VFD TRIPPING

[vanskat]

We have had tripping problems on a particular VFD (old 400hp Emerson drive system) for a few months now. Unfortunately, the trip condition shows no diagnostics to help identify the problem.

I used a datalogger yesterday to monitor and record various electrical parameters and found that the current in the "A" phase 5% of that in the other two phases and the ground cable is carrying a current equal to that of the other two phases as well.

I am surprised that our clamp on ammeter did not pick up this unbalance. Without further analysis I suspect that the ground fault relay on the ground conductor senses the imbalance, resulting in the trip condition.

Aren't those clamp on meters true RMS?

 

[busbar]

You might try all three logger currents on one phase, then roll the datalogger connections of your 3ø connections to be sure that the instrument is OK.

Clip around all three source-phase leads and see if you get an above-the-noise reading. In that case, there is likely damage to the drive—possibly an internal ground fault.

5% is 5%, but so-called true RMS ammeters are sometimes misunderstood—see that yours is not “RMS calibrated/average responding.”

 

[electricpete]

I'm curious if we're talking about the current on the input or output of the vfd.

 

[vanskat]

Pete
I was referring to the current at the output of the drive to the motor. We made some progress today in identifying sources of ground fault. Additionally, there were a couple bad SCR's, discovered when the drive was run independent of the motor.

We are about to return the pump to service with the datalogger installed and monitor electrical parameters. Hopefully we have addressed the problem, if not back to square one!

 

[vanskat]

I am still having that tripping problem even after these repairs. We have a main disconnect switch which supplies power to the drive which is protected by a 600A GE breaker.

On tripping the fuses are bypassed and the fault seems to trip the breaker within the drive. I have checked the breaker settings and fuse sizes which are identical to three other drive units which are in service.

Any comments?

 

[buzzp]

There is a bank of capacitors in the drive. Make sure these are not breaking down when voltage is applied. I have had this happen to me and it took a couple of days to figure this out.

I believe the high current in the neutral could be due to high harmonic content, especially if the drive is an older one. Just some suggestions.

 

[busbar]

Are you able to determine whether the breaker is tripping in the thermal or magnetic mode?

 

[dydt]

is the bad SCR's found on the output stage?
how is your DC bus voltage...is it dipping once you
run the motor...'could be a bad power semiconductor or bad capacitor...

dydt

 

[jbartos]

Suggestion: The waveforms monitoring at suitable diagnostic test points according the VFD Emerson test manual or procedures would help to pinpoint the root cause of the trips.

 

[vanskat]

Guys,

I think I have found the problem.

Initially the breaker had been tripping on thermal overload. An infrared scan demonstrated significant temperature rise on this particular unit. Furthermore,a t full load current it was hot to the touch.

I installed a minisplit AC unit to compliment the central unit that is already in the substation. Since then the tripping problem has not occured.

I guess more science should be applied to this theory to see if this is actually the case but troubleshooting revealed no issues. DC bus voltage was perfect. Output waveforms could not have been better and the ground current had been addressed.
Any comments?

 

[busbar]

Happy you found a solution. I hope the air conditioning buys the necessary long-term headroom.

What voltage?

 

[jbartos]

Suggestion: It appears that the input section of the drive was not properly addressed. Is more info available covering the input conversion? How many pulses, phase shifting transformers, active front end, input filtering, etc.