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.”