Check Of The Maximum Dc Link Capacitance; Basic Informations - Siemens SINAMICS G130 Engineering Manual

SINAMICS S120
Engineering Information
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Check of the maximum DC link capacitance

Basic informations

The DC link of the SINAMICS devices must be precharged via a precharging circuit integrated in the SINAMICS
S120 Infeed when the rectifier is connected to the supply voltage. At the moment of connection, the precharging
circuit limits the charging current flowing into the capacitators of the DC link. For more information about the
precharging circuits used in the various Infeed variants, see section "SINAMICS Infeeds and their properties" of the
chapter "Fundamental Principles and System Description".
In the case of S120 Basic Infeeds with lower power ratings, precharging is time-controlled and takes place by
changing the firing angle of the rectifier thyristors (phase angle control). In the case of S120 Basic Infeeds with higher
power ratings, which are equipped with rectifier diodes, and in the case of S120 Smart Infeeds and S120 Active
Infeeds, the precharging circuit comprises precharging contactors and precharging resistors, which precharge the DC
link via the rectifier diodes.
If an excessive DC link capacitance is connected, this can result in an excessive precharging current ,which can
overheat or even destroy the precharging contactor and precharging resistors.
Under unfavorable operating conditions, however, an excessive DC link capacitance can also endanger the rectifier
diodes. In this case, a critical operating condition is a short-term interruption or failure in the supply system, where the
voltage is restored shortly before the undervoltage shutdown threshold in the DC link is reached. Due to the resulting
voltage rise, recharge currents can occur in the DC link that can damage the rectifier diodes.
Situations such as these mean that the DC link capacitance of the drives (Motor Modules) connected to the S120
Infeeds must be limited and must not exceed the maximum permissible DC link capacitance values stipulated in the
technical specifications.
The influencing factors described must be evaluated differently for different Infeeds:
For S120 Basic Infeeds, the precharging circuit is the limiting factor since, due to the fact that the precharging time is
just a few seconds, excessive charging currents can occur when high DC link capacitances are connected. These
can endanger the thyristors and, in particular, the precharging resistors for the diode rectifiers.
For S120 Smart Line Modules supply voltage dips are the limiting factor. The limitations of the DC link capacitance
have to ensure that the recharging current into the DC link after supply voltage dips cannot damage the rectifier
diodes in the Smart Line Modules as described above. This effect is almost independent of the voltage as long as the
relative short-circuit voltage of the supply system is at least u
configurations supplied by Smart Line Modules, which consists of a huge number of Motor Modules, require larger
values of the line supply impedance resp. larger values of the relative short-circuit voltage. The corresponding values
for u = 4 % and u = 8 % have been incorporated into the tables below.
k k
For Active Line Modules the precharging resistor is the critical limitation, due to the fact that the line side current is
controlled by the firmware. It is, therefore, possible to define different DC link capacitances depending on the voltage
range. This has also been incorporated into the table below.
With Infeed units connected in parallel, the maximum possible DC link capacitance is determined by the number of
Infeed Modules connected in parallel multiplied by their maximum DC link capacitance. Prerequisite for this is that all
units connected in parallel are connected to the supply voltage simultaneously (e.g. by a circuit breaker).
SINAMICS Engineering Manual – May 2008
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© Siemens AG
= 4 % related to the rated current of the SLM. Drive
K