Siemens SINAMICS G130 Engineering Manual page 63

Fundamental Principles and System Description
Engineering Information
As the DC link voltage is unregulated, line voltage fluctuations cause the DC link voltage to fluctuate correspondingly.
With S120 Smart Line Modules, the connected DC link is precharged via resistors, which create losses. Due to the
power loss that occurs in the resistors during precharging, the permissible DC link capacitance of the connected
inverters (S120 Motor Modules) is limited to relatively low values. This restriction is not only required due to the
power losses, however, but also to protect the diodes in the IGBT modules against an excessive recharge current
from entering the DC link capacitors when the voltage is restored following voltage dips.
The maximum permissible DC link capacitance for the different S120 Smart Line Modules can be found in the section
"Checking the maximum DC link capacitance" of the chapter "SINAMICS S120, General Information about Built-in
and Cabinet Units".
To achieve an increased output power rating, it is possible to connect up to four S120 Smart Line Modules in parallel
(including 6-pulse and 12-pulse configurations). Further details can be found in the section "Parallel connections of
converters".
Due to the operating principle of the 6-pulse three-phase bridge circuit, the Smart Infeed causes relatively high
harmonic effects on the supply system. The line current contains a high harmonic content with harmonic numbers
h = n 6 ± 1, where n assumes integers 1, 2, 3, etc. The harmonic currents produced in rectifier operation (motor
*
operation) are identical as those of the Basic Infeed and have the same spectral distribution. The Total Harmonic
Distortion factor of the current THD(I) is typically in the range from about 30 % to 45 %. In regenerative operation, the
th
5 harmonic decreases significantly but all the others increase slightly so that the Total Harmonic Distortion factor
THD(I) only decreases by a few percent. The use of Line Harmonics Filters for the reduction of harmonic effects is
not permissible with Smart Infeeds due to the different spectrums of the current harmonics in rectifier operation
(motor operation) and in regenerative operation. A reduction of the Total Harmonic Distortion factor (THD)(I) to a
value of approx. 10% can only be achieved with 12-pulse circuits, i.e. by supplying two Smart Line Modules from a
three-winding transformer with a 30° phase displacement between its secondary voltages.
The criteria for defining of the required transformer power rating, taking into account the harmonic load as well as the
characteristics of three-winding transformers in 12-pulse operation, are described in the section "Transfomers".
SINAMICS Engineering Manual - May 2008
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