[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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FIGURE 2.2.16 Two-bushing subway. (By permission of Pacific Gas & <strong>Electric</strong> Company, San Francisco, CA.)<br />
FIGURE 2.2.18 Single-phase round. (By permission of Pacific Gas & <strong>Electric</strong> Company, San Francisco, CA.)<br />
FIGURE 2.2.19 Two-primary bushing. (By permission of Pacific Gas & <strong>Electric</strong> Company, San Francisco, CA.)<br />
FIGURE 2.2.17 Three-phase vault. (By permission of Pacific Gas & <strong>Electric</strong> Company, San Francisco, CA.)<br />
2.2.7.2 Surface-Operable Installations<br />
The subsurface enclosure provides the required ventilation as well as access for operation, maintenance,<br />
and replacement, while at the same time providing protection against unauthorized entry. Surfaceoperable<br />
enclosures have grade-level covers that can be removed to gain access to the equipment. The<br />
enclosures typically are just large enough to accommodate the largest size of transformer and allow for<br />
proper cable bending. <strong>Transformer</strong>s for installation in surface-operable enclosures are manufactured as<br />
submersible transformers, which are defined in C57.12.80 (IEEE, 2002b) as “so constructed as to be<br />
successfully operable when submerged in water under predetermined conditions of pressure and time.”<br />
These transformers are designed for loop application and thus require internal protection. Submersible<br />
transformers are designed to be connected to an underground distribution system that utilizes 200-Aclass<br />
equipment. The primary is most often #2 or 1/0 cables with 200-A elbows. While larger cables such<br />
as 4/0 can be used with the 200-A elbows, it is not recommended. The extra stiffness of 4/0 cable makes<br />
it very difficult to avoid putting strain on the elbow-bushing interface, which may lead to early failure.<br />
The operating points of the transformer are arranged on or near the cover. The installation is designed<br />
to be hot-stick operable by a person standing at ground level at the edge of the enclosure. There are three<br />
typical variations of submersible transformers.<br />
2.2.7.2.1 Single-Phase Round Submersible<br />
Single-phase round transformers (Figure 2.2.18) have been used since the early 1960s. These transformers<br />
are typically applied to serve residential single-phase loads. These units are covered by C57.12.23 (IEEE,<br />
FIGURE 2.2.20 Four-primary bushing. (By permission of Pacific Gas & <strong>Electric</strong> Company, San Francisco, CA.)<br />
1992). They are manufactured in the normal single-phase kVA ratings of 25, 37.5, 50, 75, 100, and<br />
167 kVA. Primary voltages are available from 2,400 through 24,940 GrdY/14,400, and the secondary is<br />
240/120 V. They are designed for loop-feed operation with a 200-A internal bus connecting the two<br />
bushings. Three low-voltage cable leads are provided through 100 kVA, while the 167-kVA size has six.<br />
They commonly come in two versions — a two-primary-bushing unit (Figure 2.2.19) and a four-primarybushing<br />
unit (Figure 2.2.20) — although only the first is detailed in the standard. The two-bushing unit<br />
is for phase-to-ground-connected transformers, while the four-bushing unit is for phase-to-phase-connected<br />
transformers. As these are designed for application where the primary continues on after feeding<br />
through the transformer, the transformers require internal protection. The most common method is to<br />
use a secondary breaker and an internal nonreplaceable primary-expulsion fuse element. These units are<br />
designed for installation in a 36-in.-diameter round enclosure. Enclosures have been made of fiberglass<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC