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Designing an autotransformer for  400Vdc, 1000Adc
with 2 parallel connected rectifiers

in 12-pulse operating mode  

General Information

Technical Specification

Input voltage

3 x 400/230V

Autotransformer output voltage for Udc = 400Vdc

3 x 314/182V, shifted +15 to 230V
3 x 314/182V, shifted -15 to 230V

Line output current per secondary: (Ia1,Ib1,Ic1,Ia2,Ib2,Ic2)

I1 = 388Arms
I5  = 77.5Arms,shifted 180to 182V
I7  = 55.5Arms,shifted 0 to 182V
I11 = 35Arms,shifted 180to 182V
I13 = 30Arms,shifted 0 to 182V
continuous operating mode

Frequency

50Hz

Ambient temperature

40C

Temperature rise

Max. 120K, insulation class H

--Steel & Core

M6, annealed, strips for alternated stacking (45),
rectangular cross section

 

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 500-1000Adc.

 

 

 For equal current distribution between 2 parallel connected rectifiers (without the chokes Ld1 and Ld2) the ratio Ucc_out1-out2/Ucc_in-out has to be bigger or equal 4 and Ucc_in-out > 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 3-phase commutating chokes at the AC side between the rectifiers and the autotransformer. Using of two 3-phase 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 short-circuit 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 dc-output 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 230-148.6 = 81.4V

 

Windings parameters per leg

The Large Transformers Program supports the input per leg. You have to follow the following rules

  1. The galvanic connected windings (autotransformer connection) in the input fields Connection are marked with the sign “+”.
  2. The voltages are set per winding
  3. The currents are set per output or tap
  4. The angles are ser to the input voltage

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:
Here are some examples with the first and 5. harmonic.

        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= 180-5*15=105

        Ia1.5+Ia2.5 = 2*77.5*cos(105)=40Arms, angle= 180

 

 

 

 


Harmonic

Input current*

Output current Ia1

Output current Ia2

Output current Ia1+Ia2

Output current Ia3

Output current Ia4

rms

Angle**

rms

Angle

rms

Angle

rms

Angle

rms

Angle

rms

Angle

A

el

A

el

A

el

A

el

A

el

A

el

1

670

0

388

+15

388

345

750

0

388

+15

388

345

5

25.9

180

77.5

255

77.5

105

40

18’

77.5

255

77.5

105

7

25.3

180

55.5

105

55.5

255

28.6

180

55.5

105

55.5

255

11

51.2

0

35

345

35

15

67.6

0

35

345

35

15

13

57.9

180

30

195

30

165

58

180

30

195

30

165

        - calculated by the program (view the Results of this design below.
**- shifted to the input voltage

In a new design you do not need to recalculate the angles if your rectifiers are 2 6 pulse bridges.

 

In order to get equal short-circuit voltages for both outputs (for an optimal current distribution between 2
parallel connected rectifiers) it is recommended to use the symmetrically connected winding configuration per leg

 

Input and some general results

 

 

Core

 

Windings

 

Note that all windings are calculated with Aluminum, the 148V winding with rectangular wire, all other windings with foil.


Results

Summery

        The 5. harmonic in the input current is ca. 25A. In an autotransformer operation mode with a 6 pulse rectifier for 400Vdc, 1000Adc the 5. harmonic would be 125A. The reduction factor is 5.

        The reduction factor for 7. current harmonic is ca. 3.5

        The Ucc = 0.4% is too low. Two 3 phase commutating chokes between the autotransformer autputs and the rectifier has to be used with Ucc =3.6%

        Due to the fact that the mechanical forses in the cross section of the Al-foil in the short circuit operation mode are higher than 20 N/mm2 two (for Cu-foil the limit is 60N/mm2) , two 3 phase commutating chokes with Ucc=3.6% will solve this problem too.

 

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