Do a group of small VFD's equate to one large drive i.t.o harmonics?

[veritas]

I have a set of around 20 small VFD drives (4kW) all with 6-pulse VFD's. Do all the smaller drives produce the same harmonics on the system as one large 80kW drive? or do the harmonics interact and in some way cancel each other?

Thanks.

 

[GroovyGuy]

Hi Veritas,
I think that you will find that there is no 'magical' cancellation of harmonics in the 20 x 4kW ASD scenario vs a single 80kW ASD.
The only way to cancel any of the harmonic currents is to say move half (ie 10)of the 4kW ASDs over a 30 degree phase shifting transformer (ie delta-delta) and essentially make a twelve-pulse drive (at least as far as the power system is concerned).
Otherwise if you truly have a harmonic issue, you could possible solve this by applying either passive or active harmonic filters.
GG









"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[ScottyUK]

Numerous smaller drives are equally bad, and perhaps worse, than a single larger drive because the larger drive will need to meet more stringent requirements for harmonic pollution of the supply. In the specific size ranges you're talking about I'd say about equally bad.

 

[Skogsgurra]

Yes, worse. Not better. There is (usually) not much inductance in smaller drives. Larger ones have it (also usually).

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

I like your thinking Groovy.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

He doesn't like our thinking Skogs.

 

[Skogsgurra]

I noticed that, Scotty

Bur we think like GG when it comes to high-powered drives. Hardly worth a special transformer with double secondaries if you shall supply less than 100 kW. We do it routinely when high power motors, like steel mills and large pumps at some 5+ MW are involved.
So, maybe Smoked likes our thinking at those levels.

BTW: Connecting half the load to a delta/delta and the other half to delta/wye doesn't help much if it is the LV THD that bothers you. The 5th and 7th harmonics cancellation is only effective on the MV side.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

So, maybe Smoked likes our thinking at those levels.

Even better!

I'm not actually clear on how one can rationally EVER swap out a bunch of little drives for a single big one. Seems a pretty big stretch.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

The harmonica that cancel each other out are the same whether it is one large drive or multiple smaller ones, the harmonics we worry about I 3 phase systems are the non-triplen odd orders, and they are all addidtive regardless of source.

But also I’ll concur than small drives will typically be worse because they almost never include DC bus chokes (inductors) whereas larger ones often do. That’s not a given however, many low cost mid-size drives do not use DC bus chokes as well.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Skogsgurra]

Yes, my (or our) point is that we also cancel lower odd harmonics if a delta/wye AND a delta/delta is wound on the same core. And it is not practical to have a special transformer for such a small total load. Hence the mentioning of MW drives.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[veritas]

I wish to thank all respondents. My question was more focused on the modelling of the drives in SKM's Powertools. Do I model each 4kW drive individually, or will I get the same effect harmonics wise if I model them as one 80kW drive? I've modeled them as one large drive based on the above.

In fact, I did a test run where I modelled 5 x 4kW drives in one scenario and 1 x 20kW drive in the other scenario. The results were exactly the same.

 

[veritas]

Actually something interesting I did note but cannot explain is that the presence of DOL's does effect the harmonics produced by the drives with VFD's. It actually reduces the harmonics on the system. Anyone know why?

Thanks.

 

[stevenal]

Harmonics are expressed as a percentage of the fundamental. You are increasing the denominator.

 

[veritas]

Ah, yes, makes sense - thank you.

 

[LionelHutz]

But the harmonics produced by the VFD's does not change or get reduced with DOL motors on the system. Just the total percentage of harmonics compared to the fundamental current on the system goes down.

 

[Skogsgurra]

When you ran the simulations (comparing 5x4 and 1x20 kW), did you include any magnetic components? Like line reactors or DC bus choke?

If not, you shouldn't see any difference wrt harmonics. And since you didn't see any difference, I assume that you just changed power rating.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[veritas]

No, I did not change any magnetic components.

 

[Skogsgurra]

Computes.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Marke]

Gunnar is correct, the smaller drives are often devoid of reactors, or have smaller reactors so draw a higher harmonic current, more harmonic energy, adds up.

Some VFD manufacturers include 3% or 5% reactors even in very small models, but many do not.
If the small drives do not include reactors, then you can expect that the full load THDi will be in the order of 80% to 110% depending on the reactance of the supply. If the large drives include reactors, then the full load THDi could be in the order of 40%. If the large drives do not include reactors, then the THDi % will be much that same as the small drives.

The presence of DOL motors on the supply will help to reduce the THDv (not the THDi) because the DOL motors will appear as a lower impedance to the harmonic voltages due to the "high slip" of the harmonic voltages. The harmonic currents into the motors will increase as will the slip losses in the rotor.
I have seen examples of melted rotor bars due to high harmonic voltages. Many simulators assumme that DOL motors look resistive to harmonic voltages and this is not correct.

Mark Empson
Advanced Motor Control Ltd