . . . . . . . . . . .
Weekly or monthly
develop that might affect the continuity of service.
Liquid level or pressure vacuum
Weekly or monthly
When working around a transformer, particular
Protective devices.
care must be taken in handling all tools and other
Protective alarms
Ground connections
Every 6 months
loose articles, since material dropped into the wind-
Surge arresters
Every 6 months
ings and allowed to remain can cause a breakdown.
Pressure-relief devices.
Every 3 months
a. Power transformers. For maintenance pur-
Breather . . . . . . . . . . . . . . . . . . . . . . . . . . .
poses consider the impact that the loss of a power
Auxiliary equipment and bushings
transformer will have on the facilities operation.
Load tap changer (LTC)
External inspection
Every 6 months
Utility-facility interconnection transformers and
Internal Inspection.
From 3 to 6 years
transformers with medium-voltage secondary lines
can be defined as significant impact transformers,
Solid insulation (winding)
while other power transformers can be considered
Insulation resistance.
as less significant impact transformers. There may
Polarization index (PI)
be slightly different maintenance techniques for liq-
Power factor.
From 3 to 6 years
Hi-pot (ac or dc)
From 3 to 6 years
uid and dry-type transformers, but the general ap-
Induced potential
From 3 to 6 years
proach is the same.
Transformer turns ratio
(1) Significant impact transformers. Table 7-l
Insulating liquid
is a recommended inspection and maintenance
checklist based on input from NFPA, NETA, and
Gas analysis.
manufacturer's published guides. For transformers
Dielectric strength
Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
having a less significant impact, checking should be
Acidity (neutralization number).
decreased as covered later.
Interfacial tension (IFT)
From 3 to 6 years
(2) Less significant impact transformers.
General inspection items
Power factor test.
From 3 to 6 years
Transformer readings should include load current
at peak load, voltage readings during both peak-
load and low-load periods, temperature, liquid level,
clamp-on ammeter. Reasonable accuracy and com-
and pressure/vacuum recordings. These readings
plete safety should be of the greatest importance in
should be taken not less than every 6 months along
making transformer load surveys. Readings should
with general inspection tests from table 7-l that
be taken on the secondary side of the transformer
are not annual or 3-to-6 year tests. Other tests of
whenever possible. Testing all transformers may
table 7-l may be needed dependent upon the re-
not be necessary, because similar areas and build-
sults of the 6-months' tests.
ings may have quite similar loads.
b. Distribution transformers. Porcelain bushings
(2) Dielectric tests. Dielectric tests of liquid -
should be kept clean and the transformers in-
immersed transformers need not be made on distri-
spected annually. Check for broken porcelain, loose
bution transformers of less than l00-kVA capacity.
power connections, blown fuses, and defective surge
Liquid samples should be taken at 5-year intervals
arresters. Check for leaks, hardened bushing gas-
from each liquid-immersed distribution transformer
kets, corroded or broken ground connections, rust-
of 100-kVA and greater capacity. These samples
ing of tanks, and signs of corrosion on terminals,
should be given a dielectric test. If a liquid-
bushing studs, and connectors. If the transformer is
immersed transformer has been out of service for
excessively noisy or has a ruptured gasket, then the
one year or more, dielectrically test a liquid sample
unit should be opened, internally inspected, and
from that unit before re-energizing the transformer.
When a liquid sample fails to meet the dielectric
(1) Load test. A load test should be made annu-
standard, filter the liquid until it meets the stan-
ally on transformers which supply a load that is
dard or replace with new liquid of a type and grade
known to be increasing. Transformers which supply
recommended by the transformer manufacturer.
a steady connected load should be load tested every
5 years. Load tests should be made with portable
7-5. Transformer inspections.
ammeters (dial-indicating or recording-chart type),
installed for at least 24 hours on a peak loading
Inspection and repair will vary dependent upon the
period day as determined by spot checking with a
type of transformer installed. Always expand or


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