Filtering Oil and Filling

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

insulation.

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

WARNING

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 20or 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.

bottom.

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-

MaxTop-Oil

in the transformer should be removed before the filter

cent of Full Load

Temperature-

operation is started.

(in Degrees Centigrade)

b. If it should become necessary to refill

100

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.

4-2