Advertisement
ALUMINUM TAP TERMINALS
Ventilated dry-type transformers that have aluminum windings are
equipped with aluminum tap terminals. Spring washers or spring
lock washers are included in the tap terminal hardware. When taps
are changed, the nuts should be tightened until these washers are
flattened.
Washers are flattened when there is a noticeable
increase in bolting torque.
The terminals as received are free of oxides and taps may be
changed at installation without any special precautions. If at some
future
time taps are changed,
recommended.
NOTE: Do not bend tap terminals or position the hardware in a
manner that would reduce clearance between or cause damage to
tap terminals.
1.
Apply a light coat of silicone grease (General Electric G623 or
equivalent) to contact surfaces on terminals and links.
2.
Scrub the contact areas with a wire brush, taking care not to
gouge the metal surfaces.
oxides that may have formed.
3.
Wipe clean and reapply a thin film of silicone grease.
4.
Bolt connecting link or jumper in the new position and tighten
the nut until the washer is flattened.
ENERGIZATION AND OPERATION
Only qualified personnel should perform the tests before energizing
the transformer and placing it in service. Disconnect ALL high voltage,
low voltage and neutral connections. Disconnect all auxiliary
equipment such as lightning arresters, the fan and temperature
control system, meters, or any other low voltage control system that is
connected to the windings that are involved in these tests. Please
disconnect CPT's and short the secondary of CT's if provided.
Before energizing a dry-type transformer either initially or after any
shutdown period in which the unit has cooled to ambient
temperature, the transformer should be inspected for evidence of
moisture and the insulation resistance should be checked.
should be a one minute test taken with a 500 volt megohmmeter at
approximately 25 degrees C measured from winding to ground with
the windings not being tested connected to ground.
insulation resistance in megohms equal to 200 times (rated
winding volts/1000) is a satisfactory value of resistance to permit
energizing the transformer.
minimized during shutdown periods by using strip heaters to keep
the transformer temperature above ambient.)
If acceptance test is part of customer approval process then
MADDOX recommends doing Applied potential or AC Hi-pot test @
75 % of the actual ANSI levels.
DRYING
If moisture is found on the windings, leads, or insulation or if the
insulation
resistance
measurements
transformer should be dried. To dry, put a short circuit heat run on
the unit by short circuiting the low-voltage leads and impressing
impedance voltage on the high-voltage terminals.
necessary to impress a voltage higher than the impedance voltage in
order to heat the windings sufficiently, but the total temperature of
the following procedure is
This abrading will break up any
(Absorption of moisture can be
are
unsatisfactory,
It may be
the windings as determined by resistance should not exceed 190
C (or 185 C by non-mercury type thermometer placed in the
ducts between windings and in contact with them).
The heat run should be continued for a minimum of eight hours.
Insulation resistance is a function of insulation temperature as
well as moisture content, thus for comparison, readings must be
taken at the same temperature. During the course of a drying heat
run , the insulation resistance may initially decline as the insulation
heats up. After the insulation temperature stabilizes, the insulation
resistance will began to increase. When the insulation resistance
stabilizes, the drying procedure should be terminated.
transformer should then be allowed to cool to ambient temperature
after which the insulation resistance should again be measured and
compared with minimum requirements.
An alternate to the short circuit drying heat run is to use a procedure
in which hot air is directed into the bottom of all windings.
The temperature of this air should not exceed 150° C but should
be as close to this temperature as possible to minimize the drying
time. If the transformer is equipped with fans, they should be
removed to prevent damage to the fan motors by exposure to this
hot air.
Oven drying is not recommended because an oven temperature
that would be effective in a reasonable length of time could
damage the tin plating on the current carrying terminal surfaces.
For further information on drying, refer to ANSI Guide C57.94,
"Guide for Installation and Maintenance of Dry-Type Transformers".
FAN COOLING
NOTE: Fans are for temporary overloading only.
When fans are provided they are intended for use only when
the transformer is required to carry a load in excess of its self-
cooled rating.
transformer
is carrying a load significantly less than its self-
cooled rating.
insulation
system
moisture
absorption
insulation system.
MAINTENANCE
This
Refer to the "ENERGIZATION AND OPERATION" section.
The transformer should be inspected at periodic
An
and corrective measures taken when necessary. The frequency at
which these inspections are made depends on operating conditions
and the importance of continuity of service. In clean, dry locations
an annual inspection may be adequate. In areas where the air is
contaminated with dust or chemical fumes, three-month intervals
may be required.
To inspect the transformer, de-energize, lockout and properly
ground all circuits supplying power to the transformer and
it's accessories
CAUTION: Failure to de-energize the transformer and it's
accessories before removing the front and rear panels may
result in serious personal injury or death. Inspection should be
made for accumulation of dirt on insulating surfaces and in places
which would restrict the flow of air, for loose connections, for the
the
condition of terminal boards and for the general condition of the
transformer. Observation should also be made
overheating and of voltage creepage over insulating surfaces
as indicated by tracking or carbonizing.
- 5 -
Fans
should
not
be
Such operation may keep some parts of the
at
a temperature low enough to permit
and
may result in serious injury to the
and
remove the front
MIT100-15A
The
operated
when
the
intervals
and rear panels.
for
signs
of
6/2011
Advertisement
