Equal DC currents through star 3-phase transformer - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: November 02, 2020, 10:55:14 am
content: Normally, DC current through a transformer winding is bad news. It will saturate very easily. What though if you got a three phase transformer with a star primary and passed equal currents into each primary winding and out the star point? The direction of current flow being the same in each winding so that if for example you had 1 amp into each phase you would have 3 amps out the star point.Would the three magnetic fields in the transformer core cancel each other out or would they add up somehow and possibly saturate the core?

author: sandalcandal
author_rank: Unknown
timestamp: November 02, 2020, 11:24:06 am
content: I don't think you're going to get any combination of adding or subtracting \$iN\$ three times and end up with zero. Even with two opposite polarity windings, unless they are interleaved you'll get a local DC magnetic field under the coil. Normally with DC offset saturation you're worried about DC voltage, not DC current.

author: Circlotron
author_rank: Super Contributor
timestamp: November 02, 2020, 11:36:59 am
content: I'm just revisiting a thought experiment of having three single phase full-wave rectifiers spaced 120 deg apart feeding three choke input filters then combining the outputs in a way to achieve lowest ripple. The idea is that the ripple output of a choke input filter can be nearly sinusoidal, and feeding these three to a star "transformer" and getting the DC output from the star point.

author: sandalcandal
author_rank: Unknown
timestamp: November 02, 2020, 12:04:02 pm
content: In that case, I think you'd want them interleaved with the same polarity (on quite a large core so it doesn't saturate). You want them all parallel so that energy from one dipping or peaking will be shared to the other two phases/windings. Additional coupling from interwinding capacitance would be helpful in this case too. Not sure how that's any better than just comm'ing them together then feeding into a single "phase" filter capacitor + inductor though. Maybe if you want three separate but sorta coupled outputs? I suspect it might result in more losses (and more ripple) due to more energy sloshing about as magnetic fields.It should be possible for you to simulate all these situations and factors in spice.Edit: Modern high performance motherboards often use >11 phase power distribution systems for the CPU to get rock solid voltage rails with very high transient loads, they might be an interesting area for exploration.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 02, 2020, 02:31:39 pm
content: Reference to some of the ISO, USA standards on rectifiers,will show diagrams of many of the three phase connections, along with main parameters.Usually in your example, the 3 DC chokes would be reduced to just one choke to save cost, with no loss of ripple reduction.And similarly, the rectifier would be just a 3 phase bridge.The industrial rectifier systems I worked on, up to 10 MVA, were always phase displaced groups of bridges, to reduce the DC ripple and the harmonic demand.One transformer with Y & D secondaries will give 12 pulse, then add one more transformer phase displaced to give 24 pulse.A good reference, but old, a Siemens book : Moltgen "Line Commutated Thyristor Converters"does analytical analysis of the main topologies in an understandable way.DC, even small transient amounts in any leg of a 3 phase GOSS rectifier transformer is problematic, ( I know it!)which is likely why primary phase control is not very common.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 02, 2020, 03:34:31 pm
content: Quote from: Circlotron on November 02, 2020, 10:55:14 amI think there is a misunderstanding that needs to be corrected:This is only valid for any DC current in the primary, but not of in the secondary.Indeed, the magnetizating inductance is locate at the primary and not at the secondary.Remember the equivalent diagram of a single-phase transformer:A dysimetric current at the secondary create a DC component of the current, but, if the primary voltage is not affected by this dysimetric current, there is little or no influence on the magnetization of the magnetic circuit and nothing happens.

author: David Hess
author_rank: Super Contributor
timestamp: November 03, 2020, 02:30:15 pm
content: A three phase transformer is made up of three single phase transformers so there is no cancellation between cores.

author: Yansi
author_rank: Super Contributor
timestamp: November 03, 2020, 02:32:15 pm
content: Quote from: David Hess on November 03, 2020, 02:30:15 pmThis and exactly this.You are not going to get any DC cancellation using any regular 3ph transformer.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 03, 2020, 04:01:03 pm
content: The concern with half-wave rectification, both in a single-phase transformer and in a three-phase transformer, is not the magnetic field, but the misuse of copper due to the high RMS value of the current.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 03, 2020, 04:01:50 pm
content: Furthermore star (Y) secondaries feeding a set of half wave rectifiers, will produce what was called "yoke flux"which makes the whole transformer like a DC solenoid. eg north at the top, and south at the bottom.That topology might be similar to the effects mentioned in Circlotron's first post.I did not get any hits searching "yoke flux" but I remember the old guys talking about it,and the reference book I refered to earlier has a detailed description.Half wave circuits were used in mercury arc rectifiers which had 3 or 6 anodes and a common cathode.I suppose the transformer designers knew about it at the time and reduced the working flux density accordingly.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 03, 2020, 04:46:05 pm
content: I also have this book "Line Commutated Thyristor Converters" from Siemens.On pages 38 to 48, we speak about "converter transformers" and, even if they speak about a yoke flux and premagnetisation that make the magnetising current higher than during operation with normal three phase load, there is no question of saturation of the transformer by the secondary current.It is, of course, recommended to use a zig zag secondary coupling with mono-alternance rectification.A three phase transformer should also have at least one winding delta connected to eliminate third harmonic current.It should be remembered that a three-phase 3-column transformer is not symmetrical in terms of the magnetic circuit and that the central phase has a lower short-circuit impedance than the 2 other phases for this reason.