Possible cap bank switching?

[LMAO]

Symptoms: Customer experiencing VFD failure or fault. In case of failure the IGBTs are getting destroyed; and faults seem to be caused by VFD controller detecting overvoltage.

Please see attached waveforms taken at 480V utility. We started monitoring the line after the VFDs failures and timing of the these data captures match the latest VFDs faults. Sampling frequency is 2.5kHz. I suspect capacitor bank switching because of the sudden drop in voltage followed by oscillation (seems to be about 500Hz) and current spike.

 

[Besoeker3]

We always de-tuned capacitors to avoid this. In our case, being a 50Hz country, we used 177 Hz. For USA 225 Hz might be better.

 

[LMAO]

Sorry I was not clear; I am not sure if the problem is caused by cap bank switching. We have not even asked the utility if they are bringin capacitor banks on/off line. I was just wondering if anything else could cause the voltage and current to behave like this.

 

[Jraef]

A ringing transient like that on all 3 phases at the same time would make me thing a cap bank kicking in. It could also be a grid switching transient. Depending on where you are, those are getting more and more common now. Here in California on hot days it can be 20-30 per day, compared to when I was doing power quality studies in the late 70s and we would see one or two per month.

If you are thinking it is utility booster caps, good luck getting the utility to admit it. In my experience they don’t like to admit it for some reason. I had a project that was losing Toshiba VFDs right and left and had come to that conclusion, but the utility denied they were using booster caps. Fortunately this was out in the desert near Vegas so I could follow the power lines along the road until I saw the cap bank and took pictures of it to prove it was there. Then they admitted it and installed detuning reactors. The problem went away.

Do your VFDs have line reactors? That helps too, especially if this is a grid switching transient.

 

[Hv&Lv]

It definitely looks like cap bank.
I would call the utility and see what size caps they are switching online..
900kvar and up banks tend to cause problems. We limit to 600kVar and cycle them
Ride the line out, see where they are.

 

[LMAO]

Thanks everyone for your advice. It is kind of delicate issue. I am on the engineering side and can only give technical advice; we have different people dealing with customer, and they have to be careful to not sound like they're blaming the utility without concrete evidence. I'll post update as we learn more.

 

[synchro]

I agree that it looks like capacitor bank switching.
Where was the current measured? The current peaks are quite high relative to the voltage peaks, and so that's suggestive of a low load impedance at higher frequencies. Are there PF correction caps on the customer side? If the current spikes are mainly going into VFD bus caps, then as Jraef suggested, line reactors might reduce the peak current and the resulting increase in DC bus voltage somewhat.

 

[Jraef]

What can cause damage to the transistors is the ringing nature of the transient. Whenever the ringing drops BELOW the forward conduction threshold of the rectifier diodes, they stop conducting and feeding into the DC bus. If the motor is running when that happens, it depletes the bus caps, so the next diode pulse has to make up for it and the capacitor charging current pulled by that diode to recharge the caps gets very high (remember, there would be no pre-charge resistor in play any longer). Then the dI/dt on the DC bus can cause the parasitic turn on of the transistors at the wrong time causing their destruction. Slowing the rise time of the line side transient with an inductor (reactor) also slows the dips on the ringing, helping to prevent the diodes from not conducting into the bus at every sine wave.

It’s of course MUCH more complex than this, I’m just doing this for those that don’t live it every day but want to understand how the transistors can be damaged by something that takes place on the line side of a VFD (and why line reactors are so important).

 

[LMAO]

Utility claims they do not switch capacitors. I think it would be a huge liability issue on their part if they are straight up lying to us so I guess I should take their word...?

I am nor sure what else could be the culprit here.

 

[zbang]

Utility claims they do not switch capacitors

They also probably claim they don't cause fires, either. (PG&E had a bit of a bother over that one.)

 

[ptonsparky]

I do know that our local POCO would tell us when they had cap banks switch if we asked them. It was up to us to install line reactors. They always helped with overvoltage faults. Keep in mind our VFDs were small and limited in scope.

 

[Hv&Lv]

Utility claims they do not switch capacitors. I think it would be a huge liability issue on their part if they are straight up lying to us so I guess I should take their word...?

I am nor sure what else could be the culprit here.

I have them all over our system.
Some are at Substation.
Surely they don’t have all of them fixed?

I would ride the line back and look for them myself