[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 2.2.3 Single-phase transformer with composite hood. (By permission of ABB Inc., Jefferson City, MO.)
FIGURE 2.2.4 Cross section of enameled round wire after flattening. (By permission of ABB Inc., Jefferson City, MO. )
2.2.2.9.2 Vacuum Processing
With the coil still warm from the bonding process, transformers are held at a high vacuum while oil
flows into the tank. The combination of heat and vacuum assures that all moisture and all air bubbles
have been removed from the coil, providing electrical integrity and a long service life. Factory processing
with heat and vacuum is impossible to duplicate in the field or in most service facilities. Transformers,
if opened, should be exposed to the atmosphere for minimal amounts of time, and oil levels should never
be taken down below the tops of the coils. All efforts must be taken to keep air bubbles out of the
insulation structure.
2.2.3 General Transformer Design
2.2.3.1 Liquid-Filled vs. Dry Type
The vast majority of distribution transformers on utility systems today are liquid-filled. Liquid-filled
transformers offer the advantages of smaller size, lower cost, and greater overload capabilities compared
with dry types of the same rating.
FIGURE 2.2.5 Three- and four-legged stacked cores and five-legged wound core. (From IEEE C57.105-1978, IEEE
Guide for Application of Transformer Connections in Three-Phase Distribution Systems, copyright 1978 by the
Institute of Electrical and Electronics Engineers, Inc. The IEEE disclaims any responsibility or liability resulting from
the placement and use in the described manner. Information is reprinted with the permission of the IEEE.)
2.2.3.2 Stacked vs. Wound Cores
Stacked-core construction favors the manufacturer that makes a small quantity of widely varying special
designs in its facility. A manufacturer that builds large quantities of identical designs will benefit from
the automated fabrication and processing of wound cores. Figure 2.2.5 shows three-phase stacked and
wound cores.
2.2.3.3 Single Phase
The vast majority of distribution transformers used in North America are single phase, usually serving
a single residence or as many as 14 to 16, depending on the characteristics of the residential load. Singlephase
transformers can be connected into banks of two or three separate units. Each unit in a bank
should have the same voltage ratings but need not supply the same kVA load.
2.2.3.3.1 Core-Form Construction
A single core loop linking two identical winding coils is referred to as core-form construction. This is
illustrated in Figure 2.2.6.
2.2.3.3.2 Shell-Form Construction
A single winding structure linking two core loops is referred to as shell-form construction. This is
illustrated in Figure 2.2.7.
2.2.3.3.3 Winding Configuration
Most distribution transformers for residential service are built as a shell form, where the secondary
winding is split into two sections with the primary winding in between. This so-called LO-HI-LO
configuration results in a lower impedance than if the secondary winding is contiguous. The LO-HI
configuration is used where the higher impedance is desired and especially on higher-kVA ratings where
higher impedances are mandated by standards to limit short-circuit current. Core-form transformers are
always built LO-HI because the two coils must always carry the same currents. A 120/240 V service using
a core-form in the LO-HI-LO configuration would need eight interconnected coil sections. This is
considered too complicated to be commercially practical. LO-HI-LO and LO-HI configurations are
illustrated in Figure 2.2.8.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC