VFD DC bus problem on line fault

[Sofistioelevib]

Hi guys,
we had a 3 phase current unbalace problem due to a ph to ph short (high negative sequence).
In this situation 2 phase increased and one goes near to 0.
This problem could be induce on a thyristor dc bridge some type of malfunction?
This could be cause a broken on thyristor or motor?

thank you

 

[waross]

The motor, acting as an induction generator, will backfeed into the lower voltage phase if it still has a connection to the system.
Other than that, a few details may help with a better answer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Simple answer: If you had that kind of a fault and after that a problem with a motor or thyristor/SCR drive. Yes,
Not much need to speculate as to why, what or how.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricuwe]

I fully agree to Skogsgurra. However, a good DC-drive should detect the loss of a phase and trip before being damaged. There are also relays available to provide this kind of protection.

The post from Waross would be the explanaition for an induction motor running direct on line.

 

[jraef]

I can’t quite understand your post. Are you asking if a shorted thyristor or on the bridge rectifier of a VFD could be the CAUSE of the Ph-to-Ph Fault you saw, or the other way around? I would be more inclined to believe that a fault on the line side would CAUSE the failure in the rectifier. Bridge rectifiers are fairly simplistic robust devices, but all thyristors are subject to failure from severe line transients, such as Ph-to-Ph faults ahead of them.

It would be extremely unlikely to have been caused by the motor, but that depends a lot on the age and topology of your VFD.

And by defining it as a VFD, it cannot be a DC drive unless you used the wrong terminology. A VSD could be a DC drive, but the “F” in VFD means “Frequency”, and there is no frequency involved in a DC system


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[LionelHutz]

Saying you have a current imbalance issue almost sounds like you are describing a continuous issue and not a fault. If it's a continuous issue, then I would expect the thyristors for one phase are not be triggered, turning the 3-phase rectifier into a 1-phase rectifier.

A single thyristor not being triggered also results in a large current imbalance, but the bad phase would not go to zero current since there is still current flowing for 1/2 of the cycle. This negative or positive only current can mess with getting accurate current metering via current transformers though.

It's fairly simple to diagnose which device is bad by using an oscilloscope to show the current on each phase.