Incorrect Ground Current Measurement

[ZeroSeq]

Hello,
I've had a ground fault in my system for which an event recorder (SR750) is giving me ground currents that are not possible. This is a 6900V delta system with a 400A (10ohm) neutral grounding resistor. The phase and ground currents shown from the recorder are as follows:
A = 2810A
B = 3216A
C = 6854A
G = 1265A

I know that mathematically, the zero sequence current is 1285.69A. However, the maximum current possibly flowing through ground is 400A, as set by the NGR.
Has anyone run into this scenario before? I am assuming it is just a measurement anomaly, but unsure as to what is causing it, whether it be the CTs, the AC signal filters, the firmware, etc.

Any insight would be greatly appreciated.

Thank you.

 

[Skogsgurra]

I may be out on too deep waters here. But how can you have a Neutral in a delta system? Where is the 10 ohm resistor connected?


Gunnar Englund

http://www.gke.org

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

 

[dpc]

Unless you have a CT from the neutral going to the relay, the "G" value is probably just the phasor sum of the three phase currents. This had to involve more than a ground fault since your C phase was over 6800 A.

I haven't fiddled with an SR750 or its event reports for quite a while. If you have waveform capture, that might be more revealing. Otherwise, you may need to resort to looking at the relay manual or talking with Multilin. I would not assume there is a relay error until you know how the "G" value is determined and eliminate the other possibilities. Relay error is probably the least likely problem.

 

[ZeroSeq]

Sorry my mistake, the main sub transformer is delta/wye with a 10ohm NGR. There are no single phase loads.

 

[ZeroSeq]

There is a core balance CT going into the SR750 relay (no CTs on the NGR are used in this relay). Yes, dpc, the fault progressed into LLG at some point.

 

[stevenal]

To calculate I0 you apparently assumed 120 degree angular displacement. Why would you assume balanced phase angles when magnitudes are unbalanced? Where does one attach an NGR to a delta system?

 

[ZeroSeq]

Thanks for the responses. Yes, it is probably incorrect to assume balanced angles in an unbalanced fault. However, it seems that the relay has made the same mistake?
I am just curious if anyone has ran across this issue before.

 

[dpc]

I'm still not convinced the "G" represents the flux summation CT current. I'd do some additional research as to the meaning of the "G" current.

 

[Kiribanda]

ZeroSeq,
If the secondary of the Dyn transformer is connected with a 400A rated NGR, then there is no way that during a ground fault the max allowable ground fault current is greater than
400A. The ground fault current = 3 x zero seq current. Therefore, it looks like there is multiplication factor issue.

 

[dpc]

If the SR750 is not correctly computing the ground fault current, I have to believe that GE/Multilin must be aware of this. I would send your fault data to them and ask them to explain. Perhaps your relay needs a firmware upgrade.

 

[jghrist]

Check the CT wiring. Under normal load, does the SR750 metering show correct phase angles?

 

[ZeroSeq]

Thanks for all the suggestions. I am in contact with Multilin, and will update with the response I receive.