Chapter 2 – Twelve–Pulse Technology
II F2
2 - 6 DCS 600 Manual for 12 – Pulse Operation
The sizing of a T-reactor is done in a similar way to that of an auto-
transformer with centre tap. The type rating/size S is determined by
current
(I
RMS mot
) for the copper or aluminium cross sections of the
windings and voltage (U
AC conv
) for the number of windings and the
cross section of the iron core:
S = K x U
AC conv
x I
RMS mot
with K = f (Line frequency, DC voltage)
To be able to deliver the correct T-reactor for the drive the following
data will be needed: - AC converter input voltage
- Line frequency
- Motor duty load cycle
If the drive is used for non motoric applications a much more detailed
information is needed, such as resistance , inductivity and capacity of
the load as well as requested du/dt characteristics of the converter
output voltage.
2.3 Line/power transformers used in 12 – pulse configurations
The following details have to be taken into consideration for line/
power transformers used in 12 – pulse configurations:
• When selecting and dimensioning the 12 – pulse line trans-
former, care must be taken to ensure that overvoltages are kept
away from the converter’s power section when mains supply is
fed to the transformer. For this reason, the transformer should
have an earthed shielding winding between the primary and the
secondary windings which is also very useful for suppressing
capacitive feedthrough during switching operations on the pri-
mary side. In addition, the shielding winding also suppresses
cable-related interferences (EMC) to the network.
• Another method for avoiding overvoltages is to switch on the
components gradually. First feed the mains supply to the
transformer, and then allow the power to flow between the
transformer and the converter. Another option is to incorporate
an R-C element on the secondary side.
• Dimensioning is determined solely by the electrical conditions
involved, like mains voltage, converter supply voltage, converter
rated current and frequency.
• For transformers with a relative short circuit voltage
S
transformer rated
u
k
> 5.25 % *)
S
converter rated
a medium voltage synchronization is recommended.
• For load cycles of 200 % the formula is changed to
S
transformer rated
u
k
> 2.6 % *) .
S
converter rated
*) New values under preparation!
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