Siemens SINAMICS S120 Function Manual page 507

Infeed concepts - parallel (one CU) and redundant parallel (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. With the
redundant variant of the infeed parallel connection, each infeed is controlled by a separate
Control Unit and is thus completely independent. With the non-redundant variant, a single
Control Unit generally controls all parallel-connected power units which then, from a practical
viewpoint, function like a single, high-output infeed.
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
(https://www.automation.siemens.com/mcms/infocenter/dokumentencenter/ld/Documentsu2
0Catalogs/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.
6-pulse, 12-pulse infeed
When separate Control Units are used, pre-charging may not be synchronized accurately
enough, i.e. a converter system must be able to pre-charge the total capacity of the drive
line-up. Pre-charging power for the DC link in parallel operation must be dimensioned so that
the capacitance of the DC link can be fully charged by a single converter system. Otherwise
a separate pre-charging device must be provided.
Drive functions
Function Manual, (FH1), 01/2013, 6SL3097-4AB00-0BP3
Function modules
7.10 Parallel connection of power units
505