Quantcast Transformer insulating liquids

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
(3) Test voltage. An insulation test is not in-
or silicone fluids in less-flammable liquid-insulated
transformers. Askarel or PCB units should have
tended to be a destructive test. The test voltage
been removed, or be in the process of being re-
used must be restricted to a value commensurate
with apparatus voltage rating and condition of insu-
moved, from most installations. Tetrachloroethylene
(Wecosol) fluid used in nonflammable fluid-
lation being tested. This is particularly important in
insulated transformers, can evaporate to produce
the case of small, low-voltage transformers or those
toxic fluids and its use should not be an allowable
units containing an excessive amount of moisture.
option in facility design manuals/specifications.
b. Polarization index (PI) test. A PI test or dielec-
b. Air and moisture are the major enemies of in-
tric absorption test is a continuation of the insula-
sulating liquids. The oxygen in the air will cause
tion resistance test in which the voltage is applied
for a longer period of time. For good insulation, the
the formation of acids and sludge in the oil. Mois-
resistance values will increase with time. The polar-
ture, in as small an amount as 10 parts per million
ization index is the ratio of the lo-minute to the
by volume, can reduce the dielectric strength of in-
l-minute readings. An index below 1 indicates poor
sulating oil to below its acceptable value. All con-
insulation. An index between 1 and 2 indicates that
tainers and equipment used for handling insulating
the insulation is questionable. An index of a 2 and
liquids must be clean. Equipment used with min-
higher indicates good insulation.
eral oil should never be used with less-flammable
liquids, as any mineral oil residue will change the
c. Power factor test. The power factor of an insu-
less-flammable liquid's fire-point characteristics.
lation is a measure of the energy components of the
Transformers should not be retrofilled with a differ-
charging current. The test indicates the power loss
ent type of insulating liquid, as this can cause the
caused by leakage current through the insulation.
transformer to malfunction. Likewise, great care
The equipment to be tested should be disconnected
must be used to ensure that any liquid used to "top
and all bushings should be cleaned and dried. The
off" a transformer is compatible in all respects with
test should be conducted when the relative humid-
the liquid already in it. The best practice is to con-
ity is below 70 percent and the temperature is above
sult the transformer manufacturer or the manufact-
32 degrees F (0 degrees C). On transformer tests,
urer of the liquid intended to be used for topping off.
the power factor of each winding with respect to
In the following paragraphs, where no values are
ground, and each winding with respect to its other
given, consult the manufacturer.
winding, should be measured. Evaluation of the
c. Sampling. Samples should never be taken from
data obtained should be based on comparison of
data with any previous tests on the same trans-
energized transformers, except by means of an ex-
former or on test data from similar units.
ternal sampling valve. If the transformer has no
external sampling valve, the unit must first be de-
d. High-potential test. A high potential test is a
energized and a sample taken internally,
voltage applied across an insulation, at or above the
direct-current equivalent of the 60-hertz operating
(1) Obtaining the Liquid. The methods of ob-
crest voltage. The maximum direct-current test
taining liquid samples are covered in ASTM D 923.
voltage for periodic testing between windings, and
Oil samples should be taken from the bottom of the
from winding to ground, should not exceed the origi-
transformer, while less-flammable liquid samples
nal factory alternating-current test voltage. Good
should be taken from the top. The samples should
insulation will exhibit a gradually rising leakage
stand in tightly sealed containers for 24 hours prior
current with an increase in test voltage. If the leak-
to testing.
age current increases rapidly, the test should be
(2) Sampling considerations. The test validity
halted because a breakdown of the insulation is
is dependent upon the validity of the sample. Use
clean, dry, glass containers with nonrubber wax-
sealed stoppers to prevent leakage. Take samples
7-8. Transformer insulating liquids.
when the oil is at least as warm or warmer than the
The insulating liquids used in liquid-immersed
surrounding air, and always on a clear windless day
transformers not only provide insulation, but serve
when the relative humidity does not exceed 70 per-
to transfer heat from the windings. The liquid must
cent. Run about one quart (one liter) of liquid
be kept free of contaminants and moisture, just as
through any supply valve to clean it thoroughly.
the air insulation of dry-type transformers must be
Vent any sealed transformer which has a vacuum.
kept clean and dry; otherwise, the medium's insu-
Place the sample in a refrigerator's freezing com-
lating ability is reduced.
partment overnight. A cloudy sample indicates free
a. Types of liquid. The insulation liquids used by
water and another sample should be taken to deter-
most facilities will be mineral oil, in oil-insulated
mine whether the water was in the sample con-
tainer or in the oil.
transformers, and fire-resistant petroleum (RTemp)


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