[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

Care must be taken when specifying these values to the transformer manufacturer. The impedance
value, whether it is commutating impedance or short-circuit impedance, and kVA base are extremely
important.
Use ANSI/IEEE C57.18.10 as a reference for commutating impedance. The tables of circuits in this
reference are also useful.
2.4.5 Secondary Coupling
Three-winding transformers with one primary winding and two secondary windings, such as Circuit 31
(Figure 2.4.4), Circuit 45 (Figure 2.4.5), and Circuit 46 (Figure 2.4.6), can be constructed as tightly
coupled secondary windings or as loosely coupled or uncoupled secondary windings. The leakage reactance
is common to the tightly coupled secondary windings but is independent for the uncoupled
secondary windings.
If two separate transformers were constructed on separate cores, the couplings would be completely
independent of one another, i.e., uncoupled. When the windings of one of these transformers are
combined on a common core, there can be varying degrees of coupling, depending on the transformer
construction.
Some transformers must be made with tightly coupled windings. This is commonly accomplished by
winding the secondary windings in an interleaved or bifilar fashion. This is almost always done with lowvoltage,
high-current windings. Higher-voltage windings would have high-voltage stresses from this type
51
H 1
FIGURE 2.4.9 ANSI Circuit 51. (From ANSI/IEEE C57.18.10-1998. © IEEE 1998. With permission.)
51A
H 1
H 2
H
2
N 1 R
3 N 2 R
2
30¡
H R1
R R
3 5 6
WYE, SIX PHASE, PARALLEL DOUBLE WYE
WITH SINGLE INTERPHASE TRANSFORMER.
H 3
30¡
N 1
R
3
R 1
S 1
N 2 N 3
R2 R 4
R 5
R6
N 3 N 0
N 4
S
R 3 4
S 2 S
4
S 1
N 0
S
3
S 5
S 2
S 5
S 6
N 4
S
4
S 6
50
H
2
N 1
R R
3 2
N 2
N 3
N 0
S 3
N
S 4
2
WYE, SIX PHASE, PARALLEL DOUBLE WYE
WITH TWO INTERPHASE TRANSFORMERS.
FIGURE 2.4.10 ANSI Circuit 51A. (From ANSI/IEEE C57.18.10-1998. © IEEE 1998. With permission.)
H
1
H
3
R1
R
4
S1
S 4
52
N 0
R R
5 6
S 5
DELTA, SIX PHASE, PARALLEL DOUBLE WYE
WITH SINGLE INTERPHASE TRANSFORMER.
S 6
H 1
H
2
H
2
H H
3
1 H
3
30¡
R
5
R 1
N
1
R 3
R 9
N
2
R 11
N
3
R7
R2
R 6
R
10
R 4
R
12
N 4
R 8
FIGURE 2.4.7 ANSI Circuit 50. (From ANSI/IEEE C57.18.10-1998. © IEEE 1998. With permission.)
50A
H
1
H
2
H
3
R 1
N 1
R 5
R 6
R
4 S1
DELTA, SIX PHASE, PARALLEL DOUBLE WYE
WITH TWO INTERPHASE TRANSFORMERS.
FIGURE 2.4.8 ANSI Circuit 50A. (From ANSI/IEEE C57.18.10-1998. © IEEE 1998. With permission.)
R
3
N 2
R
2
N 3
N 0
S
3
S 5
S 2
S 6
N 4
S
4
WYE DELTA, TWELVE PHASE, QUADRUPLE WYE
FIGURE 2.4.11 ANSI Circuit 52. (From ANSI/IEEE C57.18.10-1998. © IEEE 1998. With permission.)
of construction. However, the higher-current windings also benefit from the harmonic cancellation resulting
from the close coupling of the secondary windings. This type of construction is usually required with
Circuit 45 and 46 transformers and other single-way double-wye transformers. Such transformers would
suffer from three-pulse harmonics without the tight coupling of the secondary windings.
Secondary coupling is used in all types of traction-duty transformers. This offers the advantage of low
commutating impedance with high short-circuit impedance. Harmonic cancellation also is accomplished
in the secondary windings.
The degree of coupling affects the magnitude of the fault current that can be produced at short circuit.
If secondary circuits are paralleled, unbalanced commutation impedances can produce high fault currents.
The degree of coupling can also affect the voltage regulation at high overloads, as can be seen in
traction service or high-pulse duty.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC