Howard Small Power - Manual (Page 12)

Document 2.4.54 Revision 0
July 2006 Small Power Substation Transformers
12
PRDs can be supplied with an optional alarm/signal
switch. Refer to the schematic wiring diagram
furnished with the transformer.
Fluid Level Gauge
The fluid level gauge indicates the fluid level inside the
tank. The transformer tank is filled with fluid at the
factory to the normal level at a reference temperature
of 25°C.
Fluid Temperature Gauge
The fluid temperature gauge indicates the fluid
temperature at the top of the fluid column. The
temperature gauge is mounted in a dry, leak proof
well, permitting removal of the gauge without lowering
the fluid level in the tank. The gauge is furnished with
a resettable drag-hand pointer which 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 models, by pressing a
reset button.
During normal operation the fluid temperature
gauge should read less than the sum of the ambient
temperature and the rated temperature rise. For
example, a 50°C gauge reading at an ambient
temperature of 25°C yields a top oil temperature of
75°C, which is below the transformer’s 85°C rated
temperature rise.
Winding Temperature Gauge
Transformers may be furnished with a gauge that
displays estimated winding hot-spot temperature. The
gauge uses a probe that measures the temperature of
the transformer fluid plus auxiliary heat from a heating
element that is energized by a current transformer
mounted inside the transformer tank.
The gauge’s temperature probe is mounted in a dry
well, permitting removal of the gauge without lowering
the fluid level. The gauge is furnished with a resettable
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 models, by means of a
pushbutton.
Winding temperature gauges can be supplied with an
alarm/signal switch. Refer to the schematic wiring
diagram furnished with the transformer.
Thermal Overload Relay
Transformers may be supplied with a thermal overload
relay which is similar in operation to the winding
temperature gauge described above. The relay provides
alarm/signal contacts that close at approximately 100%
and 110% rated load.
Rapid Pressure Rise Relay
The optional rapid pressure rise relay (RPRR) monitors
the rate of pressure rise in the transformer’s gas space
and signals when the rate exceeds a safe threshold.
This signal can trigger circuit breaker operation to
de-energize the transformer and can send an alarm.
The RPRR will not be actuated by normal pressure
variations during operation of the transformer. A
seal-in relay latches the output relay and illuminates
an indicator lamp until the unit is manually reset. The
seal-in relay and associated circuitry is mounted in a
separate control cabinet.
If the RPRR has activated, internal arcing has likely
occurred in the transformer tank, and the cause of such
arcing must be investigated. The following steps are
suggested to determine if there are any damages to the
transformer and if so, to what extent.
1. Using a combustible gas detector, check for
combustible gas products of decomposition
in the gas space. Refer to the operating
instructions that came with the gas detector.
2. Test the RPRR and seal-in relay panel to make
sure they are operating properly. Refer to
instructions furnished with RPRR and seal-in
relay panel.
3. Test for insulation power factor, insulation
resistance tests, and transformer turns ratio.
4. Remove the hand hole cover and inspect the
tank interior for appearance and smell. A
burning odor may be present.
5. Make any other tests which may be appropriate
considering results of the tests listed above.
Transformer Cooling Fans
Optional cooling fans may be provided to increase
the transformer load capability without overheating
the transformer windings. Operation of the fans is
controlled by a contact in the fluid temperature gauge
or winding temperature gauge (when furnished) and
by an “auto/manual” control switch. Controls for the
fans are contained in a control cabinet mounted on the
transformer.