18 Pulse VFD w14% VTHD????

[Ceast]

I've spent the last 3 days collecting PQ data on a AB 7000 drive. 17th & 19th & 35th & 37th & 53rd & 55th harmonics all way above limits. The control is intermittently stopping/faulting and shutting down the drive. I don't have enough experience with these drives to begin to troubleshoot. Any thoughts?

 

[Marke]

Hello Ceast

The first question that I would ask, is what is the THDv when the drive is not running?

Secondly, where is the measurement made relative to the drive?
and third, what is the fault current and the drive rating?

Best regards,
Mark

Mark Empson
L M Photonics Ltd

 

[Ceast]

Mark, thanks.

THDv disappears when the drives are shutdown. Data was recorded at the xfmr 15kV feeder. Xfmer=13.8/2300 delta/delta-delta-delta. Also data recorded at the 480 bus. Same high harmonic content on the 480 gear. The 480 xfmr is a Y/D. One would expect some mitigation through the 480 xfmr. Not the case.

I assume that the distortion is sourced from the drives because the signatures are multiples of 18. I don't see the distortion coming from the 480 bus.

 

[rbulsara]

I am not a drive expert but..

What is the fault code on VFD? Did you consult the drive mfr.?
Is this the only drive on this type and size? If not, compare the readings with the drives that do not have the problem.

I am sure you did some research before deciding to monitor the PQ.

Rafiq Bulsara

http://www.srengineersct.com

 

[Ceast]

Yes the manufacturer is chasing the problem. I'm doing my own follow-up for future reference and education.

2 drives on the 15kV bus both suffering the same distortion. With one drive off the VTHD is half.

 

[rbulsara]

Does the other drive fail too or only one?

Rafiq Bulsara

http://www.srengineersct.com

 

[Skogsgurra]

New equipment? Always been like this? Or did it work OK before?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[MAGTiger]

Input voltages balanced with the drive offline? When you claim the high order harms are way above limits. To what limits are you referring? What do the other harms look like?

Neil

 

[LionelHutz]

14% distorion on those higher order harmonics just does not sound right at all. A 6-pulse would not have that much higher order harmonic distortion. Remember, the drive is causing current distortion and then the current harmonics cause system voltage drops which create the voltage distortion. If you have 14% voltage distortion then you likely have quite a few orders more current distortion.

In most cases I've metered, a drive with 30% to 100%+ current distortion might only cause a few percent voltage distortion.

 

[Ceast]

See limits = IEC61000-2-12 60Hz

Balanced input voltages.

New equipment

Both drives fail.

 

[Skogsgurra]

Commissioned by supplier?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sibeen]

What is the current THD and individual current readings?

 

[electricpete]

I have very little basis and experience with vfd's to make any comment. But if you'll forgive a very blind stab, I have a vague idea that high 6th harmonic of line frequency on the dc input to the PWM might (?) cause this. Is it possible to monitor the ripple of dc on output of your rectifier / input to the PWM ?

=====================================
(2B)+(2B)' ?

 

[sibeen]

I don't know the AB product so had a quick look at the website. This seems to suggest that these drives have an active front end.

 

[Marke]

Hello Ceast

I am surprised to see that level of harmonic voltage coming from a VFD.
This suggests that the VFD has no reactors, either DC or AC and that the supply is very inductive.

The 18 pulse drive option will cancel some harmonics, but not all, however the higher order harmonic currents should not be high enough to create such high harmonic voltages.
I would check to see if there is either a DC bus choke fitted or AC line reactors. If they are not fitted, are they an option?
If you fit reactors, they must be balanced and equal on all rectifiers. You can have a DC bus choke in series with each rectifier, or a single reactor after the combined rectifier output. If AC reactors are used, they should be fitted at the input of each reactor.

With reactors fitted, I would expect the high order harmonic currents to be sufficiently attenuated to not cause any harmonic voltages of significance.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

 

[LionelHutz]

Does that VFD have any type of LC or LRC filtering on the input? I know some AB models do and those filters can trap or even resonate with the system.

Seeing the current and voltage harmonic spectrum would be more useful than those curves you posted.

 

[MAGTiger]

Multipulse transformer are sensitive to voltage imbalance. Check that the phase to phase voltages are close better than +-2% would be good. Check at the transformer input terminals. If they are balanced, then check the output terms be careful there are a lot of them and check that you are matching up the correct phases. Can you post a spectrum including evens?

Neil

 

[Ceast]

All, thanks for the input. I'm following up.

Chris

 

[peter250]

Are you seeing any problems with the DC link not staying up? With an 18-pulse diode front-end, you can lose a diode or two and the drive will still run, albeit with bad harmonics and intermittent tripping of the drive. The trips will occur during acceleration and load changes as the remaining diodes can't supply enough power to keep the drive up and running.