Howard Medium Power Substation - Manual (Page 16)

Document 2.4.06, Revision 4
June 2013 Medium Power Substation Transformers
Refer to the outline drawing for a list of accessories
furnished with the transformer.
Transformer Nameplate
A nameplate is supplied on each transformer according
to ANSI standard C57.12.00. The nameplate provides
certain physical and electrical information about the
transformer.
Control Cabinet
Refer to the wiring diagram for a description of electrical
circuits inside the control cabinet. Use caution when
testing alarm switches to prevent damage to the switch.
DANgER: THE CONTROl CABINET
CONTAINS DANGEROUS vOlTAGES. DE-
ENERIGZE THE CONTROl CABINET vOlTAGE
SUPPlY PRIOR TO PERFORMING WORK INSIDE THE
CONTROl CABINET OR ON EXTERNAl ACCESSORIES
CONNECTED TO EQUIPMENT WITHIN THE CONTROl
CABINET. FAIlURE TO DO SO COUlD RESUlT IN
SEvERE PERSONAl INJURY, DEATH, OR PROPERTY
DAMAGE.
Alarm Switch Ratings
When control wiring or alarm contacts are provided, refer
to the control wiring diagram, or the outline drawing for
contact type, contact ratings, and terminal locations.
Current Transformers
Current transformers (if provided) are mounted inside
the transformer tank around the transformer line leads.
For cover-mounted bushings, current transformers are
located underneath the transformer cover. For tank-wall
mounted bushings, current transformers are usually
located on top of the core and coil assembly. Current
transformer leads are always terminated inside a
junction box. Refer to the transformer wiring diagram to
identify the wire markings. Current transformer leads
are always shorted and grounded to the tank at the
factory before shipment.
WARNINg : CURRENT TRANSFORMER
SECONDARIES MUST BE CONNECTED TO A
lOAD OR SHORT CIRCUITED TO AvOID DAMAGING
vOlTAGES AT THE TERMINAlS. FAIlURE TO MAKE
THESE CONNECTIONS COUlD RESUlT IN SEvERE
PERSONAl INJURY, DEATH, OR PROPERTY DAMAGE.
Pressure/vacuum gauge
The pressure/vacuum gauge indicates the pressure in
the tank gas space relative to atmospheric pressure.
The pressure will normally vary as a function of
transformer temperature. 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 an
optional alarm/signal switch. Refer to the schematic
wiring diagram furnished with the transformer.
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,
WHICH COUlD DEGRADE THE INSUlATION AND
FlUID. lEFT UNREPAIRED, lEAKS COUlD lIMIT
TRANSFORMER lIFE OR CAUSE A vIOlENT FAIlURE
THAT COUlD RESUlT IN SEvERE PERSONAl INJURY,
DEATH, OR EQUIPMENT DAMAGE.
Automatic Pressure Relief Device
Medium power transformers are usually furnished with
a weatherproof, cover-mounted automatic pressure
relief device (PRD) to relieve excessive tank pressures
that might occur during operation of the transformer.
The device consists of a self-resetting, spring-loaded
diaphragm and a visual indicating flag. When gas
pressure in the tank exceeds the PRD’s specified limit,
the gas pressure will open the diaphragm, vent the
excess pressure, and trip the visual indicating flag.
After the internal pressure decreases below the PRD
reseal rating, the valve will automatically close and
reseal the transformer. The visual indicating flag must
be manually reset. The cause of PRD activation should
always be investigated, since pressure venting indicates
a potential problem inside the transformer.
COMPONENTS AND ACCESSORIES
16