Siemens SINAMICS S120 Function Manual page 521

Note
Additional information on parallel power unit connections, particularly with regard to their
configuration, can be found in the "SINAMICS Low Voltage Configuration Manual
www.automation.siemens.com/mcms/infocenter/dokumentencenter/ld/
Documentsu20Catalogs/lv-umrichter/sinamics-engineering-manual-lv-en.pdf)".
Infeed concepts - parallel (one CU) and parallel redundant (two CUs)
Some applications require redundant infeeds for a DC line-up. This requirement can be fulfilled
through the implementation of multiple, independent infeeds which are connected in parallel to
the DC line-up. Depending on how the drive is dimensioned, the DC line-up can continue
operating at between 50% to 100% output when one infeed fails.
In the cases of the non-redundant variant, a single Control Unit generally controls all parallel-
connected power units which then function like a single, high-output infeed. For the redundant
parallel connection, each infeed is controlled by a separate Control Unit and is thus completely
independent.
The type of circuit required depends on whether the redundancy requirement applies only to the
infeed itself or also includes the supply-side transformers or the supply systems (see
"SINAMICS Low Voltage Configuration Manual
infocenter/dokumentencenter/ld/Documentsu20Catalogs/lv-umrichter/sinamics-engineering-
manual-lv-en.pdf)").
6-pulse infeed
With a 6-pulse infeed, the two redundant infeeds with the same power rating are supplied from
a line supply via a two-winding transformer. As both infeeds are supplied with exactly the same
line voltage, the current distribution is largely symmetrical in normal operation, even with
uncontrolled infeeds. The infeeds can thus be dimensioned such that, taking into account a
minor current derating factor, each can carry 50% of the total current. However, if one infeed
fails, only half the output remains available. If the full output needs to be available when one
infeed fails, then each infeed must be dimensioned to carry the full current.
12-pulse infeed
For a 12-pulse infeed, the two redundant infeeds with the same power rating are supplied from
a line supply via a three-winding transformer. Depending on the transformer design, the line-
side voltages of the two infeeds will include minor tolerances of between about 0.5% to 1%.
These can cause slightly asymmetrical current distribution in normal operation when
uncontrolled infeeds are used and current derating factors must be applied accordingly. If the
full output needs to be available when one infeed fails, then each infeed must be dimensioned
to carry the full current.
In addition to the requirements of the three-winding transformer and the SINAMICS infeed, the
supply system must also meet certain standards with respect to the voltage harmonics present
at the point of common coupling of the three-winding transformer. You can find more in-depth
information about the requirements for the feeding supply network in the "SINAMICS - Low
Voltage Configuration Manual
Drive functions
Function Manual, 06/2019, 6SL3097-5AB00-0BP2
(http://www.automation.siemens.com/mcms/
(http://www.automation.siemens.com/mcms/infocenter/
Function modules
9.10 Parallel connection of power units
(http://
519