Document 2.4.126, Revision 4
October 2017
22
Three-Phase Padmounted Distribution Transformers
Many of the accessory devices described below are
optional and may not be present in any particular
transformer design. The inclusion of particular
accessory devices in any transformer design is
governed by industry standards and by individual
customer specifications.
The transformer nameplate, supplied on each
transformer according to IEEE Standard C57.12.00,
provides a circuit diagram and certain physical and
electrical information.
Pressure-Vacuum Gauge
The pressure-vacuum gauge is a dial-type
instrument that indicates the pressure in the tank
gas space relative to atmospheric pressure. The
gauge is mounted on the front panel in the terminal
compartment above the fluid level. Pressure in the
tank will normally vary as a function of transformer
and ambient temperatures. If the transformer is
lightly loaded or de-energized during times of low
ambient temperature, the gauge may indicate a
negative pressure.
Pressure-vacuum gauges can be supplied with
optional switch contacts, which can be used
to provide remote alarm of positive or negative
pressure.
Fluid Level Gauge and Sight Plug
The fluid level gauge is a dial-type device that
indicates the fluid level inside the transformer tank.
The gauge is mounted on the front panel in the
terminal compartment at the normal 25°C fluid
level.
If the gauge reads “LOW,” the cause of the low
reading should be investigated and corrected. A low
fluid level can cause overheating of the transformer
and can compromise the insulation system.
Fluid level gauges can be supplied with optional
switch contacts, which can be used to provide a
remote alarm of low fluid level.
Transformers may also be supplied with a sight
plug, which provides direct observation of the fluid
level.
Fluid Temperature Gauge
The fluid temperature gauge is a dial-type bi-metal
instrument that indicates the fluid temperature at
the top of the fluid column in degrees centigrade.
The temperature gauge is mounted on the front
panel in a dry leak-proof well, permitting removal of
the gauge without exposure to the tank fluid.
The gauge may be furnished with a red drag-hand
pointer that indicates the maximum temperature
reached since it was last reset. The drag-hand can
be reset by rotating the magnet at the center of the
dial or, on some types, by pressing a reset button.
Fluid level gauges may also be supplied with switch
contacts, which can be used to provide a remote
alarm indication of low or high temperature, or to
energize a fan control circuit.
During normal operation the fluid temperature
gauge should read less than the sum of the
ambient temperature and the rated temperature
rise (normally 85°C). Refer to IEEE Standard C57.91
for loading recommendations.
Drain Valve and Sampling Device
The drain valve and sampling device permits
draining the transformer fluid and sampling the fluid
for testing purposes. The valve is located in the
SECTION 6: OPERATION OF BUSHINGS, GAUGES
AND OTHER ACCESSORY DEVICES
WARNING
If the pressure-vacuum gauge constantly
reads zero under varying load and ambient
conditions, the transformer should be
checked for a possible tank leak. A leak
will allow moisture and air to enter the
transformer tank, which could degrade the
paper insulation and insulating fluid. Left
unrepaired, a leak could limit transformer
life or cause a violent failure. FAILURE
TO FOLLOW THESE INSTRUCTIONS MAY
RESULT IN DEATH, SEVERE PERSONAL
INJURY AND/OR DAMAGE TO THE
EQUIPMENT.
