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.