What Are Single Phase Pad Mounted Transformers?

Single-phase pad-mounted transformers are usually applied to serve residential subdivisions. Most single phase transformers are manufactured as clamshell, dead-front, loop-type with an internal 200-A primary bus designed to allow the primary to loop through and continue on to feed the next transformer.

These are detailed in the IEEE Standard C57.12.25 (ANSI, 1990). The standard assumes that the residential subdivision is served by a one-wire primary extension. It details two terminal arrangements for loopfeed systems: Type 1 (Figure 2.2.26) and Type 2 (Figure 2.2.27).

Type 1
Type 2
Both have two primary bushings and three secondary bushings. The primary is always on the left facing the transformer bushings with the cabinet hood open, and the secondary is on the right. There is no barrier or division between the primary and secondary.

In the Type 1 units, both primary and secondary cables rise directly up from the pad. In Type 2 units, the primary rises from the right and crosses the secondary cables that rise from the left. Type 2 units can be shorter than the Type 1 units, since the crossed cable configuration gives enough free cable length to operate the elbow without requiring the bushing to be placed as high.

Although not detailed in the national standard, there are units built with four and with six primary bushings. The four-bushing unit is used for single-phase lines, with the transformers connected phase-to-phase. The six-primary-bushing units are used to supply single-phase loads from three-phase taps.

Terminating all of the phases in the transformer allows all of the phases to be sectionalized at the same location. The internal single-phase transformer can be connected either phase-to-phase or phase-to-ground.

The six-bushing units also allow the construction of duplex pad-mounted units that can be used to supply small three-phase loads along with the normal single-phase residential load. In those cases, the service voltage is four-wire, three-phase, 120/240 V.

Cabinets for single-phase transformers are typically built in the clamshell configuration with one large door that swings up. Older units were manufactured with two doors, similar to the three-phase cabinets.

New installations are almost universally dead front; however, live-front units are still purchased for replacements. These units are also built with clamshell cabinets but have an internal box shaped insulating barrier constructed around the primary connections.

1 comment:

  1. what is the limit for 100kVA DT. I have an exisiting 100kva and its load is 99.755kva is this ok


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