Short-Circuit Currents On The Dc Busbar - Siemens SINAMICS G130 Engineering Manual

Example 5: Linear arrangement of the DC busbar in two sub-configurations installed a long distance apart
The two sub-configurations for this type of application should be no further apart than about 75 m. Basically distances
between sub-configurations of between around 75 m and 150 m are possible, but boundary conditions requiring
careful examination apply in this case. In this case technical clarification is generally necessary.
As a general rule, when the sub-configurations are spaced at the distances stated above, it is particularly important to
use a low-inductance DC busbar or DC cabling in order to eliminate the possible risk of oscillations between the sub-
configurations. These can occur in the oscillating circuit formed by the capacitances of the two sub-configurations
and the inductance of the DC busbar in cases, for example, where the resonant frequency of the oscillating circuit is
excited by system variables (such as the pulse frequencies of the Motor Modules) of the drive configuration. A low-
inductance DC busbar can be achieved by means of positive and negative bars or positive and negative cables
routed in parallel and as close as possible to one another.
Moreover, for the purpose of minimizing the risk of oscillations, it is better if the DC configuration is supplied by a
controlled Active Infeed.
Note:
There are basically three important limits which apply to the length of busbars and cables for SINAMICS S120 multi-
motor drives:
·
Limitation of the length of the DC busbar. This is necessary in order to eliminate the risk of undesirable
oscillations in the system, as described above.
·
Limitation of the motor cable length between each Motor Module and the associated motor. This is
necessary in order to limit the capacitive leakage currents caused by the motor cable capacitance and thus
to prevent the Motor Modules from tripping on overcurrent. For further information, please refer to section
'"Effects of using fast-switching power components (IGBTs)".
·
Limitation of the total cable length, i.e. the total length of all motor cables of all Motor Modules which are
supplied by the same Infeed via a common DC busbar. This is necessary in order to prevent the total
capacitive leakage current (total of all capacitive currents produced by individual Motor Modules), which is
caused by and depending on the motor cable length, from overloading the S120 Infeed if it returns to the DC
busbar via the line filter of the S120 Infeed or the mains and the S120 Infeed itself. For further information,
refer to chapter "General Information about Built-in and Cabinet Units SINAMICS S120", section "Checking
the total cable length for multi-motor drives". Strictly speaking, the length of the DC busbar should also be
taken into account when calculating the permissible total cable length. In practice, however, the length of the
DC busbar is negligible in comparison to the total lengths of motor cables in multi-motor drives and the DC
busbar length can consequently be ignored for the purposes of this particular calculation.
1.8.2.4

Short-circuit currents on the DC busbar

A direct short circuit on the DC busbar causes spontaneous discharge of the DC links of all the S120 Infeeds and
S120 Motor Modules connected to the busbar. Since the inductance of the DC busbar is relatively low, a very high
short-circuit current develops within a very short time. Even though this current is interrupted within a few
milliseconds by the response of the DC fuses in the Motor Modules and the line-side fuses or circuit breakers of the
Infeeds, the DC busbar must still be capable to withstand the brief peak short-circuit current without impermissible
mechanical deformation or temperature rise. When the drive configuration is dimensioned, therefore, it is important to
consider the short-circuit strength of the DC busbar.
Fundamental Principles and System Description
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