Mobile transformers or mobile substations can be used to
provide temporary service during equipment maintenance, construction,
emergency, or high load periods. Sufficient mobile units strategically placed
can reduce or eliminate the requirements for on-site spare transformers.
Several aspects should be considered in applying mobile
transformers or substations:
1. Size and maneuverability of the equipment
2. Installation location and provisions
3. Electrical clearances
4. Primary and secondary connections
5. Grounding
7. Safety
Size and
Maneuverability of the Equipment
One of the primary advantages of mobile equipment is its
ability to be used at more than one location. To accommodate installation,
adequate space has to be available to position and connect the equipment at all
intended locations.
It may be impossible to use larger units in some locations
without substantial modifications because of the lack of sufficient space.
Substation entrances and access roads should be evaluated before committing
particular equipment to the location in question. Prior planning can save much
time and facilitate installation.
Installation Location
and Provisions
The mobile transformer or substation location should permit
primary and secondary connections as short as possible to the permanent
substation equipment. It is desirable to utilize bare conductors for the
connections. Sometimes, insulated cables can be used where electrical
clearances cannot be maintained or where connections are long.
The location should permit any required connections to be
made quickly and safely without disturbing adjacent equipment. The ease and
speed of installation can be influenced by the proximity of energized
equipment. Substations for which mobile equipment has been designated should
have provisions for installation of the equipment.
The provisions can simply be terminals on permanent
substation equipment or buses for connecting the mobile equipment. It may be
desirable to include bus extensions and/or disconnect switches in some
substations to facilitate the connections, particularly if they may be made
while the substation is energized.
If low-voltage ac or dc supplies are required, permanent
facilities can be provided in the vicinity of where the mobile equipment will
be positioned. A weatherproof cabinet containing any necessary terminal blocks,
switches, or protective devices can be provided for terminating the low-voltage
circuits.
Temporary connections can be made from this cabinet to the
control cabinet on the mobile equipment. Connections into the substation alarm
system can also be provided in this or another cabinet. Terminal blocks, test
switches, indicating lamps, or any other necessary equipment can be located in
the cabinet. Provisions for grounding the equipment can consist of terminals or
ground rods connected to the main grounding grid.
Electrical Clearances
Maintaining adequate electrical clearances between the
mobile equipment, its connections, and other equipment is of prime importance.
Installation using bare conductors should not be considered for a location
unless the minimum clearances listed in Tables 4-7 and 4-8 can be maintained.
Insulated conductors can be used in some locations if the minimum clearances
cannot be maintained.
Primary and Secondary
Connections
All primary and secondary connections should be as short as
possible and should be made with bolted connections. If possible, use bare
conductors. However, for situations where minimum electrical clearances cannot
be maintained or where connections are long, insulated conductors can be
employed.
Conductors used should be sized to carry the maximum loads
expected without overheating and to sustain anticipated fault currents without
damage. They should be checked for sufficient length before connecting either
end. Temporary poles or structures may be required in some locations to
facilitate the connections and maintain clearances. It is desirable to store
any necessary equipment not part of the mobile unit at the substations, where
required.
Grounding
Adequate grounding of mobile transformers and substations is
extremely important for safe operation. At least two independent connections
should be made between the trailer and the ground system. The mobile equipment
should be connected to the substation ground grid whenever it is close to the
substation. In situations where the mobile is located a long distance from the
substation and connection to the substation ground grid is impractical, a
separate ground system has to be provided.
Auxiliary System
Requirements
Mobile unit transformers are usually designed for
forced-cooled operation. Some units can provide the low voltage necessary for
auxiliary equipment operation through the use of on-board supply transformers
and equipment. For units without these provisions, low-voltage supplies can be
obtained from the substation station service system.
Before the substation station service system is used to
supply mobile unit auxiliary systems, the voltage(s) required by the auxiliary
systems has to be checked against those available at the substation for
compatibility. The system should also be checked for adequate capacity. If an
external dc supply is necessary for power or control applications, the
substation control battery can be used. The system should be checked for proper
voltage and adequate capacity prior to utilization.
Safety
Unless the mobile equipment is completely contained within
another fenced area, a separate fence should be provided to surround the
equipment. The fence has to provide the same security and protection as would a
permanent substation fence. Gates should be provided with adequate locking
facilities.
Mobile equipment usually requires some assembly during
installation. Barriers and supports may require installation. Some supporting
members or braces used to protect the equipment during transit may have to be removed.
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