Siemens Sinamics s120 Commissioning Manual page 169

Parallel connection with one or two Control Units
If an infeed is deactivated, the pre-charging must be able to charge the remaining infeeds in
the DC link. For example, the charging time is doubled, if only one of the two infeeds
connected in parallel is activated. Configure the infeeds in such a way that one of the parallel
infeeds or, for redundant interconnection (2 Control Units), one subsystem is able to pre-
charge the whole DC link.
The connected capacitance should not be too large. However, pre-charging double an
infeed's rated capacitance (one of two infeeds previously out of operation) will work with no
problems.
Pre-charging contactor monitoring
To monitor the pre-charging contactors (for infeed failure), you must insert the auxiliary
terminal blocks on the pre-charging contactors.
The following diagram shows the basic concept of interconnection:
Image 3-36 Pre-charging contactor monitoring
The contactor states can be monitored using the logic blocks "free blocks" in the SINAMICS
drive. If one of the contactors does not pick up, an external fault message is generated.
STARTER Commissioning Manual
Commissioning Manual, (IH1), 07/2016, 6SL3097-4AF00-0BP5
3.10 Commissioning of power units connected in parallel
Commissioning
169