Page 172 - Power Electronics Handbook

164 Static switches
system almost exactly and the same equations and circuit waveforms are
applicable. It produces the lowest voltage harmonics over most of the
control range, but is restricted for use with open-star connected loads. The
circuit of Figure 8.6(b) is also similar in operation to the single-phase
systems. Thyristors are fired in pairs, spaced 120" apart, in order to give the
required phase sequence output, the load current flowing from a more
positive to a negative phase. A typical sequence of conduction could be Bl
Y2 R1, Y2 R1 B2, R1 B2 Y1, B2 Y1 R2, Y1 R2 B1 and R2 B1 Y2 over a 360"
period.
The control system shown in Figure 8.6(c) uses one thyristor and a diode
in each line. It has no counterpart in a single-phase circuit since the diode
would then supply load current during a complete half cycle, although for
three-phase systems the diode cannot conduct unless a thyristor in a more
positive line has been fired. This circuit is simpler to use since only the
thyristors require gate pulses, the diodes conducting automatically. The
circuit shown in Figure 8.6(d) is the simplest, since it only uses three power
components, but now each device has to carry a higher current than in the
other circuits, the current flowing in the components for 2400 in every
cycle, at full load. Table 8.1 summarises the device ratings for the circuits
shown in Figure 8.6.

Tabk 8.1 Device ratings for the tbree-pbsllc drepltr 0tFigw-e 8.6
~
~ ~ ~ ___ _ _ _ ~
Circuit Thyristor voltage Thyristor 1.rn.s. current Full control dekay
ndcr (percentage of a.c. line) (percentage of ax. line) angle (degrees)
844 141.4 40.8 180
8WJ) 122.5 70.7 150
8.6(c) 122.5 70.7 210
8.6(d) 141.4 76.6 210

Figure 8.7 gives the variation of r.m.s. line voltage with firing angle for
Figure 8.6(b). It is essentially similar to the single-phase operating curves
but is limited to lower control angles. Another peculiarity of three-phase
systems is that the neutral point voltage will vary with the conduction
angle, owing to imbalance in the instantaneous phase voltages, and when a
neutral line is present a current will flow. Figure 8.8 illustrates the
waveform of this current for the arrangement of Figure 8.6(b) when a
four-wire star-connected load is used. Clearly, the imbalance is the greatest
at delay angles of 90", which is illustrated in Figure 8.9, where the neutral
current is then seen to be equal to the line current.
The harmonics generated in three-phase circuits have shapes similar to
those of single-phase circuits although, as expected, they have a lower
value and the harmonic present varies with the configuration. Figure 8.10
shows the plot of harmonics for the three-phase controller of Figure 8.6(b).

8.2.3 Control circuits
The essential features of a firing circuit for phase control are shown in
Figure 8.11. The detector senses the zero voltage points of the input lines,