Ground Fault Detection / Protection in LV VFDs for High resistance system 2

[cherryg]

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!!

 

[cherryg]

Some more info......both ABB & Siemens LV VFDs do not have ground fault detection/protection capabilities in their drives (ACS800 and Simovert respectively) for hogh resistance grounded system. I believe Rockwell has a unit which satisfies the requirement. And so has Schneider. I would like to hear from you all, if you had similar experience with these drives.

 

[Skogsgurra]

Bender has a range of ground fault monitirs and relays that ca be used with VFDs.

http://www.groundfault.com/

Gunnar Englund

http://www.gke.org

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

 

[cherryg]

Skogsgurra..Are you suggesting that we fit the VFD units with a Core balance CT wired to the GF relay? I believe the location of the CT cannot be on the load side of the VFD due to harmonics.

 

[Skogsgurra]

No, not at all! I doubt if a core balance principle would work at all in an IT grid. (I know it doesn't)

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-ac-dc/

They work with a superimposed DC with changing polarity and evaluate the resulting current. They actually do an R_isol=U/I measurement. I have used them on large machine groups like paper machine drives with good results.

Gunnar Englund

http://www.gke.org

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

 

[dpc]

What is your goal? With a high-resistance ground, any ground fault will be detected by the overall system ground detection system. One of the advantages of a HRG system is that it is not necessary to shut down immediately. Tracing systems can help locate and isolate the fault. However, the unfaulted phases will be at full line-to-line voltage to ground until the fault is removed. If the AFD does not like this, it should be fully capable of providing an alarm or shutting down based on the phase to ground overvoltage condition without need of any additional protection.

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.

 

[Skogsgurra]

Customer request. And, as I said, it was for "large machine groups like paper machine drives".

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

http://www.gke.org

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

 

[dpc]

Gunnar,

My comment was meant for the OP. Sorry for any confusion. I agree your comments for larger drives.

Dave

 

[LiteYear]

There are several difficulties in detecting ground faults in HRG systems with LV VSDs. One is that most drive manufacturers will require that the common mode filter at the drive be disabled, since in a HRG system that filter provides an alternate path for fault currents that is not limited by the NER. This results in greater than normal ground currents in normal operation, which a ground fault relay has to be insensitive to. The second difficulty, and the reason why you don't see many manufacturers offering the function built-in to the drives, is that a earth fault in a HRG system is merely noise on the current sensors within the drive - the drive will typically not trip until leakage current reaches about 50% of load current.

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.