Page 196 - Power Electronics Handbook

Bi-directional converters 187

the rectifier can be operated with a small permanent bleeder load,
although as an alternative to this the interphase transformer may be
energid by a triple frequency supply current flowing through an auxiliary
winding on its core.
The thyristors in the circuit of Figure 9.8 are seen to conduct for 120"
each, so increasing their utilisation factor, with the input load current
resembling a sine wave more closely than for two- or three-pulse circuits,
and containing no d.c. component.
Two six-pulse systems, of the type shown in Figure 9.8, can readily be
connected together, through a third interphase transformer, to form a
twelve-pulse converter, as shown in Figure 9.10. The principle of operation
is essentially unchanged, each thyristor still conducting for a 1uP period
during a cycle. However, the d.c. voltage has a fundamental ripple
frequency twelve times that of the a.c. supply and the a.c. current
approaches the mean sine wave even closer than for a six-pulse system.
Three-phase bridge converters are usually six-pulse in operation, as
shown in Figure 9.11(a), thyristors conducting current from a line at a

-pull twelve-pulse bi-directional converter with interphase
3 9.10 Push Push-pull twelve-pulse bi-directional converter with interphase transformer
9.10
C C

R- R- TH5
TH5
Load
YO
YO Load
80
80
TH2
TH2
(a)
(a) 0 0
voltages

..
-9.11 Bridge six-pulse bi-directional converter: (a) circuit; (b) and (c) waveforms