Siemens SINAMICS G130 Engineering Manual page 196

Fundamental Principles and System Description
Engineering Information
12-pulse parallel connection of S120 Basic Line Modules
With 12-pulse parallel connections, up to four Basic Line Modules are supplied by a three-winding transformer on the
line side. In this case, an even number of modules, i.e. two or four, must be divided between the two secondary
windings. The Basic Line Modules of both secondary windings are controlled by a common Control Unit, despite of
the 30° phase-displacement. This is possible because the Basic Line Modules produce their gating impulses for the
thyristors, which must have a phase displacement by 30° due to the 12-pulse circuit, by independent gating units in
the individual Basic Line Module, which are not synchronized by the Control Unit.
12-pulse parallel connection of S120 Basic Line Modules
As Basic 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
·
Difference between relative short-circuit voltages of secondary windings Δv
·
Difference between no-load voltages of secondary windings ΔV ≤ 0.5 %.
·
Use of symmetrical power cabling between the transformer and the Basic 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
double-tier transformer is used and only one BLM is connected to each secondary winding of the
transformer).
A double-tier transformer is generally the best means of meeting the relatively high requirements of the three-winding
transformer. When other types of three-winding transformer are used, it is advisable to install line reactors.
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 Basic 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 Basic 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.
SINAMICS Engineering Manual – November 2015
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Ó Siemens AG
≥ 4 %.
k
≤ 5 %.
k
= 2 %. (Line reactors can be omitted if a
k