
Designing
an autotransformer for 400Vdc, 1000Adc 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:
The
output currents, set in the following table were calculated by hand using the
parameters of a 6 pulse rectifier: ·
Ia1.1 = Ia3.1 = 388Arms,angle +15°
·
Ia2.1 = Ia4.1 = 388Arms,angle 15°
·
Ia1.1+Ia2.1 = 2 x 399 x cos(15°) = 750Arms, angle = o°
·
Ia1.5=Ia3.5 = 0.155*500=77.5Arms, angle= 180+5*15=255°
·
Ia2.5=Ia4.5 = 0.155*500=77.5Arms, angle= 1805*15=105°
·
Ia1.5+Ia2.5 = 2*77.5*cos(105°)=40Arms, angle= 180°
·
 calculated by the program (view the Results of this
design below.
