Medium voltage network1
Three-winding transformer2
12-pulse drive3
No grounding of transformer secondary windings is permitted4
No load unbalance between transformer secondary windings is permitted5
Large 6-pulse drives (possible sources of harmonics)6
No (or only minimal) voltage distortion in medium voltage network is permitted.7
a. Path of harmful effect (harmonics) to one leg of the 12-pulse supply unit through transformer
delta-delta connections
b. Path of harmful effect (harmonics) to one leg of the 12-pulse supply unit through transformer
delta-wye connections.
Use identical supply cables between the transformer secondary windings and the
drive (type, size, length, quantity).
Make sure that there is no (or only minimal) voltage distortion in the medium voltage
system. The distortion has a negative effect on the operation of the transformer and
the 12-pulse drive. Especially 5th and 7th order harmonics are harmful. They can
decrease the DC output voltage of either 12-pulse leg of the supply unit. This causes
load unbalance and decreases the load capacity of the drive. Similar transformer
connection of the source drive and the 12-pulse drive tend to convey the harmful effect
more effectively. This strengthens the load unbalance between the two legs of the
12-pulse drive. Refer to the illustration above. Loads such as large 6-pulse drives can
cause 5th and 7th harmonics.
If you must connect a load other than a 12-pulse drive to the transformer, make sure
that the load is identically shared between the secondary windings.
Guidelines for planning the electrical installation 91
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