Adj Freq Drive Type selection criteria - 6-pulse, 12-pulse, etc. 1

[dpc]

In the water and wastewater treatment world, we encounter some owners (and consultants) who want to establish specific requirements for AFDs based on hp size.

I have seen specifications requiring 12-pulse drives for anything larger than 100 hp and 18-pulse for anything larger than 250 hp. The concern is to minimize harmonics on the ac system.

I'm curious as to what others are seeing in specifications and what you would recommend.

What are your views on 12-pulse (and higher) AFDs versus incoming line reactors, filters, etc and what criteria would you apply in making this decision?

TIA,

Dave

 

[Skogsgurra]

I do not see 12-pulse below 600 - 800 kW (that would be around 800 - 1050 HP). The reason is probably because all these drives have their own transformer or a few drives use one transformer that is mostly in the 1200 - 1800 kVA, sometimes up to 2200 kVA, range.

There are (in Europe) standards saying that you should not have more than 10 percent voltage THD at the point of common coupling (PCC). The limit is 8 percent for medical buildings, computer centers and other more sensitive places.

The more conservative guys try to keep voltage THD below 4 percent. But that does cost some effort and money. Actually, I have never had any problem with any equipment due to THD. HF pollution from inverters and thyristor controllers, yes, but never harmonics. I once popped in to get a chat with a guy at a paper mill. And since I had this (then) new Fluke 41 analyzer, we had a look at his mains. He had 14 percent THD - and he had never noticed!

Actually, it is up to you and your customer to decide what your THD upper limit should be. If you are cautious, make it 4 percent. If you are economical, make it 10 percent.

The biggest risk you will meet is that low-voltage PFC capacitors can resonate with the transformer. You have to check that. On the other hand, it is not a good idea to put the capacitors on the low voltage side if you have lots of VFDs and thyristor controllers.

Gunnar Englund

http://www.gke.org

 

[jraef]

I think the subject is too important for generalizations. There are many ways to deal with the harmonics created by VFDs, all of which have merits. The best approach is to look at each situation and determine the best for that single project, taking into consideration the scope, utility requirements, budget, likelihood of proper maintenance, future growth etc. etc. etc. Here is an example when there are multiple drives at one location.

Using higher pulse count front-ends is a way of having inherent harmonic mitigation that is not dependant on external components and future changes in the system. For an installation with multiple medium size drives however, having each drive contain a 12 pulse front end will be considerably more expensive than having 6 pulse front ends on the drives and one active filter system that reacts to whatever is created by whichever drives are running. Then again if the system is expected to grow a lot later, it may not be practical to buy a large enough active filter to accommodate future growth, so having the mitigation be part of the VFD puts the future mitigation costs into the future budgets. A side issue is that an active filter becomes the single point mitigation device, so if it goes down you have nothing, whereas if one drive goes down you still have mitigation built in to each of the remaining drives.

If you have only one drive at a pump station, that can be another issue all together. A reactor with a passive filter may be all that is required there, depending on the existing conditions and proximity to other grid connected users. In some remote areas, people don't get exited about harmonics at all.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[dpc]

What I generally see in the US is reference to IEEE 519 which limits voltage distortion at the point of common coupling with the utility to 5% THD. It also has limits on current distortion - these can be more difficult to meet.

For a supplier of an AFD to a system that probably already has multiple drives, the difficulty is in sorting out the added distortion from the existing, so these performance-based spec requirements can descend into finger-pointing pretty quickly, unless some type of harmonic spectra of the existing system is provided, which is usually not done.

jraef,

I was thinking more of larger facilities, not pump stations. Pump stations do have different requirements, since they often have to run on a standby generator that might be about the same kW rating as the AFD. We had a lot of problems with harmonics on these pump station installation twenty years ago due mostly, I think, to interference with the electronic voltage regulators. Lately, I don't hear of too many harmonics issues.

 

[NNG]

I support Jraef’s views for generalization with harmonic mitigation on drive’s applications in industries. It’s up to customer to choose their applicable drive configuration (cost, size, and manufacturer).
As many drive’s manufacturers (ABB, AB, SquireD and etc.) offer now a preliminary harmonic study of the customer power distribution system to evaluate the harmonic levels within their system and select correct type of the harmonic mitigation technique (6p, 12p, 18p , passive and active HF).
The 12 pulse Drives are mostly gone in past due to the most of the drive manufacturers are using 18 pulse configurations with phase-shifting AutoXFMR’s now. This 18p AutoXFMR has less size and cost vs. its Isolation version.
I’m disagree that the impact of the Harmonics in power distribution system can not effect on proper functionality of the system as overheating, malfunctioning and other others.
If the harmonic study for a large system will not perform properly than these problems will show up later.
Please, see attached brochure as sample for the drive applications:

http://literature.rockwellautomation.com/idc/groups/literature/documents/br/drives-br007_-en-p.pdf

Thanks, NNG.

 

[dpc]

I do have a suspicion that part of the reason for specification of 12-pulse and 18-pulse at specific kW ratings is to give a competitive advantage to a favored manufacturer.

In most cases these drives are specified as part of an overall construction specification, so the drive supplier is a third or fourth tier subcontractor in many cases.

I also think part of the motivation of specifying an 18-pulse drive is that there is perceived to be no need to even evaluate harmonics, since impact on the system should be minimal. It's a good CYA approach for consultants, since it reduces contractual interfaces and potential risks. I just feel that in many instances it is complete overkill, especially when no consideration is given to use of filters in place of 18-pulse drives.

 

[DickDV]

dpc is right on concerning 12 and 18 pulse drives being overkill and a protective reaction on the part of a&e firms, in many cases.

The curious part about 12 and 18 pulse drives is that equivalent harmonic mitigation can be acheived with lower initial cost, lower part count, and higher system efficiency by using filters, as dpc mentions.

Mirus is a Canadian company that produces filters certified to produce 12 pulse and 18 pulse results with 6 pulse drives and, having used their products and evaluated the resulting harmonics, I can heartily recommend them. There may be others. Mirus just happens to be one that I am familiar with.

It is noteworthy, I believe, that these types of filters almost never end up on spec's as a suitable alternative to the more complex and expensive 12 and 18 pulse drives.

 

[LionelHutz]

I personally see a lot of specifications stating that the drive installation must meet limits as specified in IEEE519-1992. Typically, there is nothing on how to meet the limit but just to meet the limits. The other thing I see a lot is that there is usually never a requirement to measure any harmonics and you can openly quote a system that likely won't meet the limits with acceptance. It seems to me that the IEEE-519 clause is being used as an escape clause in case there is a problem with the installation more than a requirement for the installation.

Dick, I take it you don't live in Canada? We see Mirus appearing a lot in Canadian specifications. Their older designs had some issues I wasn't too impressed with but they seem to be much better now and they do seem to work.

I think that the problem with the harmonic levels is that for the most part there is no clear fixed number for the harmonic levels produced by the different front-end configurations. We can tell you which harmonics but not their levels unless details about the system are known. This makes many decide it's easier to go for the lowest harmonic producing piece of equipment than to actually figure out what is needed to just meet the set limits.

If you want to achieve IEEE-519 harmonic control without doing any system analysis then you're basically stuck with using 18-pulse or possibly a Mirus filter. These are the only 2 I know that will (or at least come close) meet IEEE-519 distortion limits at their connection terminals which means they won't go over the limits when installed in the system. However, this approach is usually overkill since any other linear loads on the system will allow the VFD to produce more harmonics while the system still meets the limits at the PCC. This is because the point of common coupling (PCC) is usually specified as the utility connection. Some of the small utility pumping stations are all VFD pump load except for a couple of lights and the PLC system so each VFD pretty much does have to meet the limits individually anyways.

 

[LionelHutz]

I forgot about active front-end VFD's also being capable of meeting IEEE-519 at their input terminals.

I'd also be curious if the patented 18-pulse transformer that AB mentions in that brochure linked by NNG is actually an AB patent.

 

[ozmosis]

The specifications I see where the statement is "you must meet IEEE519-1992" is usually just that. There is usually never any indication on the site fault capacity, other background harmonics, transformer ratings/impedance etc. When you ask the questions on above, the reply is usually limited and yet you are still expected to 'comply'. You have to comply with a standard that is not a product standard but related to the system, and yet you are supplying a product. The stock answer is: this drive will generate zero harmonics until connected to your supply. Once you tell me what your supply is then I will tell you what the harmonic distortion will be from this product!
So, why do 12/18/sometimes AFE units get supplied into installations that may not need it? To cover your/contractors backside usually.

 

[NNG]

On main questions from this forum: “…In the water and wastewater treatment world, we encounter some owners (and consultants) who want to establish specific requirements for AFDs based on hp size.”
My opinion in general without harmonic study:
1. 1 HP up to 10HP 6 p VFD (Individual) & 6p VFD + DC link Ind. or Input LR (system)
2. 10 HP up to 50HP 6 p VFD + DC link Ind. or Input LR (Individual) & same (system)
3. 50 HP up to 200HP 6p VFD + Passive Harm Filter or 18 p VFD (Individual) & 18p only (system)
4. 200HP up to 1000HP 18p VFD or AFE (Individual) & 18p VFD or AFE (system)
5. above 1000HP – customer specific installations

“… What are your views on 12-pulse (and higher) AFDs versus incoming line reactors, filters, etc and what criteria would you apply in making this decision?”:

Pros and Cons on input harmonic level of the VFD”s:
6 p VFD – Current THD above 50% (low cost, smaller size, reliability)
6 p VFD + DC Link Ind - Current THD around 25% (low cost, smaller size, reliability)
6 p VFD + Input LR - Current THD around 25% (low cost, smaller size, reliability)
6 p VFD + Passive Harm Filter Current THD >5% (low cost, size, resonant problems)
18p VFD + AutoXFMR Current THD <5% (high cost, size, reliability)
AFE Current THD <5% (high cost, size, less reliability)

Otherwise, as I mentioned before, the consultants can request an estimation of the harmonic levels in preliminary study through their spec. That will be determine exactly what type of VFD can comply with the harmonic limits.

 

[DickDV]

As I suspect most on this forum already know, IEEE 519 is a network spec and not a component spec. This fact is lost on many spec writers and, naturally, the resulting spec is a bit ridiculous.

All this handwringing about what to buy to meet the network spec is reduced greatly when the drive supplier is willing, sometimes without charge, to do a network analysis of the intended future system with advanced power quality software. Of course, someone has to measure the lead lengths, other loads on the system, transformer impedance, etc, etc, but, the results are very accurate generally. The drive manufacturers that I am close to will do this without charge as long as they can bid on the drive.

When the neither the consulting engineer or the customer is willing to gather the data necessary to do a proper analysis, the common reaction is to spec a drive (typically 18 pulse) way beyond the real requirement just to be nice and safe (and comfortable). Who cares, it isn't my money!!

Anyone familiar with Robicon drives knows just how far this can go---they build 30 and 36 pulse drives!!! In American, at least, if a little is good, alot must be better!!!

Harmonics, especially current harmonics, can be genuinely troublesome and cannot be responibly ignored. But the solution should be approached with equal responsibility. Often it isn't.

 

[NNG]

DickDV,
great job to summarize our efforts in this endless post!
Thanks, NNG.