ABB DCS800 Hardware Manual page 29

Conclusion for the field supply
Basically, similar conditions apply for both field supply and armature-circuit supply.
Depending on the power converter used (diode bridge, half-controlled bridge, fully
controlled 4-quadrant bridge), some of the fault sources may not always be
applicable. Due to special system conditions, such as supply via an autotransformer
or an isolating transformer, new protection conditions may additionally apply.
The following configurations are very often used:
In contrast to the armature-circuit supply, fuses are never used on the DC side for
the field supply, since a fuse trip might under certain circumstances lead to greater
damage e.g. (small, but long-lasting overcurrent; fuse ageing; contact problems;
etc.).
Semiconductor fuse F3.1 (super-fast acting) should be used, if conditions similar to
those for armature-circuit supply are to apply, like for example protection of the field
supply unit and the field winding.
F3.1
The F3.2 and F3.3 fuse types serve as line protectors and cannot protect the field
supply unit. Only pure HRC fuses or miniature circuit-breakers may be used.
Semiconductor fuses would be destroyed, for example, by the transformer's starting
current inrush.
F3.2
F3.1
F3.3
F3.1
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Configuration for field supplies
Configurations for field supplies
Planning the electrical installation
29