[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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FIGURE 2.2.31 Radial-style dead front. (By permission of Pacific Gas & <strong>Electric</strong> Company, San Francisco, CA.)<br />
FIGURE 2.2.34 Mini three-phase in clamshell cabinet. (By permission of ABB Inc., Raleigh, NC.)<br />
FIGURE 2.2.32 Small loop-style dead front. (By permission of ABB Inc., Raleigh, NC.)<br />
FIGURE 2.2.33 Large loop-style dead front. (By permission of ABB Inc., Raleigh, NC.)<br />
the primary compartment is on the left, and the secondary compartment is on the right, with a rigid<br />
barrier between them. The secondary door must be opened before the primary door can be opened. The<br />
primary cables are terminated with separable insulated high-voltage connectors, commonly referred to<br />
as 200-A elbows, specified in IEEE Standard 386. These plug onto the primary bushings, which can be<br />
either bushing wells with an insert, or they can be integral bushings. Bushing wells with inserts are<br />
preferred, as they allow both the insert and elbow to be easily replaced. Units with a secondary of 208Y/<br />
120 V are available up to 1000 kVA. Units with a secondary of 480Y/277 V are available up to 2500 kVA.<br />
2.2.8.5 Additional Ratings<br />
In addition to what is shown in the national standards, there are other variations available. The smallest<br />
size in the national standards is the 75 kVA unit. However, 45 kVA units are also manufactured in the<br />
normal secondary voltages. Units with higher secondary voltages, such as 2400 and 4160Y/2400, are<br />
manufactured in sizes up to 3750 kVA. There is a new style being produced that is a cross between singleand<br />
three-phase units. A small three-phase transformer is placed in a six-bushing loop-style clamshell<br />
cabinet, as seen in Figure 2.2.34. These are presently available from 45 to 150 kVA in both 208Y/120 and<br />
480Y/277V secondaries.<br />
2.2.8.6 Pad-Mount Common Elements<br />
2.2.8.6.1 Protection<br />
Most distribution transformers include some kind of primary overcurrent protection. For a detailed<br />
discussion, see Section 2.2.13, <strong>Transformer</strong> Protection.<br />
2.2.8.6.2 Primary Conductor<br />
Pad-mounted transformers are designed to be connected to an underground distribution system that<br />
utilizes 200-A-class equipment. The primary is most often #2 or 1/0 cables with 200-A elbows or stress<br />
cones. It is recommended that larger cables such as 4/0 not be used with the 200-A elbows. The extra<br />
stiffness of 4/0 cable makes it very difficult to avoid putting strain on the elbow-bushing interface, leading<br />
to premature elbow failures.<br />
2.2.8.6.3 Pad<br />
Pads are made out of various materials. The most common is concrete, which can be either poured in<br />
place or precast. Concrete is suitable for any size pad. Pads for single-phase transformers are also<br />
commonly made out of fiberglass or polymer-concrete.<br />
2.2.8.6.4 Enclosure<br />
There are two national standards that specify the requirements for enclosure integrity for pad-mounted<br />
equipment: C57.12.28 (ANSI/NEMA, 1999) for normal environments and C57.12.29 (ANSI, 1991) for<br />
coastal environments. The tank and cabinet of pad-mounted transformers are commonly manufactured<br />
out of mild carbon steel. When applied in corrosive areas, such as near the ocean, they are commonly<br />
made out of 300- or 400-series stainless steel. In general, 300-series stainless steel will outperform 400-<br />
series stainless steel, which significantly outperforms mild carbon steel in corrosive applications.<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC