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.

 

[Jraef]

Do you mean 35% * original rating or 65% * original rating?

Cheers, Wayne

De-rate BY 65%, so TO 35% of the drive rated output. So if you have a 10A 3 phase motor, you select a VFD rated at 10/.35 =28.571 minimum VFD rating, not 20A as most people think.

OR, you can use the 50% current de-rate, and make sure the TEMPERATURE never exceeds 50% of the drive ambient operating temperature in Celsius. So if the drive is rated 50C (122F), the ambient cannot exceed 25C (77F). That’s tough without adding an air conditioner, which would cost more than just over sizing the drive.

 

[Jraef]

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?

I ran into a situation where the MOVs where failing, shorting, and eventual failure was an abrupt explosion. I couldn't get to the bottom of why the MOVs keep failing like this. It was an ungrounded service so I assumed it was a restriking ground that was pushing the voltage higher than the MOV was rated and eventually it flashed over and failed.

The most common failure mode for MOVs is to fail shorted, but depending on the magnitude of the fault, that can result in the silicon oxide material inside vaporizing instantly, exploding the ceramic covering and spewing SiO2 particles all over the place, which is conductive, often resulting in further collateral damage of other devices in the vicinity.

The failure of MOVs in VFDs is because ALL VFDs are designed for use on solidly grounded Wye systems, what’s called a TN or TT system in IEC terminology. So in the front end of the drive (rectifier), the MOVs are connected in a Wye (Star) configuration with the center point connected to ground. Plus the MOVs (and other line side components) are often** selected to NOT see more than the L-G voltage. If you are connected to a Delta power system, the L-G potential can be the same as the L-L potential, so 1.732x higher than the design voltage. If everything is perfect and there are no faults, the MOVs are OK, but if there is a fault to ground ANYWHERE on the line side, the Wye point of the drive MOVs becomes the Wye point for the ENTIRE SOURCE, for as long as it takes for that fault current to vaporize the MOVs, usually 1/2 of a cycle.

** Delta power sources only exist in North America, they are not used in IEC countries, so it’s something that is not “normal” for them. Drives DESIGNED FOR the N.A. market will usually have a simple way to disconnect the ground reference point of the MOVs to lessen the chances of them failing catastrophically, plus the drives will use line side components that are CAPABLE of operating at the L-L potential. This unfortunately is not universally true, and many Asian based low cost drives that were NOT designed this way, don’t have that ground reference disconnect nor do they use the higher rated components. But rather than just stating this outright, the instructions allude to it by saying words to the effect of “This drive is intended to only be used on a source with TN, TN-S, TN-C or TT earthing system.”, which few people in N.A. understand. So if you hook it up to a Delta system, the MOVs blow up from a GF event somewhere else on the system. Then if it was still under warranty, they will point to that nebulous statement and say “Oh, you should have used a drive isolation transformer. So sorry, but no warranty.”

 

[tortuga]

** Delta power sources only exist in North America, they are not used in IEC countries, so it’s something that is not “normal” for them.

I know VFD manufactures used that excuse but there is delta connections to be found in other countries, its kinda like here not super common and perhaps not for anything new, I think a 230V corner grounded delta can be found in Norway, Denmark and parts of Sweden, for a residential or light commercial service the consumer would get only two phases with 230V between them.
They never ground the mid point like we usually do.