28
CAUTION
It is hazardous to attempt drying out a transformer un·
less constant attention is given the job.
An
unattended
transformer could be destroyed if the coils are allowed to
overheat.
A recommended method of drying the windings and
insulation is by circulating current through the windings.
The LV winding should be short-circuited and sufficient
voltage impressed across the high voltage winding to cir
culate enough current through the coils to maintain the
coil temperature at 8 0°C to 9 0°C as measured by winding
resistance. About one-fifth of normal full-rated current is
generally sufficient to do this. The impressed voltage nec
essary to circulate this current varies within wide limits
among different transformers. This voltage will generally
be approximately
I
/2 percent to 1 - 1 /2 percent of the
normal voltage of the winding at normal frequency. The
cooling tubes or radiators should be blanketed to prevent
air circulation and thus heat loss. Otherwise , the power
requirements will be excessive .
The transformer should be placed in its case with the
liquid and with the handhole cover removed to allow free
circulation of the air in the gas space .
For complete information on determination of dryness
and additional methods of drying out, order Instruction
Leaflet 4 8 - 6 2 0-1.
Minimum Insulation Resistance in Oil at 20°C
L-L Voltage
aass KV
Megohms
1 .2
3 2
2. 5
68
5
1 3 5
8 .66
2 3 0
15
4 1 0
2 5
6 7 0
34. 5
9 3 0
4 6
124 0
6 9
1 8 6 0
Fig.
1 8
8.4
Vacuum Filling of Transformer
The evacuation and liquid-filling operations specified in
the following are intended to be accomplished as contin
uous uninterrupted processes.
Insulation Resistance Temperature Correction
Transformer
Correc-
Transformer
Correc-
Tempera-
tion
Tempera-
tion
ture °C
Factor
ture °C
Factor
9 5
8 9 . 0
3 5
2.5
9 0
6 6.0
3 0
1.8
8 5
4 9.0
2 5
1.3
8 0
3 6 .2
2 0
1.0
7 5
2
6
. 8
1 5
0.7 3
7 0
2 0. 0
10
0.5 4
6 5
14. 8
5
0.4 0
6
0
1 1.0
0
0.3 0
5 5
8 . 1
- 5
0.22
s o
6
.
0
- 1 0
0. 1 6
4 5
4.5
- 1 5
0. 1 2
4 0
3.3
Fig.
1 9
During the vacuum and liquid -filling operations, the
temperature of the core and coils must be above 0°C
( 3 2°F). The temperature of the liquid when it is added to
the transformer during vacuum-filling, must be
I
0°C
(S 0°F) or higher.
When vacuum filling a transformer on which a Sudden
Pressure Relay is mounted , care must be taken that the
Relay is not filled with oil. Also pull vacuum and break
vacuum at 1 /4 psi/second maximum to avoid any possi
bility of straining the bellows. The Sudden Pressure Relay
must be correctly installed according to the instructions
in Section 5 . 1 0 to prevent failure during vacuum treat
ment. If the transformer is shipped with a dummy plate
mounted in place of the Sudden Pressure Relay , the
transformer should be filled with liquid before the Relay
is mounted.
CAUTION
Transformer oil should always be handled as a flammable
liquid. It should also be remembered that closed trans
former tanks may under some conditions accumulate ex
plosive gases, and that oil-handling procedures may gen
erate static electricity. Safety precautions should include
purging all gas spaces with nitrogen before oil-fdling or
ftltering and grounding the transformer, its bushings, and
all oil-handling equipment. Otherwise, static electricity
could ignite the oil.