[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 2.2.26 Typical Type 1 loop-feed system. (By permission of ABB Inc., Raleigh, NC.)
FIGURE 2.2.29 Radial-style live front. (By permission of Pacific Gas & Electric Company, San Francisco, CA.)
FIGURE 2.2.27 Typical Type 2 loop-feed system. (By permission of ABB Inc., Raleigh, NC.)
FIGURE 2.2.30 Loop-style live front. (By permission of Pacific Gas & Electric Company, San Francisco, CA.)
FIGURE 2.2.28 Single-phase live front. (By permission of Pacific Gas & Electric Company, San Francisco, CA.)
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, as shown in Figure 2.2.26 and Figure 2.2.27. Older units were manufactured with
two doors, similar to the three-phase cabinets. New installations are almost universally dead front;
however, live-front units (see Figure 2.2.28) 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.
2.2.8.2 Three-Phase Pad-Mounted Transformers
Three-phase pad-mounted transformers are typically applied to serve commercial and industrial threephase
loads from underground distribution systems. Traditionally, there have been two national standards
that detailed requirements for pad-mounted transformers — one for live front (ANSI C57.12.22) and
one for dead front (IEEE C57.12.26). The two standards have now been combined into one for all pad
mounts, designated IEEE C57.12.34.
2.2.8.3 Live Front
Live-front transformers are specified as radial units and thus do not come with any fuse protection. See
Figure 2.2.29. The primary compartment is on the left, and the secondary compartment is on the right,
with a rigid barrier separating them. The secondary door must be opened before the primary door can
be opened. Stress-cone-terminated primary cables rise vertically and connect to the terminals on the end
of the high-voltage bushings. Secondary cables rise vertically and are terminated on spades connected to
the secondary bushings. Units with a secondary of 208Y/120 V are available up to 1000 kVA. Units with
a secondary of 480Y/277 V are available up to 2500 kVA.
Although not detailed in a national standard, there are many similar types available. A loop-style live
front (Figure 2.2.30) can be constructed by adding fuses mounted below the primary bushings. Two
primary cables are then both connected to the bottom of the fuse. The loop is then made at the terminal
of the high-voltage bushing, external to the transformer but within its primary compartment.
2.2.8.4 Dead Front
Both radial- and loop-feed dead-front pad-mounted transformers are detailed in the standard. Radialstyle
units have three primary bushings arranged horizontally, as seen in Figure 2.2.31. Loop-style units
have six primary bushings arranged in a V pattern, as seen in Figure 2.2.32 and Figure 2.2.33. In both,
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC