Howard HI-104 Single-Phase Pad Transformer - Manual (Page 24)

Document 2.4.96, Revision 0
March, 2013
24
Single-Phase Pad-Mounted Compartmental-Type Distribution Transformers
SECTION 6: OPERATION OF TERMINAL COMPARTMENT, BUSHINGS, GAUGES AND
ACCESSORY DEVICES
Some of the 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 user specifications.
Hot-Stick Operable Devices
Some devices such as draw-out expulsion fuses,
dry-well canister fuses, dead-front high-voltage elbow
terminations, rotary load-break switches and auto-
matic pressure relief valves are designed to be oper-
ated with a live-line tool (hot stick or shotgun stick).
Do not attempt to operate by hand any device that is
designed to be operated with a live-line tool. Inspect,
test and operate the live-line tool according to the in-
structions provided by the live-line tool manufacturer.
Pressure-Vacuum Gauge
The pressure-vacuum gauge is a dial-type instru-
ment that indicates the pressure in the tank gas
space relative to atmospheric pressure. The gauge is
mounted on the front panel in the terminal compart-
ment above the fluid level. Pressure in the tank will
normally vary as a function of transformer and ambi-
ent temperatures. If the transformer is lightly loaded
or de-energized during times of low ambient temper-
ature, the gauge may indicate a negative pressure.
The pressure-vacuum gauge may be provided with
optional switch contacts, which can be used to pro-
vide a remote alarm.
▲
! WARNING
FAILURE TO FOLLOW THE INSTRUCTIONS BELOW
COULD RESULT IN DEATH OR SERIOUS PERSON-
AL INJURY, AND COULD ALSO RESULT IN DAM-
AGE TO THE EQUIPMENT.
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.
Fluid Level Gauge and Sight Plug
The optional 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 read-
ing should be investigated and corrected. A low fluid
level can cause overheating of the transformer and
can compromise the insulation system.
The fluid level gauge may be provided with optional
switch contacts, which can be used to provide a re-
mote alarm of low fluid level. Transformers may also
be provided with an optional sight plug instead of a
gauge to allow direct observation of the fluid level.
Fluid Temperature Gauge
The optional fluid temperature gauge is a dial-type
bi-metal instrument that indicates the fluid tempera-
ture at the top of the fluid column. 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.
The fluid level gauge may also be provided with
switch contacts, which can be used to provide a
remote alarm 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 (usually
85°C rise). Refer to IEEE Standard C57.91 Guide for
Loading Mineral-Oil-Immersed Transformers for load-
ing recommendations.
Drain Valve and Sampling Device
The optional drain valve and sampling device permits
draining the transformer fluid and sampling the fluid
for testing purposes. The valve is located in the ter-
minal compartment at the bottom of the front panel.