Page 194 - Power Electronics Handbook

Bi-directional converters 185

When the firing angle is delayed by a beyond the natural commutation
point of the thyristors, the mean d.c. voltage is seen to decrease and then
to reverse (net inversion) for a > 90". The ripple voltage also increases and
the negative commutation periods across the thyristor decrease, each
thyristor still conducting for a 120" period, although, as expected, these are
shifted relative to the input supply voltage so that phase angle Q equals the
delay angle a. The d.c. component of the input current is unchanged,
therefore the waveforms are essentially as for the two-pulse system except
that now the d.c. voltage is fabricated from three parts, for each cycle of
the input voltage, instead of two.
Three-phase systems may be extended to six pulses, as shown in Figure
9.7, by creating a six-pulse system in which each line conducts to neutral
for 60" during a cycle. This results in poor utilisation of the transformer and
devices, with a consequent increase in their r.m.s. to mean current ratio, so
that a much better solution is given in Figure 9.8. In this circuit two
three-pulse systems are operated in parallel through an interphase
transformer, also called an absorption coil or phase equaliser. This is a
centre-tapped auto-transformer and its action is such as to cause it to
absorb the instantaneous voltage difference across its windings and to
produce a mean potential at its centre point. Therefore the d.c. voltage will
contain a six-pulse ripple, as shown in Figure 9.9, the frequency of the

a.c. supply
1
'*''.X, f----Tl

Load

9.7 Push-pull six-pulse bi-directional converter

tI
Interphase

Load

Group A Group B
ebprc 9.8 Push-pull six-pulse bi-directional converter using an interphase transformer