

Designing a
autotransformer for
400Vdc, 1000Adc in
12pulse operating mode General
Information
Technical
Specification
Creating
Input
4 input screens are used
to set the input parameters for the designing of a transformer: ·
Winding parameters
per limb ·
Core ·
Environment ·
Other parameters and 3 screens for
selection and set up of material : ·
wires ·
steels ·
cores. Windings parameters per
phase
The following 3 phase autotransformer
circuit is often used to drive 2 parallel connected 6 pulse bridge rectifiers
in order to compensate the 5. and 7. current
harmonics on the input voltage side of the autotransformer. The parallel
connection of the rectifiers is normally used if the output current Id is
over 5001000Adc. For equal current
distribution between 2 parallel connected rectifiers (without the chokes
Ld1 and Ld2) the ratio
Ucc_out1out2/Ucc_inout has to
be bigger or equal 4 and Ucc_inout > 4%.
Normally this condition can be not realized by using the autotransformer and
you need to consider using of Ld1&Ld2 chokes and/or 2 3phase commutating
chokes at the AC side between the rectifier and the autotransformer. Using of
2 3phase commutating chokes is very effective for suppressing all current
harmonics. Note that the
shortcircuit voltage of a rectifier autotransformer is a complex issue reflecting: ·
the rectifier
protection in a short circuit operation mode of all secondary winding, a
group of windings or of only one winding. ·
the commutation
operation mode of a group of windings ·
the voltage drop of
the dcoutput voltage ·
the current
distribution between the parallel connected rectifiers It has to be prescribed
by the user of the transformer The following 50Hz vector diagram of the autotransformer
was created as follows: ·
Input voltage per phase is 230V ·
The output voltage per phase for 400Vdc rectifier voltage is 182V ·
The first current harmonic is 388A (for rectifier DC current 500Adc) · The zag voltage is 182 x sin(45°)/sin(120°) = 148.6V · The zig voltage is 182 x sin(15°)/sin(120°) = 54.4V ·
The voltage on the W1 winding is 230148.6 = 81.4V ·
The input current is 2 x 388 x 148.6/230 = 482A (blue) ·
The zig current is 2 x 388 x cos(15°) *54.4/230 = 177A (red) Windings parameters per
leg
The Large Transformers
Program supports the input per leg. You have to follow the following rules
6.
The set current harmonics are calculated for the worst
case: Ucc= 0 and Ld = ∞
using the following table:
In
order to get equal shortcircuit voltage for both outputs and a good current
distribution between 2 parallel connected rectifiers it is recommended to use
the following winding configuration per leg
uInput & Otput
