Checking The Maximum Dc Link Capacitance; Basic Information - Siemens SINAMICS G130 Engineering Manual

SINAMICS S120
Engineering Information

6.6 Checking the maximum DC link capacitance

6.6.1

Basic information

The DC link of SINAMICS drives is precharged by means of a precharging circuit in the SINAMICS S120 Infeeds as
soon as the rectifier is connected to the line supply voltage. The precharging circuit limits the charging current flowing
into the capacitors of the DC link. For further details about precharging circuits used in the different Infeed types,
please refer to the previous section and to section "SINAMICS Infeeds and their properties" in 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 setting 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, the period where the precharging current flows can become too
long, thus causing overheating and possibly destruction of the precharging contactor and precharging resistors.
Under unfavorable operating conditions, however, an excessive DC link capacitance can also endanger the rectifier
diodes. Looking at this aspect, 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 as desribed above mean that the DC link capacitance of the drive configuration (Motor Modules)
connected to the S120 Infeeds must be limited and not exceed the maximum permissible DC link capacitance values
according to the technical specifications.
The influencing factors described above must be evaluated differently for different Infeed types:
In the case of S120 Basic Infeeds, the precharging circuit is the limiting factor. This is because the precharging time
limit of a few seconds means that excessive charging currents or excessive periods of charging current flow would
occur in the case of high DC link capacitances, posing a risk to the thyristors and / or the precharging resistors in the
diode rectifiers.
In the case of S120 Smart Infeeds 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 v = 4 % related to the rated current of the
k
SLM. Drive 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 v = 4 % and v = 8 % have been incorporated into the tables below.
k k
In the case of Active Infeeds the precharging resistor is the critical limitation, due to the fact that the line side current
is always under control 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. This can be ensured by a common
circuit breaker or by different circuit breakers with interlocking control.
SINAMICS Engineering Manual – November 2015
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