Ground Fault Detection / Protection in LV VFDs for High resistance system
Ground Fault Detection / Protection in LV VFDs for High resistance system
(OP)
I am sure some of you have come across this problem. It seems that only few vendors offer ground fault detection/protection in their LV VFDs, for high resistance grounded system (5A). Please provide your feedback, and possible solutions. Thanks!!






RE: Ground Fault Detection / Protection in LV VFDs for High resistance system
RE: Ground Fault Detection / Protection in LV VFDs for High resistance system
http://www.groundfault.com/
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Ground Fault Detection / Protection in LV VFDs for High resistance system
RE: Ground Fault Detection / Protection in LV VFDs for High resistance system
If you browse Bender's site (I have done that for you) then you will see that there are many different types of ground fault monitors and measurement devices. Have a look at these: http://207.56.194.41/ground-fault-relay-ungrounded...
They work with a superimposed DC with changing polarity and evaluate the resulting current. They actually do an Risol=U/I measurement. I have used them on large machine groups like paper machine drives with good results.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Ground Fault Detection / Protection in LV VFDs for High resistance system
I have heard of issues with some AFDs during normal operation on ungrounded or HRG systems, due to the lack of a zero sequence current path, but I'm not really clear on how much of an issue that really is.
RE: Ground Fault Detection / Protection in LV VFDs for High resistance system
Those groups usually have their dedicated transformer, sometimes several transformers in the 1600 - 2200 kVA size range. So, the monitor is only for the drives and motors in this machine or group and not for anything else.
If you have found a solution for a single VFD, or if there are VFDs that takes care of the problem, then I do not really see the problem.
There are some problems with the IT type grid an VFDs. One of them is that the line-ground voltage contains lots of PWM components and they couple to motors that were not supposed to see anything but a sine (plus a few percent harmonics) and that causes bearing EDM in some cases. If there are DC drives on the same transformer, you get a really nasty situation because the coupling capacitance from armature to rotor is ten or hundred times more effective than it is from stator winding to rotor in an ASIM.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Ground Fault Detection / Protection in LV VFDs for High resistance system
My comment was meant for the OP. Sorry for any confusion. I agree your comments for larger drives.
Dave
RE: Ground Fault Detection / Protection in LV VFDs for High resistance system
That said, it can and really should be done. As already mentioned, Bender have some solutions and so do others. Invariably a core-balance CT arrangement is used - there's nothing inherent in a HRG that prevents this method. Load or line side is an important question though, and ultimately it comes down to a detection verses nuisance trade-off. If the relay is unreliable on the load side then it can't be used. But even if the relay is reliable on line side, it's unlikely to detect ground faults on the load side. You'll have to look at your entire protection scheme and I suspect, take a risk reduction approach.
Note also that a narrow bandwidth (traditional) ground fault relay, while potentially more reliable, may also be blind to many load side ground faults. That's where the wide-bandwidth relays comes in. Bender have one and so do Startco. In Australia, Ampcontrol (disclosure: my employer) have one too, which is specifically designed for operation with VSDs.