[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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Advanced Position
3.5.6 Paralleling of Transformers
Transformers having terminals marked in the manner shown in Section 3.5.2, Polarity of Single-Phase
Transformers, can be operated in parallel by connecting similarly marked terminals provided that their
ratios, voltages, angular displacement, resistances, reactances, and ground connections are such as to
permit parallel operation.
The difference in the no-load terminal voltages of the transformers causes a circulating current to flow
between the transformers when paralleled. This current flows at any load. The impedance of the circuit,
which is usually the sum of the impedances of the transformers that are operating in parallel, limits the
circulating current. The inductive circulating current adds, considering proper phasor relationships, to
the load current to establish the total current in the transformer. As a result, the capacity of the transformer
to carry load current is reduced by the circulating current when the transformers are paralleled. For
voltage ratios with a deviation of less than the 0.5%, as required by the IEEE standards, the circulating
current between paralleled transformers is usually insignificant.
The load currents in the paralleled transformers divide inversely with the impedances of the paralleled
transformers. Generally, the difference in resistance has an insignificant effect on the circulating current
because the leakage reactance of the transformers involved is much larger than the resistance. Transformers
with different impedance values can be made to divide their load in proportion to their load ratings
by placing a reactor in series with one transformer so that the resultant impedance of the two branches
creates the required load sharing.
When delta–delta connected transformer banks are paralleled, the voltages are completely determined
by the external circuit, but the division of current among the phases depends on the internal characteristics
of the transformers. Considerable care must be taken in the selection of transformers, particularly singlephase
transformers in three-phase banks, if the full capacity of the banks is to be used when the ratios
of transformation on all phases are not alike. In the delta–wye connection, the division of current is
indifferent to the differences in the characteristics of individual transformers.
Series Unit
Main Unit
3.6 Transformer Testing
FIGURE 3.5.5 Two main type of PSTs: top) single core; bottom) dual core.
The other common PST circuit used is shown in Figure 3.5.5b. This PST design requires two separate
cores, one for the series unit and one for the main or excitation unit. For large power, this PST design
requires two separate tanks with oil-filled throat connection between them. This type of design does not
have the technical limitations of the single-core design. Furthermore, another tap winding connected in
quadrature to the phase-shifting tap windings can be readily added to provide voltage regulation as well
as phase-shift control. This enables essentially independent control of the real and reactive power flow
between the systems. However, the cost of this type of PST design is substantially higher because of the
additional core, windings, and internal kVA capacity required.
3.5.5 Three-Phase to Six-Phase Connections
The three-phase connections discussed above are commonly used for six-pulse rectifier systems. However,
for 12-pulse rectifier systems, three-phase to six-phase transformations are required. For low-voltage dc
applications, there are numerous practical connection arrangements possible to achieve this. However,
for high-voltage dc (HVDC) applications, there are few practical arrangements. The most commonly
used connections are either a delta or wye primary with two secondaries: one wye- and one deltaconnected.
Shirish P. Mehta and William R. Henning
3.6.1 Introduction
Reliable delivery of electric power is, in great part, dependent on the reliable operation of power transformers
in the electric power system. Power transformer reliability is enhanced considerably by a wellwritten
test plan, which should include specifications for transformer tests. Developing a test plan with
effective test specifications is a joint effort between manufacturers and users of power transformers. The
written test plan and specifications should consider the anticipated operating environment of the transformer,
including factors such as atmospheric conditions, types of grounding, and exposure to lightning
and switching transients. In addition to nominal rating information, special ratings for impedance, sound
level, or other requirements should be considered in the test plan and included in the specifications.
Selection of appropriate tests and the specification of correct test levels, which ensure transformer
reliability in service, are important parts of this joint effort.
Transformers can be subjected to a wide variety of tests for a number of reasons, including:
• Compliance with user specifications
• Assessment of quality and reliability
• Verification of design calculations
• Compliance with applicable industry standards
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC