Page 29 - Application Guide Semiconductor Fuse Link

Regenerative loads

Two possible situations can be considered

• If only F1 fuses are installed then the d.c. fault current will be interrupted by two of these
fuses acting in series. They assist each other to some extent, depending upon the melting
time, which is infl uenced by the inductance of the fault loop.

• If fuses have been installed in the a.c. line (F2) rather than at F1, it will be necessary to have
d.c. side fuses F3 installed to interrupt the d.c. fault. Ferraz Shawmut recommends that two
F3 fuses should be used. The d.c. fuses must be rated for the full d.c. voltage at the L/R time
constant of the circuit.

Note that if a shoot-through fault occurs when in the rectifying mode, the d.c. fault current would
be counter-clockwise around the loop containing devices 6 and 5 (see Fig.21). This is in the opposite
direction to the pre-existing load current, and causes the current through the healthy device 6 to be
reduced rapidly to zero, when it turns off and interrupts the fault. For this reason F3 fuses are not
needed unless the converter is regenerative.

Commutation failure (non-diametric fault)

Another important type of fault which can occur in the regenerative mode is a commutation failure (or
non-diametric fault) which occurs when an incoming thyristor fails to switch on. This failure produces
a combination a.c.+ d.c. fault condition in which fault current fl ows round a loop formed by the a.c.
supply, the two previously-conducting thyristors, and the d.c. side. This is illustrated in Fig.22. In this
case devices 3 and 6 were conducting normally, but then the current failed to commutate from 6 to
2. The a.c. voltage applied to the loop then reverses and adds to the voltage on the d.c. side.

F3
F

1 3 5

F1

F2
a

b
DC

c

AC
Supply

2 4 6

Fig.22 Commutation failure in inverting mode (non-diametric fault)

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