CLICK HERE TO GET INPUT FILE: RaleInput_106

Designing an autotransformer for 400Vdc, 1000Adc with 2 parallel connected rectifiers in 12-pulse 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 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 galvanically 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:

In order to get equal short-circuit 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.

Input & Test & Otput

The 148V abd the 82V windings are connected as autotransformer. The 54.4V secondary windings are wound outside.
All windings are wound with Cu foil.

In the test mode are rounded the turns and the foil thicknesses.

Finally here in the test mode is recalculated the nominal operating mode; click here.