[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

• Vault-type transformers are suitable for occasional submerged operation.
• Subway transformers are suitable for frequent or continuous submerged operation.
From the definitions, the vault type should only be used when a sump pump is installed, while the
subway-type could be installed without a sump pump. The principal distinction between vault and subway
is their corrosion resistance. For example, the 1994 version of the network standard, C57.12.40, required
the auxiliary coolers to have a corrosion-resistance equivalence of not less than 5/16 in. of copper-bearing
steel for subway transformers but only 3/32 in. for vault-type transformers. In utility application, vault
and subway types may be installed in the same type of enclosure, and the use of a sump pump is predicated
more on the need for quick access for operations than it is on whether the transformer is a vault or
subway type.
2.2.7.1.1 Transformers for Vault Installation
2.2.7.1.1.1 Network Transformers — As defined in IEEE C57.12.80 (IEEE, 2002b), network transformers
(see Figure 2.2.13) are designed for use in vaults to feed a variable-capacity system of interconnected
secondaries. They are three-phase transformers that are designed to connect through a network protector
to a secondary network system. Network transformers are typically applied to serve loads in the downtown
areas of major cities. National standard C57.12.40 (ANSI, 2000a) details network transformers. The
standard kVA ratings are 300, 500, 750, 1000, 1500, 2000, and 2500 kVA. The primary voltages range
from 2,400 to 34,500 V. The secondary voltages are 216Y/125 or 480Y/277.
Network transformers are built as either vault type or subway type. They incorporate a primary switch
with open, closed, and ground positions. Primary cable entrances are made by one of the following
methods:
FIGURE 2.2.14 Single-phase subway. (By permission of Pacific Gas & Electric Company, San Francisco, CA.)
• Wiping sleeves or entrance fittings for connecting to lead cables — either one three-conductor or
three single-conductor fittings or sleeves
• Bushing wells or integral bushings for connecting to plastic cables — three wells or three bushings
2.2.7.1.1.2 Network Protectors — Although not a transformer, the network protector is associated with
the network transformer. The protector is an automatic switch that connects and disconnects the
transformer from the secondary network being served. The protector connects the transformer when
power flows from the primary circuit into the secondary network, and it disconnects upon reverse
power flow from the secondary to the primary. The protector is described in C57.12.44 (IEEE, 2000c).
The protector is typically mounted on the secondary throat of the network transformer, as shown in
Figure 2.2.13.
FIGURE 2.2.13 Network transformer with protector. (By permission of Pacific Gas & Electric Company, San
Francisco, CA.)
FIGURE 2.2.15 Three-bushing subway. (By permission of Pacific Gas & Electric Company, San Francisco, CA.)
2.2.7.1.1.3 Single-Phase Subway or Vault Types — These are round single-phase transformers designed to
be installed in a vault and capable of being banked together to provide three-phase service (Figure 2.2.14).
These can be manufactured as either subway-type or vault-type transformers. They are typically applied
to serve small- to medium-sized commercial three-phase loads. The standard kVA ratings are 25, 37.5,
50, 75, 100, 167, and 250 kVA. Primary voltages range from 2,400 to 34,500 V, with the secondary voltage
usually being 120/240. Four secondary bushings allow the secondary windings to be connected in parallel
for wye connections or in series for delta connections. The secondary can be either insulated cables or
spades. The units are designed to fit through a standard 36-in.-diameter manhole. They are not specifically
covered by a national standard, however they are very similar to the units in IEEE C57.12.23 (IEEE,
2002c). Units with three primary bushings or wells, and with an internal primary fuse (Figure 2.2.15),
allow for connection in closed-delta, wye, or open-wye banks. They can also be used for single-phase
phase-to-ground connections.
Units with two primary bushings or wells, and with two internal primary fuses (Figure 2.2.16) allow
for connection in an open-delta or an open-wye bank. This construction also allows for single-phase
line-to-line connection.
2.2.7.1.1.4 Three-Phase Subway or Vault Types — These are rectangular-shaped three-phase transformers
that can be manufactured as either subway-type or vault-type. Figure 2.2.17 depicts a three-phase vault.
These are used to supply large three-phase commercial loads. Typically they have primary-bushing well
terminations on one of the small sides and the secondary bushings with spades on the opposite end.
These are also designed for radial installation and require external fusing. They can be manufactured in
any of the standard three-phase kVA sizes and voltages. They are not detailed in a national standard.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC