VFD, Load Side Reflected Wave on an Ungrounded Delta 3

[MikeNice]

I thought this was interesting and wondered if anyone in the forum had any experience with it. I've been on the job and out of college for a year, so I have limited experience in most issues. The following is a simplification of the local system and the problem/issue description is how it was described to me by a senior engineer.

System of interest:
(1) 3ph, 460V, 150HP motor w/ ABB VFD combination in service since 2000
(4) 3ph, 460V, 10HP motors w/ AB PowerFlex40 VFDs recently installed -- ~100 ft. distance from drive to motors
(1) 3ph, 6.9KV/480V ungrounded delta, SUS w/ numerous other loads that will be neglected
(1) MCC feeding to the VFDs

Description:
A few weeks ago the ABB drive failed due to an internal circuit board being scorched. As indicated above, the ABB drive had only been in service since 2000. The question asked was why is this drive failing already? A voltage monitor was setup to measure any spikes at the ABB drive. It was set to trigger at a 5% deviation from system voltage. Upon returning (x) time later, it was discovered that there were intermittent ~1000V pulses hitting the drive.

After some deduction, and some motor starting it was found that the ~1000V was only seen when one of the four 10HP motors being fed from a newly installed VFD was started. After a Megger test, it was found to have had a grounded phase. The individuals who installed the VFDs had grounded them to the motors (this is where my understanding becomes foggy). Because the system is an ungrounded delta, when the motor phase grounded it had the ground to the VFDs ground bus as a return path but had no where to go to ground?

The (4) 10HP motors are fed from the MCC, which is fed from the SUS. While the 150HP motor is fed directly from the SUS. The reason I included the distance from VFD to motor is that I have read that reflected wave becomes a problem at greater distances.

Any comments or explanations are welcome. Thanks.

Michael

 

[Skogsgurra]

The 1000 V spikes are not at all uncommon. They are rather the rule. They do not have any bad influence on the drives, but can affect the motor insulation if the motors are not inverter rated.

Having one motor phase grounded(!?) is a bad idea. But I still do not see why it should influence the drive for another motor. What parts of the drive were hit?

Are there any filters? On inputs or motor side?

Drives fail. Some very soon and some "never" - or after a very long time. It is the old bell curve again. Before knowing more about your case, I would say it was either a spontaneous failure or some abuse of that seven year old drive. I have had random failures occur after one or two years. MTBF numbers are not always THE truth.

Gunnar Englund

http://www.gke.org

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

 

[MikeNice]

Hey Gunner.. thanks for the reply.

No filters, no isolation transformer. I'm not sure what parts of the drive were hit.

The (4) 10HP motors were connected to the ground buses on their respective VFD. After talking with AB, I was told that these drives have protective MOVs that are referenced to ground and that the onboard jumper needs to be removed to protect the drive. Furthermore, it was explained to me that anything that's referenced to ground would have seen similar voltage levels. Since the ABB drive on the 125HP motor had some reference to ground it also saw ~1000V.

From what I understand, they ended up installing a 1:1 isolation transformer (delta/wye) to the ABB drive, grounding the wye, which will allow a path for any other extreme voltages to flow?

 

[Skogsgurra]

You may be interested in a discussion we had a few months ago. It is in thread237-179482

Gunnar Englund

http://www.gke.org

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

 

[DickDV]

Speaking with good familiarity with ABB and lesser familiarity with other brands, it is safe to say that installation of any inverter on an ungrounded delta power source is asking for trouble.

At minimum, you must disconnect the internal filter and protection networks that reference ground inside the drives. Both ABB and AB make provision for this with screws etc that must be removed. Doing this will at least allow the drives to survive on a floating system. This was probably not done to the ABB drive and, on the first event grounding on of the supply phases, the protection network went up in smoke. It's very predictable.

Doing the above changes in the drives is not the only problem. The drives, (I don't believe it matters much what brand) must have some way of knowing where ground is in order to effectively do ground fault protection to the motor. Unfortunately, this point of reference doesn't exist when the grounded networks in the drive are removed so output side ground fault protection is not possible. It must be turned off in the drive software or it will be the source of many nuisance faults.

Bottom line---ungrounded distribution is a bad deal when inverters are involved. Ideally, the drives should be provided with an isolation transformer arranged delta to wye. It sounds like this is what you have already done and there is no better solution.

If the above isn't bad enough, running drives directly off ungrounded sources is just asking for common mode noise problems since there is no direct path for output drive pulses to pass thru the ground system back to the DC bus. An isolation transformer takes care of that nicely too. This pulsing-thru-ground problem is dealt with more completely in the thread Skogsgurra mentions above.

 

[LionelHutz]

Well, if the 10hp drives are still on the delta system then the problem isn't fixed yet.

The motor had a grounded phase? The VFD alternates by switching the motor leads between the positive and negative rails of the DC buss at it's carrier frequency which is typically about 2kHz to 8kHz. So, as it is switching, each side of the DC supply would have been alternately grounded. This would mean that as the different diodes in the rectifier conducted they were grounding the different input phases. So, your ungrounded supply would have been swinging all over the place compared to ground.

As others have said, the phase to ground protection probably wasn't disconnected or can't be disconnected in the ABB drive so it failed when subjected to this.

I'm surprized that didn't cause problems with the 10hp AB drive.

 

[DickDV]

I would expect the AB drives to go bad too. Plus, if the motor has a grounded phase, the ground fault protection must be turned off in the drive, as it needs to be just to get the drive to run. One problem just leads to another!

LionelHutz, you are precisely right about the floating supply leads getting yanked to ground when the diodes conduct. Now imagine how much excess current flows in those diodes if there is significant capacitance between those supply phases and ground.

 

[LionelHutz]

I know, I'm actually surprized that charging and discharging the system capacitance like that has not either been tripping or damaged that drive.

I didn't say the motor was still a problem - I just assumed they would have fixed the motor. Maybe I should not have assumed that. It really should be fixed without running it more.

 

[MikeNice]

Yeah, I'm sure the motor has been replaced as well. Thanks for everybodys input on this.

Michael

 

[MikeNice]

I spoke with the our electrical systems specialist who was able to explain the phenomena that had occured on this system. Apparently the carrier frequency of the drive was 2khz and the resonant frequency of the building was hiting in right around the 33rd harmonic @ 1980hz. This was causing the floating system to experience voltages at ~2kV p-p. WOW!

Michael

 

[Skogsgurra]

Really! That would be worth a trip. Just to see it happen. Is there a report on it?

Very interesting. Indeed. Not at all impossible. But 1980 Hz is a very high frequency. A low impedance transformer and short cables could no doubt give frequencies like that. I have had 880 Hz once. But that's the highest I have seen. But I haven't seen them all.

Can you fill us in on transformer rating, X and cable lengths?

Gunnar Englund

http://www.gke.org

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

 

[MikeNice]