Siemens SINAMICS G130 Engineering Manual page 64

Fundamental Principles and System Description
Engineering Information
Instantaneous short-circuit protection (I = Instantaneous)
The instantaneous short-circuit protection I can be set on electronic trip units ETU15B. The value should be set as
low as possible to ensure the fastest possible response of the circuit breaker to a short circuit to minimize damage to
the unit. Since the permissible range of operating currents for SINAMICS converters is limited to a maximum of
approximately 1.45 times the rated input current, the relative instantaneous short-circuit current should equal
approximately 2 times the referred rated input current of the converter or infeed. Based on this requirement, the
setting value is thus as follows:
This value is also specified in the relevant operating instructions.
The relationships described above are explained below using the LSI characteristic of the electronic trip unit ETU25B
that is used in the Line Connection Modules of SINAMICS S120 Cabinet Modules. It is important to note that the
setting values for currents I
current I .
n
LSI characteristic of electronic trip unit ETU25B and recommended settings for rotary coding switches
The characteristic shown by the blue line limits the utilizable range of input currents of the converter or the infeed.
Currents to the left of the blue characteristic are permissible input currents while those to the right of the blue line
occur only in the event of a fault.
It is therefore necessary to set the tripping characteristic of the electronic trip unit such that it is always positioned to
the right of the blue characteristic, but is also as close as possible to the blue characteristic. The first condition must
be fulfilled in order to ensure that the permissible range of input currents can be fully utilized in operation, while
fulfilment of the second condition ensures the fastest possible response of the circuit breaker to faults. The
characteristic shown by the red line meets these criteria and is achieved by application of the specified setting values
The characteristic shown by the red line is placed on top of or between the characteristics shown by the black lines
which represent the limit settings of the electronic trip unit.
SINAMICS Engineering Manual – November 2015
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Ó Siemens AG
and I are relative values in each case that are referred to the circuit breaker rated
R sd
I =0.7....1.0 (unit-specific)
R
I = 2
sd
t = 0.
sd
I = 2.
i