24 pulse AC-DC Converter

[derek22r]

I was asked a question by a colleague of mine did not know the answer. I'm sure someone here can help.

He is purchasing a 4KV AC/DC converter (not sure what for) and the vendor quoted him a 24 pulse system. This threw up a red flag because naturally the 24 pulse system is much more expensive than 6 pulse. His question was basically what is the difference a 24 pulse and 6 pulse converter.

In my research I found that using a 24 pulse system cancels out all of the lower order harmonics (2-22). I also saw that this is advantagous in HVDC systems. This is what I don't understand. Why would eliminated lower order harmonics matter more in a high voltage system?

 

[Jraef]

24 pulse essentially means they have 4 separate 3 phase rectifiers feeding a common DC bus. They put transformers in front of each of them and they deliberately create phase shifts between each one by virtue of using Delta and Wye windings with careful phasing of the primaries and secondaries. So when each rectifier draws its non-linear current, the harmonics created on one are at a different phase angle than the others and end up canceling out some of the harmonics created on the neighboring transformers. 12 pulse is better than 6 pulse, 18 pulse is better than 12 pulse, 24 pulse is the best.

 

[G._S._Ohm]

. . .naturally the 24 pulse system is much more expensive than 6 pulse.

How 'clean' does your DC need to be?

 

[Besoeker]

24 pulse essentially means they have 4 separate 3 phase rectifiers feeding a common DC bus. They put transformers in front of each of them and they deliberately create phase shifts between each one by virtue of using Delta and Wye windings with careful phasing of the primaries and secondaries. So when each rectifier draws its non-linear current, the harmonics created on one are at a different phase angle than the others and end up canceling out some of the harmonics created on the neighboring transformers. 12 pulse is better than 6 pulse, 18 pulse is better than 12 pulse, 24 pulse is the best.

Yes. We generally use two transformers each with a delta and a wye secondary. The primaries are delta with slightly extended windings to give +7.5deg on one and -7.5deg on the other.

I suppose it might be worth pointing out that, with a balanced system, the lowest order harmonic should the the pulse number -1 and so, for 24-pulse, the lowest would be the 23rd. The series goes 24n ? 1. Or 23, 25, 47, 49 etc.
The significance of this is that the magnitude of the harmonic current reduces roughly as the inverse of the harmonic number.

 

[Besoeker]

In my research I found that using a 24 pulse system cancels out all of the lower order harmonics (2-22). I also saw that this is advantagous in HVDC systems. This is what I don't understand. Why would eliminated lower order harmonics matter more in a high voltage system?

It is advantageous particularly on higher power systems because the remaining (higher order) harmonic currents are lower magnitude and cause less distortion on the supply.
If the equipment is rated for 4kV, it is probably quite high power.
Something taking a few hundred Watts generally won't have much of an impact on the supply even with a high percentage of harmonic current.

 

[Jraef]

...
Something taking a few hundred Watts generally won't have much of an impact on the supply even with a high percentage of harmonic current.

Until you start adding hundreds or even thousands of them together, i.e. electronic lighting ballasts, PC power supplies, UPS systems and small VFDs in large facilities.

 

[Besoeker]

Until you start adding hundreds or even thousands of them together, i.e. electronic lighting ballasts, PC power supplies, UPS systems and small VFDs in large facilities.

But then it would more than a few hundred Watts of course.....
I did think about adding the point you made but thought it was an unnecessary addition in the context of the question asked in the first post.

 

[Jraef]

But then it would more than a few hundred Watts of course.....
I did think about adding the point you made but thought it was an unnecessary addition in the context of the question asked in the first post.

Yeah, you're probably right. That wasn't germane to the OP.