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From the twelve-pulse bridge subsection of the Wikipedia article on High Voltage DC converters:

The phase displacement between the two AC supplies is usually 30° and is realised by using converter transformers with two different secondary windings (or valve windings). Usually one of the valve windings is star (wye)-connected and the other is delta-connected. With twelve valves connecting each of the two sets of three phases to the two DC rails, there is a phase change every 30°, and harmonics are considerably reduced. For this reason the twelve-pulse system has become standard on almost all line-commutated converter HVDC systems, although HVDC systems built with mercury arc valves usually allowed for temporary operation with one of the two six-pulse groups bypassed.

I've looked at this and this answer, but I'm not familliar with the terminology or notation of phasor diagrams and their use in transformer winding connections.

Is there a simple way to explain how a star (wye)-connected and delta-connected set of windings can provide a phase change every 30°? How does this produce twelve equally-spaced pulses per cycle?

Image: "12 pulse bridge with thyristor valves" from here.

enter image description here

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  • \$\begingroup\$ I'm voting to close my own question as a duplicate of Why does a delta/wye transformer make 30 degrees phase shift?. SE found a third question my search missed, and the answers there do a good job of addressing my question. I'm not sure if it's better to close-as-duplicate or to delete; duping helps bring future readers to high quality answers, but it exposes me to down votes. Hopefully people will recognize I'm trying to "do the right thing". \$\endgroup\$
    – uhoh
    Aug 5, 2017 at 5:38
  • \$\begingroup\$ I did try a simple explanation but it was not well-received... \$\endgroup\$
    – Solar Mike
    Aug 5, 2017 at 5:50

2 Answers 2

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Draw a delta, and draw a star across it from the same three vertices. This is effectively a phasor diagram.

Measure the angle between each leg of the star and either delta from the same vertex. You should find it's 30 degrees.

So between three phases of a delta waveform and three phases of a star waveform, and their six complements, you will find 12 waveforms equally spaced at 30 degrees.

Now, note that a single phase bridge rectifier provides two output pulses per cycle, one from the waveform's peak, and one from its complement (i.e. a positive pulse from the negative peak).

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  • \$\begingroup\$ Did you see the part where I said "...but I'm not familliar with the terminology or notation of phasor diagrams and their use in transformer winding connections"? I was looking for an answer that did not go in that direction. I did not understand how connecting windings in a transformer like that can make these shifted waveforms. The answer for which I've marked this as a duplicate explained it to me. None-the-less, +1 for leaving an answer. \$\endgroup\$
    – uhoh
    Aug 5, 2017 at 12:05
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I think the quoted "a star (wye)-connected and delta-connected set of windings can provide a phase change every 30°?" is badly worded. I would say that each phase in turn drives the output voltage.

The arrangement in your illustration is not familiar to me but would work as shown below in Figure 1. Note that it is effectively two series connected DC supplies with their ripple voltages offset by 30° due to the delta/star arrangement.

enter image description here

Figure 1. 6-phase delta-star rectification.

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  • \$\begingroup\$ ...and yet it answered my question and solved my problem. \$\endgroup\$
    – uhoh
    Aug 5, 2017 at 11:53
  • \$\begingroup\$ Good. I've improved the graphic a bit by adding vertical reference lines and added a sentence to the second paragraph. \$\endgroup\$
    – Transistor
    Aug 5, 2017 at 11:57
  • \$\begingroup\$ oh, no. I was addressing your first sentence; "I think the quoted a star (wye)-connected... is badly worded" ...and yet it answered my question and solved my problem. I marked my question as a duplicate because that answer turned up later after I'd done my original search and posted the question, and it literally answered my question and solved my problem. I understand what a drawing of bunch of 30° shifted sine waves looks like, it's the way that star and delta winding configurations behave that had my stymied. Anyway +1 for your answer nonetheless! \$\endgroup\$
    – uhoh
    Aug 5, 2017 at 12:02

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