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.

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.

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.

Assembly and installation should be in strict accordance with the manufacturer’s instructions. The equipment should be positioned on a level site and blocked to prevent movement. Ground slope at the installation location should not exceed the manufacturer’s recommendations.

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