Howard HI-102 Overhead Transformer - Manual (Page 19)

Document 2.4.95, Revision 0
May, 2013
19
Fluid-Filled Overhead Distribution Transformers
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 low-voltage circuit breakers,
switches and automatic pressure relief valves are
designed to be operated 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 instructions provided by the live-line
tool manufacturer.
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 transformer tank 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.
The pressure-vacuum gauge may be provided with
optional switch contacts, which can be used to
provide a remote alarm.
▲
! WARNING
FAILURE TO FOLLOW THE INSTRUCTIONS BE-
LOW COULD RESULT IN MINOR OR MODERATE
PERSONAL INJURY, AND COULD ALSO RESULT
IN DAMAGE 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 fluid level gauge is a dial-type device that
indicates the fluid level inside the transformer tank.
The gauge is mounted on the transformer tank at
the normal 25°C fluid level. Transformers may be
provided with a fluid sight plug instead of a fluid level
gauge to allow direct observation of the fluid level.
The top fluid level should be visible in the sight plug.
If the gauge or sigh plug indicates a low fluid level, 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. The fluid level gauge may be provided with
optional switch contacts, which can be used to
provide a remote alarm of low fluid level.
Be aware that fluid temperature and orientation of
the transformer tank will cause the fluid level to vary.
Transformers are filled to a level that corresponds to
a fluid temperature of 25°C. The actual fluid level will
increase with increasing temperature. The fluid level
indication will also vary when the transformer is not
installed in a level orientation.
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. The temperature gauge is
mounted on the transformer tank near the top of the
fluid column.
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 temperature 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 (normally
a sum of 85°C). Refer to IEEE Standard C57.91,
IEEE Guide for Loading Mineral-Oil-Immersed
Transformers and Step-Voltage Regulators, for loading
recommendations.
SECTION 6: OPERATION OF BUSHINGS, GAUGES AND
ACCESSORY DEVICES