Siemens SINAMICS G130 Engineering Manual page 113

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:
•
Three-winding transformer must be symmetrical, recommended vector groups Dy5d0 or Dy11d0.
•
Relative short-circuit voltage of three-winding transformer u
•
Difference between relative short-circuit voltages of secondary windings Δu
•
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 u
a 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 only means of meeting the requirements of a three-winding transformer for
this application. Line reactors must always be installed if other types of three-winding transformer are used.
Alternative solutions for obtaining a phase displacement of 30 °, such as two separate transformers with different
vector groups, cannot be used due to the inadmissibly high tolerances involved.
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.
Fundamental Principles and System Description
Engineering Information
≥ 4 %.
k
= 2 %. (Line reactors can be omitted if
k
SINAMICS Engineering Manual - May 2008
© Siemens AG
≤ 5 %.
k
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