the transformer is at operating temperature, preferably
above 40° .
4-5. Drying a Transformer
a. The first step in drying a transformer consists of
(2) Starting with the hot sample, rinse a clean
removing any free water and the water in solution as
dry test tube with the liquid. Fill half full and stir
previously explained in paragraph 4-4. The moisture
continuously with a centigrade thermometer while cooling
to approximately 20° Cool as much as possible in the
remaining in the windings and insulating oil can then be
driven off by heating the transformer. Exercise caution
ambient air and complete the cooling by momentarily
when heating the transformer to avoid damaging the
dipping the test tube in an ice bath, removing and stirring
The maximum winding temperature as
and then redipping, etc.
determined by resistance measurements should not be
(3) Observe the sample carefully and note the
allowed to exceed 950C.
temperature at which initial cloudiness appears. Wipe the
outside of the tube with a clean rag or paper towel to
Any drying method which involves heating
facilitate observation of the slight moisture c(loud that
may form. Compare to clean insulating oil at ambient
an oil-filled transformer when it is exposed
temperature in a similar tube if necessary. Examination
to the atmosphere creates a serious fire
for the presence of a cloud should preferably be made
hazard. No smoking or open flames should
against dark background and not directly into the sunlight.
be permitted near the transformer and
(4) If cloudiness appears at 20°or above, high
suitable fire extinguishers of the carbon
to excessive moisture content is indicated.
dioxide type must be on hand before
(a) Inspect the unit for free water on the
beginning the dryout.
b. Heating the transformer can be accomplished by
(b) If free water is present, remove water
shorting one winding and applying a suitable voltage on
as indicated in paragraph 4-4.
the other. Full-load current can be obtained by applying
(5) If cloudiness does not appear until the
the impedance voltage of the transformer. Be sure to
temperature is below 20° , an acceptable moisture
load the entire winding. If the transformer is at room
content range is indicated.
temperature at the start of drying, 125 per cent load may
be applied until the top liquid temperature reaches 650C.
4-4. Filtering Oil and Filling
At this point, the current should be reduced in accordance
with the following table:
a. If test results indicate that moisture or other
contaminants are present they can usually be removed by
Max. Allowable Short-
passing the liquid through a filter system. Any free water
Circuit Amps. (in Per-
in the transformer should be removed before the filter
cent of Full Load
operation is started.
(in Degrees Centigrade)
b. If it should become necessary to refill
transformer, make sure all joints are tight and vent the
tank to the atmosphere while filling. In order to prevent
aeration. it is preferable to fill the transformer through
Since the windings are at a higher temperature than the
the drain valve with a filter system. Fill to the level
insulating liquid, the insulation may be damaged if these
indicated on the transformer nameplate in accordance
values are exceeded. Filtration during the heat run will
with the prevailing temperature. If tests on the insulating
not greatly hasten the drying process, because at these
oil are satisfactory and no filter system is available fill the
temperatures the filter press loses its ability to remove
transformer through a cover opening. Strain the oil
any appreciable amount of moisture.
through two or more thickness of muslin or other closely
c. The air space in the transformer must be
woven cotton cloth which has been thoroughly washed
thoroughly ventilated to remove the water vapor given off.
and dried to remove the sizing. Use at least one set of
This can be done by removing manhole covers, the
cloths for each transformer.
pressure relief device, or the entire cover. If drying is
c. A transformer contaminated with moisture will not
done indoors, provide good ventilation to drive exhaust
only have moisture suspended in the insulating oil, but
vapors from the room. If the cover is left in place, it
also in the windings and insulation. The most efficient
should be thoroughly insulated to prevent condensation.
temperature for filtering moisture from the transformer is
The required temperature can be more readily obtained
between 20° nd 40° but at this temperature the transfer
by blanketing the transformer with heavy paper, cloth,
of moisture from the windings and insulation to the
building felt, etc.
insulating oil is quite slow. In order to completely dry the
d. Take liquid samples every four hours and make
transformer the filtering operation should be followed by a
tests of the dielectric strength. Samples should be
short-circuit heat run.