Howard Medium Power Substation - Manual (Page 17)

Medium Power Substation Transformers
Document 2.4.06, Revision 4
June 2013
17
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 the extent of damage to the
transformer, in any.
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 manhole 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.
WARNINg : DO NOT ATTEMPT TO
DISASSEMBlE A PRESSURE RElIEF DEvICE.
TO DO SO COUlD RESUlT IN EQUIPMENT DAMAGE
AND SEvERE PERSONAl INJURY.
PRD’s 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.