Page 139 - Power Quality in Electrical Systems
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Methods for Correction of Power-Quality Problems 121
+ +
a
–
Source
Load
b
c +
– –
Figure 8.10 A 12-pulse rectifier supplied by a three-winding transformer with
delta and wye secondary windings.
A common application of transformers is the input to a 12-pulse rec-
tifier, as shown in Figure 8.10. The transformer has a wye and delta sec-
ondary to supply each of the rectifier bridges. The 30-degree phase shift
between the two secondary voltages serves to cancel the fifth and sev-
enth harmonics of the primary current. Another use of transformers is
to cancel harmonics, as shown in Figure 8.11. The two secondary wind-
ings are zig-zag to provide 30 degrees of phase shift between secondary
line-to neutral voltages for the two sets of equal-power loads. The same
effect can be achieved with wye-delta secondary windings with half the
loads operating at 120 V and half at 277 V, single-phase. A third example
Load
No. 1
a
+15°
Source
b
–15°
c
Load
No. 2
Figure 8.11 Harmonic cancellations for two equal loads using a transformer
with 15-degree zig-zag secondary windings.
