torslum
Electrical
- May 29, 2005
- 19
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
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