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I'm aware that 30° shift is needed in order to obtain a 12-pulse rectification, however, I'm a bit confused about the configuration of these transformer. In most HVDC systems that appear in Alstom - HVDC for Beginners and Beyond, for example, shows a WYE winding as primary:

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However, I stumbled upon this similar ResearchGate question but it was answered as the Y/Y/Δ configuration is used and that's why I got confused.

I'm doing a simulation on PLECS in stationary state of this series connected 12-pulse rectifier so as long as I can represent leakage inductances and 30° shift it's okay as I won't do neither a transient analysis nor a fault scenario. However, I want to clarify this to at least justify why I used one configuration or the other.

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    \$\begingroup\$ The combinations you use are not important. You need one winding that has the same in and out (Y-Y or D-D) and the other set that has a change from Y to D or the other way. \$\endgroup\$ – KalleMP Jun 15 '18 at 16:40
  • \$\begingroup\$ @KalleMP Thanks!. If you don't mind to clarify me something please... for these applications, Power Transformers and Converter Transformers are the same? Or does a CT include the rectifying section inside the whole 'package'? \$\endgroup\$ – Zrakk Jun 19 '18 at 4:14
  • \$\begingroup\$ A converter transformer is just a power transformer designed with the appropriate number of secondary windings (i.e. two LV windings for a 12-pulse converter) and designed with due regard to the harmonic load of the converter. The converter (rectifiers etc.) is supplied separately. \$\endgroup\$ – Li-aung Yip Dec 19 '18 at 4:00
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A 3 winding power transformer isn't equal to a 12 pulse conver transformer. A 12 pulse transformer can have 4 windings. 2 HV and 2 LV windings. The HV and LV windings are arranged in double tier for better decoupling (still a coupling exsiting) of the winding systems and to have equal short circuit voltages.

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I understand that your question is related to "why HVDC 12 pulse transformer uses Y in primary instead of a D, as converter transformer do". Looking to the converter, it needs two secondaries with 30° shifted (1 Y and 1 D). So, your answers lies on the options for the primary windings. Converter transformers usually are connected to smalls industrial systems, that requires a D connection. HVDC transformers are connected to power systems that requires a Y connection. Now, why the each systems needs D or Y, it's a more deep question, but I believe you can find.

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