Environmental Engineering Reference
In-Depth Information
3-phase AC
DC
Figure A.1
6-pulse converter bridge
3-phase AC
DC
Figure A.2
12-pulse converter bridge
Twelve-pulse bridges (Figure A.2) produce smoother AC and DC waveforms.
They are normally created from two 6-pulse bridges: one connected to a star-
configured part of the grid transformer and the other to a delta-configured part.
This ensures that commutation takes place every 30
rather than every 60
,asfor
a 6-pulse bridge.
In order to operate successfully, such converters have a commutation overlap
period and hence have short periods in each cycle when two phases are effectively
short circuited. The DC-side leakage inductances of the two phase contributions to
the DC link determine the magnitude of the short-circuit current. The larger the
current, the longer will be the commutation time. The effects are more significant
in inverter operation. If the system is weak, the system voltage will collapse during
these periods and the inverter control will lose its commutation reference. For this
reason, current source converters can only be used where both systems are rela-
tively strong in an electrical sense, as measured by fault level. Manufacturers
indicate that commutation failure starts to become an issue when the fault level is
less than six times the transferred power and is not possible below three times the
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