high leg and VFD's on roof top units

[petersonra]

What happens if you do not have a solidly grounded system is you can have a voltage to ground that exceeds the rating of the MOV as far as how much energy it can absorb.

Think about it this way. If you have a delta system, the only thing that keeps the voltage to ground steady is the capacitance in the wires. There is nothing otherwise preventing the voltage to ground from shifting, and it sometimes does. If this happens for more than a split second at a high enough voltage the MOV will try to bring the voltage down. This is how a MOV works, but if the high voltage stays for more than a few milliseconds the energy absorbing capability of the MOV is exceeded and it disintegrates, sometimes taking the VFD with it.

 

[winnie]

This might be a bit of a odd ask, but do you know of any documentation for the effects VFDs might have on MOVs? Has there been a study?

The issue that @petersonra was describing was not VFDs effecting MOVs, but rather the MOVs internal to the VFDs failing because they were used on the wrong sort of supply. The MOVs internal to the VFDs are connected 'line-ground' and selected for 'wye' supplies. If you supply such a VFD on any flavor of delta, then one or more of the MOVs will be subject to overvoltage and will fail.

I see that @petersonra also answered, and this reminds me to emphasize: the MOVs are wye connected to ground, so even with an ungrounded delta there is a very good chance that they will be subject to excess voltage, possibly simply because of capacitive coupling from the motor frame to ground at the switching frequency.

 

[Elect117]

What happens if you do not have a solidly grounded system is you can have a voltage to ground that exceeds the rating of the MOV as far as how much energy it can absorb.

Think about it this way. If you have a delta system, the only thing that keeps the voltage to ground steady is the capacitance in the wires. There is nothing otherwise preventing the voltage to ground from shifting, and it sometimes does. If this happens for more than a split second at a high enough voltage the MOV will try to bring the voltage down. This is how a MOV works, but if the high voltage stays for more than a few milliseconds the energy absorbing capability of the MOV is exceeded and it disintegrates, sometimes taking the VFD with it.

The issue that @petersonra was describing was not VFDs effecting MOVs, but rather the MOVs internal to the VFDs failing because they were used on the wrong sort of supply. The MOVs internal to the VFDs are connected 'line-ground' and selected for 'wye' supplies. If you supply such a VFD on any flavor of delta, then one or more of the MOVs will be subject to overvoltage and will fail.

I see that @petersonra also answered, and this reminds me to emphasize: the MOVs are wye connected to ground, so even with an ungrounded delta there is a very good chance that they will be subject to excess voltage, possibly simply because of capacitive coupling from the motor frame to ground at the switching frequency.

I appreciate both of your replies.

I ran into the issue in Meters. The meter would fail on ungrounded services and my guess was an overvoltage event that flashed over or shorted the MOV. Since the failures where catastrophic to the meter I couldn't tell exactly what was happening. Nothing would trip since the services were on CTs and the current in the service wasn't the problem. The MOV was always melted or burnt through. The rest of the meter would follow and the wiring and meter would melt. It appeared to occur randomly. It happened in more than one plant and on different ungrounded configurations. I always thought it had something to do with an overvoltage as mentioned in both your posts. There could have been a momentary rise in voltage to "case" (ground) in the meter. I was just hoping there was a paper on it in VFDs to see if there is something I could do to develop a fix so it doesn't keep happening.

 

[kwired]

I appreciate both of your replies.

I ran into the issue in Meters. The meter would fail on ungrounded services and my guess was an overvoltage event that flashed over or shorted the MOV. Since the failures where catastrophic to the meter I couldn't tell exactly what was happening. Nothing would trip since the services were on CTs and the current in the service wasn't the problem. The MOV was always melted or burnt through. The rest of the meter would follow and the wiring and meter would melt. It appeared to occur randomly. It happened in more than one plant and on different ungrounded configurations. I always thought it had something to do with an overvoltage as mentioned in both your posts. There could have been a momentary rise in voltage to "case" (ground) in the meter. I was just hoping there was a paper on it in VFDs to see if there is something I could do to develop a fix so it doesn't keep happening.

A little out of my expertise on details but could you transform to a low capacity grounded voltage system for the metering circuit and still measure the current of the ungrounded system with CT's? There would likely be offset between voltage and current because of the transformation but probably is a way to deal with it to get correct results.

 

[jim dungar]

see if there is something I could do to develop a fix so it doesn't keep happening.

Provide a ground reference for the system, through some type of impedance or an artificial neutral. Wye-delta transformers are not uncommon on ungrounded MV metering circuits.