VFD going to ground on a delta system

[adamscb]

Hey forum, had an interesting incident at work that I had never seen before. We had a motor on a VFD (around 200hp-300hp) go to ground. As a result, ALL of the closed WL breakers in the upstream 3000A Siemens switchgear showed high amps (around 1500-2000 amps), including the main. The feeder breakers eventually tripped out on LT. We put a power meter on the load side of the main, and saw this amperage didn't exist. This is on a delta, ungrounded 480V system. Once we narrowed down which VFD caused this, we megged the wires going to the motor/motor itself and found the ground.

We know that the VFD should have detected the ground and tripped, but what I find interesting is all of the breakers in the Siemens gear showing this "ghost" amperage. We had all the breakers primary injection tested and they passed. Has anyone seen/heard of anything like this happening before?

 

[petersonra]

Hey forum, had an interesting incident at work that I had never seen before. We had a motor on a VFD (around 200hp-300hp) go to ground. As a result, ALL of the closed WL breakers in the upstream 3000A Siemens switchgear showed high amps (around 1500-2000 amps), including the main.

All of them? Or just the one associated with the VFD? And how do you know that they showed high amps at all? Did someone go out and put a meter on them while the fault was happening?

The feeder breakers eventually tripped out on LT. We put a power meter on the load side of the main, and saw this amperage didn't exist.

After it tripped it seems unlikely that there would be any amperage at all. What feeder breakers?

This is on a delta, ungrounded 480V system. Once we narrowed down which VFD caused this, we megged the wires going to the motor/motor itself and found the ground.

We know that the VFD should have detected the ground and tripped, but what I find interesting is all of the breakers in the Siemens gear showing this "ghost" amperage. We had all the breakers primary injection tested and they passed. Has anyone seen/heard of anything like this happening before?

Whether the VFD should have detected the ground or not might well depend on the vintage of the VFD. I don't know that a grounded motor lead on the output of a VFD would even cause more current to flow than normal anyway.

I suspect there is more information available than you are telling us and you expect us to guess what happened without telling us all you know.

What was the LT setting on the breaker? I don't think you can normally set it for more than about 25 seconds. How do you know it tripped on LT?

Are you sure this is an ungrounded Delta system? If so there should be a ground detect circuit that you could check to make sure that you don't have an inadvertent ground somewhere.

How did you narrow down which VFD caused the problem? Did the VFD show it was pulling a lot of current?

 

[petersonra]

One other thing. Did you personally witness this event or are you getting your information second or third hand?

I have found that most of the time when people have reported strange things like this that it didn't actually happen the way they said it did. Often reports come in from people who think they know what happened but data logging often shows that something else is what really happened.

But I have also seen where data logging has failed to read the correct data because it was programmed wrong. Computers are infinitely more reliable than people are, but when you are relying on people to set them up sometimes they screw up and no one notices it for 10 years.

 

[adamscb]

All of them? Or just the one associated with the VFD? And how do you know that they showed high amps at all? Did someone go out and put a meter on them while the fault was happening? All of the closed breakers. The breakers showed high amps on the electronic trip unit, I was looking at them myself.

After it tripped it seems unlikely that there would be any amperage at all. What feeder breakers? Ok so those two sentences placed next to each other are misleading. What I meant to say was that when all of these circuit breakers were showing high amperage, we put the power analyzer on the load side of the main. We confirmed that these amps didn't exist.


Whether the VFD should have detected the ground or not might well depend on the vintage of the VFD. I don't know that a grounded motor lead on the output of a VFD would even cause more current to flow than normal anyway.

I suspect there is more information available than you are telling us and you expect us to guess what happened without telling us all you know. Yep, you hit the nail on the head. My intention is to withhold information to keep you guys on your toes (that was sarcasm by the way).

What was the LT setting on the breaker? I don't think you can normally set it for more than about 25 seconds. How do you know it tripped on LT? Went through the trip history on the electronic trip unit. The LT settings on the feeders varied - some were around 300 or so, some 800, some 1200. Obviously the ones set lower tripped out first.

Are you sure this is an ungrounded Delta system? If so there should be a ground detect circuit that you could check to make sure that you don't have an inadvertent ground somewhere. Yes, this is an ungrounded delta system, I can read the nameplate of a transformer. There isn't, which I understand is a code violation, but I wasn't involved with designing this system.

How did you narrow down which VFD caused the problem? Did the VFD show it was pulling a lot of current? We started motors on the system, and noticed when the problem started happening.

 

[adamscb]

One other thing. Did you personally witness this event or are you getting your information second or third hand? I witnessed everything myself.

I have found that most of the time when people have reported strange things like this that it didn't actually happen the way they said it did. Often reports come in from people who think they know what happened but data logging often shows that something else is what really happened.

But I have also seen where data logging has failed to read the correct data because it was programmed wrong. Computers are infinitely more reliable than people are, but when you are relying on people to set them up sometimes they screw up and no one notices it for 10 years.

 

[synchro]

Are you sure that there isn't another ground fault that you didn't detect? If there was one it could cause fault currents to flow on an ungrounded system. But it's puzzling why there's a big discrepancy in the current readings on the breakers and the power meter. Does the power meter show what would be expected when the loads are running?

 

[adamscb]

Are you sure that there isn't another ground fault that you didn't detect? If there was one it could cause fault currents to flow on an ungrounded system. But it's puzzling why there's a big discrepancy in the current readings on the breakers and the power meter. Does the power meter show what would be expected when the loads are running?

We've checked single-line to ground voltages, and everything checks out. And that big discrepancy is what has me confused as well. The power meter showed what we would expect, rather than the 1500A on each feeder, and the 2000 or so amps drawn by the main that was being shown by all of the electronic trip units. This power meter was a Fluke, $6-7k.

Also another odd thing - these high amperages on the feeders and the main on the ETUs - consistent across all three phases. It's not like one leg was showing high while the others were normal.

 

[synchro]

The output voltages of a VFD switch between the + and - rails of the internal DC bus at kHz rates, and with a high level of harmonic content on this "carrier" frequency. And so if an output line of a VFD has a ground fault, this could cause a substantial amount of high frequency noise on both of the DC buses relative to ground. And then during the time intervals when the input rectifiers of the VFD are conducting, that could transfer the high frequency noise onto the 480V system where it could propagate to other connected circuits.
This is speculation on my part, but perhaps such noise is causing electronic breakers to falsely show high levels of line currents. But perhaps your $6-7k Fluke meter is good enough to reject high frequency noise?
Maybe you could look over any documentation about the Fluke meter, and then see if it mentions an ability to reject high frequencies.

 

[petersonra]

What does siemens say

 

[petersonra]

There is something we are missing here but I can't quite put my finger on it. I'm not a big fan of blaming electrical noise for every anomaly that we can't explain.

What output current was the VFD reporting during this anomaly?

By any chance is there a capacitor bank nearby upstream of the switchgear? Maybe own by the utility?

 

[winnie]

In the case of a VFD on an ungrounded system, a ground fault downstream of the VFD is connecting different phases to ground at the switching frequency.

This would be more than a little noise, basically a 700V peak to peak square wave superimposed onto the electrical system. It wouldn't surprise me if there were several amps of switching frequency current flowing.

If the electronic trip units had some sort of frequency dependent current sensing then this could cause errant high current readings.

If the motor winding were faulted near the neutral point, then there might be no apparent ground fault imbalance as far as the VFD was concerned.

Jon