[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

TABLE 2.4.7 Required Number of Six-Pulse Windings and Connections
Pulse
Number
Number of Six-Pulse Windings and Typical Connections
1 2 3 4 5 6 7 8
6 delta — — — — — — —
or wye
12 delta wye — — — — — —
18 delta wye wye — — — — —
–10 +10
18 delta delta wye — — — — —
+10 –10
18 delta delta delta — — — — —
+20
–20
24 delta delta delta wye — — — —
+15
–15
24 delta wye wye wye — — — —
+15
–15
24 delta delta wye wye — — — —
+7 1 /2 –7 1 /2 +7 1 /2 –7 1 /2
30 delta delta delta wye wye — — —
+12
–12 +6 –6
36 delta delta delta wye wye wye — —
+10
–10 +10 –10
48 delta
+15
delta
+7 1 /2
delta delta
–7 1 /2
delta
–15
wye
–7 1 /2
wye wye
+7 1 /2
FIGURE 2.4.19 A 12-pulse Circuit 31 5450-kVA, 4160-V delta primary to 2080-V delta and wye secondaries, castcoil
transformer in case. (Photo courtesy of Niagara Transformer Corp.)
Using different phase shifts on the single winding of the transformer, whether the primary or secondary
winding, can increase the number of phase shifts. For instance, two 12-pulse transformers can make a
24-pulse system by using a delta primary on one transformer and a wye primary on the other. In cases
where it may be desirable to have an interchangeable spare, it is sometimes beneficial to use two 15
phase-shifted primary windings. The spare transformer can then be made with a reconnectable winding
for 15 shift.
It is important to note that harmonic cancellation is generally not perfect. This is due to several factors,
such as unbalanced loading, inaccurate phase shifts, differences in commutating impedances, and tap
changes. That may be acceptable at some times but not at others. It is common to assume a 5% residual
of lower harmonics to accommodate these realities.
When the phase shift is incorporated in the primary winding, the degree of shift will vary somewhat
as taps are changed on the transformer unless a tap changer is used in the main part of the shifted
winding and the extended part of the shifted winding. Even then, there may be a slight shift. These
problems may vary by about a degree of shift over the tap range of most transformers (Figures 2.4.19,
2.4.20, 2.4.21, 2.4.22, and 2.4.23).
2.4.11 DC Current Content
If dc current is present in either the supply side or the load side of the transformer windings, it must be
specified to the transformer manufacturer at the time of quotation. Some rectifier circuits, such as cycloconverters,
have the possibility of dc current in the load current. A small amount of dc current can
saturate the core of a transformer. The effects of this may be core and core-joint overheating, core-clamp
heating from fields and circulating currents, winding hot spots, and even tank heating. Noise and
vibration are also often present.
FIGURE 2.4.20 A 24-pulse dry-type transformer. 7000-kVA drive duty transformer, to be located in a tunnel, made
of two 3500-kVA 12-pulse transformers. Each core and coil has 1100-V delta and wye secondary windings. Each core
and coil has a 13,200-V extended-delta primary winding. One core and coil has its primary winding shifted –15,
while the other core and coil has its primary winding shifted +15. One reconnectable primary spare core and coil
can be used to replace either core and coil assembly. (Photo courtesy of Niagara Transformer Corp.)
2.4.12 Transformers Energized from a Converter/Inverter
Transformers energized from a converter/inverter are often subject to considerably distorted voltages. If
voltage harmonics are known to be above the limits of IEEE 519, they must be specified. Variablefrequency
applications are generally considered to be at constant volts per hertz. If the volt-per-hertz
ratio is variable, the degree of variation must be specified. The flux density of the core is the governing
factor, not the maximum value of the sinusoidal voltage.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC