Page 28 - Application Guide Semiconductor Fuse Link

Regenerative loads

Inversion in line-commutated inverters

If the delay angle of the 3-phase bridge va vb vc
converter is increased above 90° the d.c.
output voltage reverses polarity. The current
through the rectifying devices remains of voltage
the same polarity and as a result the power
delivered to the d.c. side becomes negative.
0
In other words, power is being transmitted
from the d.c. side to the a.c. side. This is
the regenerative or inverting mode and
the waveforms are shown in Fig.20. The
current waveforms are drawn as before
on the assumption that the d.c. current is vb vc va
approximately constant. In the regenerative 3 5 1 3
mode, this is only possible for a sustained
period if there is a source of energy in the
2 4 6
d.c. side, e.g. a battery, generator or rotating
machine with stored kinetic energy. The Fig.20 Converter waveforms in inverting mode
regenerative mode enables energy savings
for example when lowering crane loads by
returning energy to the a.c. system. If the converter operates in the regenerative mode certain types
of fault conditions require the fuses to have an adequate d.c. interrupting capability.

DC shoot-through fault in inverting mode

Fig.21 illustrates the d.c. shoot-through (or diametric) fault. Devices 3 and 6 are conducting normally,
but the polarity of the output voltage is reversed, compared with Fig.18. If, during this period, device
5 fails to block, a d.c. short-circuit current will fl ow clockwise around the loop which contains devices
6 and 5.

F3
F

1 3 5

F2 F1
a Fault

b DC

c

AC
Supply

2 4 6

Fig.21 DC shoot-through fault in inverting mode (diametric fault)

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