Fundamental Principles and System Description
Engineering Information
SINAMICS Engineering Manual – November 2015
Ó Siemens AG
198/528
12-pulse parallel connection of S120 Smart Line Modules
As Smart Line Modules have no electronic current sharing control, three-winding transformer, power cabling and line
reactors must meet the following requirements in order to provide a balanced current. Furthermore, no additional
loads may be connected to only one of the two low-voltage windings as this would prevent symmetrical loading of
both low-voltage windings. Furthermore, the connection of more than one 12-pulse Infeed to a three-winding
transformer should be avoided.
· Three-winding transformer must be symmetrical, recommended vector groups Dy5d0 or Dy11d0.
· Relative short-circuit voltage of three-winding transformer v
k
≥ 4 %.
· Difference between relative short-circuit voltages of secondary windings Δv
k
≤ 5 %.
· Difference between no-load voltages of secondary windings ΔV ≤ 0.5 %.
· Use of symmetrical power cabling between the transformer and the Smart Line Modules (cables of
identical type with the same cross-section and length)
· Use of line reactors with a relative short-circuit voltage of v
k
= 4 %.
A double-tier transformer is generally the best means of meeting the relatively high requirements of the three-winding
transformer. Alternative solutions for obtaining a phase displacement of 30 °, such as two separate transformers with
different vector groups, should be used only if the transformers are practically identical (excepting their different
vector groups), i. e. if both transformers are supplied by the same manufacturer.
The current reduction from the rated value for individual Smart Line Modules in a parallel connection is 7.5 %. This is
also valid for the simplest form of a 12-pulse parallel configuration, if only one Smart Line Module is connected to
each secondary transformer winding, because also in this configuration the transformer’s tolerance can lead to an
uneven current distribution.
Since the three-winding transformer possesses a star winding and a delta winding, and the delta winding has no
neutral point which could be usefully grounded, 12-pulse parallel connections of S120 Basic Line Modules are
connected to two ungrounded secondary windings and thus to an IT system. For this reason, an insulation monitor
must be provided.
Due to the phase displacement of 30º between both secondary winding systems and the control of both systems by
separate Control Units, it is generally not possible to ensure, that both systems contribute equally to the precharging
of the connected DC link. In order to safely ensure that the individual subsystems are not overloaded during
precharging, the 12-pulse parallel connection of Smart Line Modules should be dimensioned such that each
subsystem is able to precharge the entire DC link on its own.
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