External connections
Transformers that require no internal inspection or repair
may be connected to the system. Transformers must be
connected and operated as indicated by the transformer
nameplate. Clean bushings and terminals before making
connections. Remove dirt, grease or foreign material.
Contamination can cause failure of the bushings.
Cables connected to the transformer terminals are to have
sufficient flex to allow normal pad movement due to ground
freeze/thaw and settling. Insufficient cable flex may cause
premature failure of the bushings.
Ground connections
The transformer tank must be connected to a permanent,
low-resistance ground. If the tank is not solidly grounded
and the transformer is connected to the power distribution
system, then the tank should be regarded as energized.
An energized tank is extremely dangerous. Contact with an
energized tank can shock, burn, or cause death.
Three-phase pad-mounted transformers are equipped with
two grounding pads for making the ground connection.
The pads (see Figure 3) are located on the faceplate of the
transformer behind the terminal compartment doors. One
pad is in the primary compartment and the other in the
secondary compartment.
Grounded wye-grounded wye transformer
connections
Grounded wye-grounded wye winding connection
transformers are designed for power distribution systems
that have a grounded neutral connector. All windings
designed for grounded neutral operation must be
permanently and solidly grounded to the system neutral
without electrical resistance.
High voltage connections
Make connections as shown in the diagrams and
information on the transformer nameplate. Transformer
neutrals must be connected to system neutrals. Leads
and connections that are not used must be insulated from
ground and all other leads.
Livefront transformer connections
Externally clamped porcelain bushings are normally provided
for high voltage connection to three-phase pad-mounted
livefront transformers. The bushings have tin-plated
eyebolt terminals suitable for both copper and aluminum
conductors. The bushings are positioned to allow vertical
takeoff of primary cables entering from below.
Deadfront transformer connections
Deadfront three-phase pad-mounted transformers are
equipped with universal bushing wells, one-piece bushings,
or bushing wells with factory-installed inserts. Universal
bushing wells must be field equipped with bushing well
inserts before loadbreak cable connections can be made.
Bushing well inserts must be compatible with the universal
bushing well. Use of improperly rated inserts can cause
premature failure of the transformer or its components.
Read the manufacturer’s instructions furnished with the
inserts before installing to determine if they are compatible.
Low voltage connections
Spade terminals are the standard low voltage connectors
used for three-phase pad-mounted transformers. The low
voltage bushings are externally clamped to the transformer
tank wall.
WARNING
Hazardous voltage. Can cause severe injury, death, or
damage to equipment. Ground transformer following
industry-accepted safe grounding practices before
making other electrical connections. De-energize
power distribution wires that will be connected to
transformer. Verify that wires are de-energized at the
transformer before connecting to transformer.
Figure 3. Grounding pad.
4 Three-phase pad-mounted compartmental type installation and maintenance instructions MN202001EN August 2015
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