12 or 24 pulse drive

[cmelguet]

Hi,

I am working on some MV drive specifications, the idea is to select the same drive for different applications in the plat. I would like to know what to look for in order to decide if to select a 12 or a 24 pulse drive (18 is not possible since the manufacture is already selected). The application is for ciclone pumps and conveyors between 1000 and 2000 kW motors. Thanks and regards.

 

[jraef]

LOL, mfr already selected, NOW you want to know about the right technology... What do they recommend? Seems to me if they have a lock on the project, they at least owe you a decent report on what is best for YOU. Personally if it were me, I would have looked at an Active Front End drive so that I could mitigate the harmonics AND provide line regeneration on the conveyor drive. Any conveyor needing 1000kW or more is going to have a LOT of mass, and stopping that may be an issue.

All animosity aside though, it really depends TOTALLY on your need for harmonic mitigation. 24 pulse will provide less THD / TDD than 12 pulse. Plain and simple, that is ALL it is about. If you need the least amount of harmonic distortion possible, go for 24 pulse. If you need some but can't afford 24 pulse or you want to reduce the component cost and thus increase the MTBF (a little), then go for 12 pulse.


"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[dpc]

I agree with jraef's comments. The only advantage to the 24 pulse rectifier section is reduced harmonic distortion on the input. Everything else argues in favor of the 12-pulse - smaller, cheaper, fewer components, etc.

 

[cmelguet]

Thanks jaraef and dpc, my question is more about, how can i justify the requirement for a 24 pulse variable speed drive instead of a 12 pulse one ??. The conveyors will not use regenerative breaking (conveyors manufacture desicion). The plant will have an harmonic filter installed in the 33 kV main substation busbar where it connects to the utility service. So from a IEEE 519 point of view is not a problem the amount of harmonics that the utility will see on the PCC.

Regards

 

[jraef]

Then get a 6 pulse drive. Really, the ONLY reason you have a multi pulse front end on a drive is for harmonics mitigation. If you have an active filter system further up stream, then why waste the money here?

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[electricuwe]

jraef, a harmonic filter is just a bunch of capacitors and inductors tuned to trap harmonics. An active filter uses fast switching power semiconductors to do this more flexible.

Harmonic filters at 33 kV are tremdously expensive and active filtering is not available at this voltage level.

Using multipulse rectification is usually the least expensive way to mitigate harmonics.

 

[dpc]

I don't think the drives are 33 kV. He indicated they were installing harmonic filtering at the 33 kV utility interface, regardless of the VFDs.

 

[jraef]

electricuwe,
I don't disagree with you, but I am basing this on the OP's statement

"... The plant will have an harmonic filter installed in the 33 kV main substation busbar where it connects to the utility service. So from a IEEE 519 point of view is not a problem the amount of harmonics that the utility will see on the PCC."

He has ALREADY decided that harmonics is not an issue for him, so I'm taking his point and saying that if that's true, then why consider a harmonic mitigation strategy?

Do I think he is correct that a harmonic filter on the 33kV bus is a good solution for the variable nature of a loaded / unloaded / partially loaded large MV VFD? No, but that was not the question he asked.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[LionelHutz]

The filtering requirements and expense of any harmonic filtering would certainly be reduced if a 12-pulse is used compared to a 6-pulse.

The 12-pulse would be a good choice if a harmonic filter is required for other equipment anyways.

The 24-pulse would be a good choice if the harmonic filter is being considered only for this new VFD equipment, since the 24-pulse would remove the need for using a harmonic filter.

 

[MAGTiger]

This is a cost/benefit analysis.
"Using multipulse rectification is usually the least expensive way to mitigate harmonics". Come on, line reactors are the least expensive way to mitigate harmonics. This analysis can't be made without costs, target THD and realistically you need to know the existing harmonic signature in the operation.


Neil

 

[electricuwe]

When answering I assumed Voltage Source Drive topology (as this is the common topology in the power range nowadays)

To jraef:

The question is for what purpose the 33kV harmonic filter is to be installed . Even if it is needed regardless of the choice for the drives, additional low order harmonics from the drives will need to be considered for it's design.

To MAGTiger:
In this case where the individual drives rating is in the range of 1000..2000 kW and every drive needs an own transformer, the additional expenses for going 12-pulse are very low. Line reactors are fine for reducing harmonics in drives connected to the LV-grid, but only as long as the requirements are not to stringent.

 

[jraef]

electricuwe,
I know. My point was that his approach to this was flawed from the outset, because he had ALREADY determined that "harmonics was not important" (paraphrased) and then wanted to compare the benefits of two technologies that were essentially ONLY relevant to harmonics. In otherwords he was putting the cart before the horse.

But of course it's all moot, I must have scared him off...

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[cmelguet]

Hi to all, sorry for the late replay, i was on hollidays for a few days. I will be more clear: The main substations is on 33 kV, where the design contemplates passive filters to compensate harmonics for big cicloconverters (100 MVAr compensation). On the other hand the VDF`s are connected at 4.16 kV bus bar which is taken from a 33 kV/ 4.16 kV transformer. There are several 4.16 kV busbars around the plant and to these busbar not only VFD`s are connected. There are also DOL motors on 4.16 kV.

Since we have a big passive filter at the 33 kV main station there is no problem from the utility point of view. But what happend with the harmonics inside the plant? What about the motors connected to the same busbar that the VFD`s? Shoud I take care of these equipment in order to select a 24 pulse VFD`s instead of a 12 one?. Is the transformer feeding the 4.16 kV busbar, were the VFD´s are connected a reason to go for a 24 pulse?. Regards and thanks for the answers so far.