[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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