Page 30 - Application Guide Semiconductor Fuse Link

Regenerative loads

The fault current is driven around the loop with 2 fuses at F1 or F2 in series. The required a.c. and d.c.
voltage ratings for these fuses depends on the magnitude of the fault and the extent to which the 2
fuses at F1 or F2 assist each other. For very large inductances on the d.c. side, the prearcing time can
be long, and no assistance should be assumed.
The required fuse voltage rating in this case depends upon assumptions made about the severity of
this type of fault, and the extent to which the 2 fuses in series share the breaking duty. Equipment
designers have different rules for this situation. Depending upon the application the required
minimum voltage ratings should lie within the following ranges :

Minimum AC voltage rating must be KAC times the maximum a.c. line voltage. (KAC = 1.0 to 1.7)
Minimum DC voltage rating must be KDC times the maximum d.c. side voltage. (KDC = 0.6 to 1.0)

Contact Ferraz Shawmut Technical Services for further details. The commutation fault does not need
to be considered for high power converters with multiple devices per leg. In this case all faults
involving the d.c. side are cleared by a d.c. breaker and the fuses must coordinate with the breaker
- the fuses need only to be rated for internal converter faults.

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For d.c. faults it may not be possible to provide I t protection of the devices. In this case the fuses will
only provide safety isolation in the event of a fault.

Anti-parallel connection

Fig.23 shows a circuit commonly used to regenerate power in d.c. drive systems, by reversing the
direction of current fl ow, rather than reversal of polarity of the load voltage. This is achieved by using
back-to-back thyristors which gives full directional control of the bridge.

Fig.23 Reversible bridge with anti-parallel thyristors

The fuse selection rules previously given for the standard 3-phase bridge can also be used for this
circuit. Similar considerations also apply to double bridges, in which two basic 3-phase bridges are
connected in series or parallel, to give a higher voltage or current output.

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