For a 12 pulse diode rectifier is it necessary a transformer for a phase shift of 30 degrees. What is the difference between the Y/Y/D and D/Y/D transformers for 12-pulse rectifier?
Actually the diff. is in the source side. just concern the diff. of the first part Y/Y and D/Y
Dear Aripriharta,
Thank you. I thought about it, but I was not sure.
phase difference of 30 degrees is necessary in order to get 12 pulse output without this phase shift you will not get 12 pulse.
Actually, both systems can be used, but the D/Y/D is more usual due to the delta-wye transformer connection will trap all the triplen harmonics (third, ninth, fifteenth, twenty-first, etc.) in the delta of grid side (primary). In this way, in order to ensure Power factor and Harmonic Distortion requirements in the grid side is better D/Y/D than the Y/Y/D connection.
There is a classical reference in this field, that can support you in this task:
Derek A. Paice. Power Electronics Converter Harmonics : Multipulse Methods for Clean Power. Wiley-IEEE Press.
If three wire system is used in the primary side (primary being in STAR or DELTA), triplen current harmonics can not be present. And the requirement of third harmonic current due to nonlinear B-H curve is taken care of by the DELTA winding in the secondary. So even if STAR is used in the primary, the STAR point will be at ZERO potential and not have the third harmonic voltage.
BUT, still the DELTA winding on the primary is better for few other reasons.
Thank you for your comments! Your comments helped me alot.
The reason why Y/Y transformers are normally not used is that the neutral is not available at the primary side. Any zero sequence component experiences a high impedance.The reason is that the field pattern goes from yoke to yoke and this is a really much higher leakage impedance compared to concentric windings in normal field patterns.
One can avoid third harmonic and other zero sequence problems while using YZ configurations as well. It is rarely used, as windings from two legs are put in series and one has about 15% less voltage for the same resistance.
I used in previous times YY transformers for rectifiers with a low voltage output, with star point connected and 6 diodes to the +. It allows having 120° current blocks instead of 60°, without using a bridge. A third harmonic voltage appears on the neutral.
Thank you Alex! This explanation was very helpful!