Figure 4-22b shows the voltages for 0.
Since a transition between conducting thyristors occurs every 30, there are
a total of 12 such transitions for each period of the ac source. The output has harmonic
frequencies that are multiples of 12 times the source frequency (12k, k 1,
2, 3, . . .). Filtering to produce a relatively pure dc output is less costly than that
required for the 6-pulse rectifier.
Another advantage of using a 12-pulse converter rather than a 6-pulse converter
is the reduced harmonics that occur in the ac system. The current in the ac
lines supplying the Y-Y transformer is represented by the Fourier series