Parallel Connection Of Power Units - Siemens SINAMICS DCM Operating Instructions Manual

Connecting

6.10 Parallel connection of power units

6.10 Parallel connection of power units
Current splitting/Symmetry
It is important to ensure the current is split correctly when connecting power units in parallel.
Even distribution must be ensured to prevent power unit overloads. Appropriate derating
measures should be implemented where necessary. In view of this, it is absolutely essential
that all parallel firing pulse transformers/firing pulse amplifiers are of the same type and
design. The split of current across the power units connected in parallel should be checked
during commissioning and corrected, where necessary, by harmonizing source impedances
(line reactors, supply lines, and transformers). If unexpected problems are encountered
during this process, check whether the firing pulses at the thyristors are occurring
simultaneously. The execution times for the firing pulses must not differ by more than
200 ns. In view of this, it is advisable to compare the firing currents of the thyristors on an
oscilloscope using two small current probes. With electrical connections, the power unit
should be safely disconnected to prevent accidents.
Comments on voltage measurement/Synchronization
The best method is to connect voltage measurement equipment 1U1, 1V1, and 1W1 at the
branching points of the line supply. The connection of motor voltages 1C1 and 1D1 is
relatively unimportant.
Comments on fuse monitoring
If possible, all the fuses of the units connected in parallel should be monitored. It is
particularly important to monitor the fuses at branches not measured by a current
transformer. The SINAMICS DCM Control Module is unable to measure whether the current
has been split evenly at these points.
Note
The currents (current splits) measured by the SINAMICS DCM Control Module itself are
wholly dependent on the values determined by the current transformers. The unit itself is not
able to measure current splitting in the phase that runs through the current transformers
(mainly 1V1).
Where only one set of SINAMICS DCM control electronics is being used, with firing pulses
connected in parallel accordingly, it is not easy to determine how the current is split between
the power units. One method is to measure the current at one unit and work out the other
currents, and thereby the total current, on the basis of the known current split. The other
method involves using separate control electronics for each power unit capable of
determining the allocated direct current. These currents can then be added together and the
resulting values used elsewhere.
104
SINAMICS DCM Control Module
Operating Instructions, 06.2010, C98130-A7067-A1-02-7619