

Designing an
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 an autotransformer: ·
Winding parameters
per limb ·
Core ·
Environment ·
Other parameters and 3 screens for selection
and set up of material : ·
wires ·
steels ·
cores. Windings parameters per
leg
The following 3 phase
autotransformer circuit is often used to drive 2 parallel connected 6 pulse
bridge rectifiers in order to reduce the 5. and 7. current harmonics on the
input 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 an autotransformer and you need to consider using of
Ld1&Ld2 chokes and/or two 3phase commutating chokes at the AC side
between the rectifiers and the autotransformer. Using of two 3phase
commutating chokes is very effective for suppressing all current harmonics
and balancing the current distribution between 2 parallel connected
rectifiers. Also, the commutating chokes limit the current rise during the
commutation within the rectifier. Note that the
shortcircuit voltage of a rectifier autotransformer is a complex issue, full
supported by the software, 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 autotransformer 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 tap 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 Windings parameters per
leg
The Large Transformers
Program supports the input per leg. You have to follow the following rules
5.
The set current harmonics are calculated for the
worst case: Ucc= 0 and Ld = ∞ using the following table:
* 
calculated by the program
