Siemens SINAMICS G130 Engineering Manual page 104

Fundamental Principles and System Description
Engineering Information
With S120 Smart Line Modules, the connected DC link is precharged via resistors, which create losses. To precharge
the DC link, the Smart Line Module is connected to the supply system on the line side using a precharging contactor
and precharging resistors. Once precharging is complete, the bypass contactor is closed and the precharging
contactor opened again. It is absolutely essential that the precharging and main circuits have the same phase
sequence because, during the brief period of overlap when both contactors are closed at the same time, the
precharging resistors might otherwise be overloaded and irreparably damaged. Due to the power losses that occur in
the resistors during precharging, the DC link may be completely precharged only every 3 minutes and 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 "General Information about Built-in and Cabinet Units
SINAMICS S120".
Precharging with S120 Smart Line Modules via precharging contactor and precharging resistors
The precharging circuit comprises a precharging contactor and precharging resistors and is an integral component of
the S120 Smart Line Modules. This means that the only equipment to be provided outside the S120 Smart Line
Module is the fuse protection for the precharging circuit.
For S120 Smart Line Modules in Chassis format, the fuse protection for the precharging circuit must be provided by
the customer. The fuses recommended for this purpose can be found in section "Precharging of the DC link and
precharging currents" of chapter "General Information about Built-in and Cabinet Units SINAMICS S120".
For S120 Smart Line Modules in Cabinet Modules format, the precharging circuit is always protected by fuses located
in the Line Connection Module LCM that is connected upstream of the Smart Line Module.
The bypass contactor, which can be a contactor or a circuit breaker depending on the power rating, is always located
outside the S120 Smart Line Module.
For S120 Smart Line Modules in Chassis format, the bypass contactor must be provided by the customer.
For S120 Smart Line Modules in Cabinet Modules format, the bypass contactor (contactor or circuit breaker
depending on the power rating) is always located inside the Line Connection Module LCM that is connected
upstream of the Smart Line Module.
IMPORTANT:
It is absolutely essential that the precharging contactor and the bypass contactor are controlled by the internal
sequence control of the S120 Smart Line Module (precharging contactor via internal wiring and bypass contactor /
circuit breaker via connector -X9:3,4). When a circuit breaker is used, it is essential that breaker opening is controlled
by an instantaneous release. For this reason, only circuit breakers equipped with instantaneous undervoltage release
may be used.
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.
SINAMICS Engineering Manual – November 2015
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Ó Siemens AG