Is an isolation transformer needed on a VFD input side? 4

[edison123]

This week, we trialed a 125 KW VFD for testing LV motors in decoupled condition. During the trials, I noticed our LED lights started flickering even though they were switched off.

When asked, the OEM drive guru said there will be harmonics fed into the system, and we need to install an isolation transformer to prevent harmonics getting back into other loads.

Is this correct? If yes, what will be transformer connection, delta input/wye output, or delta/delta or wye/delta?

What will be the KVA rating of the isolation transformer?

If isolation transformer is required, can I go for 415 V input / 690 V output so that I can get a 690 V VFD output for testing some motors which are rated at 690 V?

Will a 690 V VFD give 415 V output (which will be the 95% of the motors we test) without any derating in current capacity?

 

[edison123]

Lionel. Our all copper electrical system, designed and executed by professional engineers like my dad and me, has been in place for over 45 years with zero issues, not even a fuse blowout. So, the VFD is the issue here, not our electrical system.

Bill. The motors were hardly 5 feet away from the drive that was being demoed.

 

[waross]

With the switch off, that leaves radiated energy the next most likely.
Alternately, conducted harmonics on the neutral, passing through the LED drivers and radiating from the hot conductor so as to develop a causative current.
Maybe possible but I'd go with direct radiation first.

 

[Nayeem1993]

I think you don't need an isolation transformer just only to get rid of harmonics. You can just use an inductor or inductor capacitor combination for the same purpose.

 

[LionelHutz]

Respectfully, I read flickering, not failure.

my LED lights were flickering even they were switched off and a few failed when the drive was test run.

We test drives here all the time loaded at 50-150A using SOOW cable for the connections and have never had a building issue. Loaded with no input filtering means probably injecting 50-75A of harmonics into the building power supply.

We have a local motor shop running 600-700hp motors unloaded for testing purposes on a VFD, with no filtering on their VFD and also SOOW cables for the connections, and they have no problems either.

You can keep chasing a few amps of harmonics or radiated energy, but in my opinion neither is the root cause of LED lights failing. I highly doubt either is the root cause of LED lights flickering either.

 

[System Protection]

Does the VFD have a line reactor installed?