22
The use of pressure gauges containing mercury should
be avoided unless an effective trap is placed between the
gauge and the transformer because of the possibility of
an accident resulting in mercury getting into the trans
former. For measuring vacuum, properly calibrated ther
mocouple gauges or aneroid absolute pressure gauges are
preferred.
In
most cases, it is not necessary nor desirable to open
the transformer tank . There are no internal braces to be
removed.
The voltages of the high voltage circuit and the low
voltage circuit must match the transformer voltages as
shown on the nameplate. The tap changers must be set
on the correct positions. The transformers must also be
phased out correctly before closing the circuits. Ener
gizing the transformer at the wrong voltage or out of
phase may result in damage requiring complete rebuild
ing of the internal parts.
6.2 Location and Mounting
Transformers must be placed on a foundation of suffi
cient strength to support the weight of the unit. The
foundation must be level. If the unit is not level, oil may
not circulate through all the cooling tubes. The over
heating that can result will shorten transformer life.
When a transformer is designed to allow operation at a
tilt, the degree of tilt will be noted on the transformer
nameplate. The location of the transformer, whether in
door or outdoor, should provide for adequate accessibility ,
ventilation and ease of inspection for the unit. To assure
proper air circulation for cooling, the transformer coolers
should be at least
24
inches
(61 0
mm) from any obstruc
tion. Location in areas of corrosive chemicals should be
avoided.
6.3 Assembly
Whenever possible, transformers are shipped completely
assembled, liquid filled, with all accessories installed. If
it should be necessary to open the tank to install bushings
or accessories, follow the precautions in Sections
2.5
and
6. 1
of this book.
CAUTION
The transformer may contain positive internal pressure.
Release pressure before removing manhole bolts. Internal
pressure can blow the manhole cover off in
j
uring person
nel.
If possible, fill transformers shipped in dry gas with
fluid before opening the tank. When the transformer can-
not be filled with fluid before the tank is opened, limit
the total open time to
24
hours. Refer to Sections
8.4
through
8.6
for instructions on liquid filling and ventila
tion of transformer tanks.
TO AVOID DEATH FROM SUFFOCATION NEVER
ALLOW ANYONE TO ENTER THE TRANSFORMER
TANK UNLESS AN ANALYSIS OF THE AIR IN THE
TANK SHOWS AT LEAST
1 9 .5%
OXYGEN. THE GAS
SPACE OF AN OPERATING TRANSFORMER CON
SISTS OF NITROGEN GAS. WHENEVER ANYONE IS
IN THE TANK, A PERSON SHOULD BE STATIONED
AT THE MANHOLE OUTSIDE THE TANK TO INSURE
SAFETY OF THE PERSON INSIDE.
Instructions for the installation of bushings are given in
Sections
5.13 . 1
through
5.13 .3
of this book. Complete
the assembly of all items, such as bushings, which require
work inside the transformer tank or through an open man
hole as rapidly as careful workmanship will permit. When
work is complete, all openings must be sealed. For gasket
ing instructions, see Section
8.7.
6.4 Filling in the Field
When transformers are shipped filled only with dry gas,
they must be filled under vacuum according to the in
structions in Section
8.4,
"Vacuum Filling of Trans
formers". Do not exceed the maximum pressure (positive
or negative) that the transformer tank is designed to with
stand as shown on the nameplate. To do so may cause the
tank to collapse.
6 .5 Air Terminal Chamber and Switchgear Adapter
Installation
Air terminal chambers are designed to provide adequate
electrical insulation by maintaining a clean dry environ
ment around connections and to protect personnel from
dangerous high voltage. Installed chambers must be
sealed tight to assure safe operation.
Poor installation may allow water and other contamina
tion to enter the chamber which can cause exposed ter
minals to flashover, cause transformer failure, and put
personnel in danger.
The unit substation-switchgear adapter is designed to
permit greater flexibility in transformer design while al
lowing the designer of metal clad switchgear to use stand-