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Transformer 7.5 degrees phase-shift explained?

Transformer 7.5 degrees phase-shift explained?

Transformer 7.5 degrees phase-shift explained?

(OP)
Dear all,

I'm very familiar with phase-shift transformers supplying for instance large VFD- or other rectifier-loads. I totally understand how and why a phase-shift of 30dgs eliminates the 5th and 7th harmonic on the primary-side on for-instance a three-winding transformer. Of course, the same thing would also work if you have one Dy- and one Dd-transformer on the same network with approximately the same amount of current being drawn in both.

However, I've seen configurations with several three-winding transformers connected to a point of common coupling where the primaries are said to have a D+7,5dgs and D-7,5dgs. The 5th and 7th are efficiently eliminated on the primary side due to the 30dgs phase shift between the two secondary windings, and the clue is to obtain a so-called "pseudo 24-pulse system". I totally understand the theory behind the 15 degrees phase shift giving a 24-pulse system, and of course it is pseudo since it is only working well with balanced load on the different transformers.

So.... To the point, gentlemen; Can anyone give a good, practical explaination on what is being done in the transformer design to obtain a +7,5 or -7,5? I understand how 30 degrees is obtained with the Y/D, but I've never got a good answer on how the 7,5 occurs.


Br,
torslum

RE: Transformer 7.5 degrees phase-shift explained?

One way to obtain a phase shift is to have an additional secondary winding on each leg of the core. The main winding and then a smaller winding. The smaller winding is connected in series with the main winding of one of the other phases. For example,  the main winding on the A phase leg is connected to the smaller winding on the B phase leg. Since the B phase leg of the transformer is not in phase with the A phase leg it causes a phase shift.
 

RE: Transformer 7.5 degrees phase-shift explained?

Draw a delta and tap each winding at 87.6%. You get 7.5 degrees shift by my calculation. Any amount of shift is possible by this method.

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