Page 26 - Application Guide Semiconductor Fuse Link

Converters with multiple parallel devices

Converters for high current applications may use several rectifying devices in parallel in each leg of
the converter, as shown in Fig.19. Such circuits are commonly used for electrochemical and heavy-
duty traction applications.

IF/4
Fault
F1

a IF

b

c

AC
Supply

Fig.19 High current three phase bridge rectifi er

In this case each device has a fuse in series with it (F1) so that each device is protected and to ensure
that if a device failure occurs, only one fuse will operate. The faulty device will be isolated, the
converter will be able to continue to operate, and the faulty device and blown fuse can be replaced
at some later time. There is no possibility of protecting parallel devices with fuses in the a.c. line,
since the fuses would be too large. The current on the d.c. side of the converter is very high and is
often controlled with a d.c. breaker. Alternatively, in very large rectifi ers, a d.c. short-circuit (external
fault) may be cleared by a breaker on the a.c. side (sometimes in the primary side of the supply
transformer). It is often required that the fuses should survive the clearance of an external fault by a
breaker without deterioration (see section 7). The general principles for fuse selection are the same
as described in the previous section, but with the following additional requirements.

Device redundancy

If there are n devices in parallel per leg, the current in each fuse is ILEG /n, and the required continuous
current rating is based on this value. However, as previously mentioned, it is common to operate
the converter for a considerable period of time after one fuse has opened. If this is the case, the fuse
current will be ILEG /(n-1), and its continuous current rating must be based on this higher value.

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