Fault Analysis Help 3

[rockman7892]

We recently had a fault on one of our 4.16kV motor starters that was pretty big in magnitude. It appeas the fault happend due to a loose bus connection where the starter section's vertical bus terminates on the contactor stabs. The attached picture shows a screw being missing most causing a loose connection on this bus, and as soon as motor was started fault occured. It is assumed that the loose connection was arcing, and somehow this arcing caused a L-L fault with one or both of the other phases.

I say a L-L fault because looking at my upstream relays it shows what appears to be a phase fault of aprox 22kA. Our upstream breaker actually tripped on a ground fault but the phase overcurrent element picked up when the 22kA or so was seen. I guess somehow this L-L fault produced some ground current as well.

Our system is a 4.16kV primary distribution system which is LRG to 400A at the utility substation.

I have the event reports/waveforms from both my main breaker (multilin 750) and the utility's 387E schweitzer relay. The 387E relay is monitoring the utility transformer differential with winding 1 being the primary side, and winding 2 being the secondary side. Winding 3 is looking at the transformer tank ground connection.

The utility says that they saw the same 22kA current magnitude on the secondary but claim they saw about 2000A on this tank ground CT. I question how they can see this magnitude of ground current if we are resistance grouned limiting current to 400A? Can this current get above 400A?

I'm by no means an expert at interpreting these event reports, and wanted to see if anyone was interested in looking at these event reports to see what appears to be the sequence of events and analysis of the fault. I am interested to learn from others input. I also do not have the correct software to view utilites relay files.

I will attached multilin 750, and schweitzer 387E relay in following posts.

Thanks for the help.

 

[rockman7892]

Attached is Multilin SR750 relay event waveform.

 

[rockman7892]

Attached is utility Schweitzer 387E relay file

 

[desertfox]

Hi rockman7892

There is nothing in the attachment you posted.

desertfox

 

[desertfox]

I mean the first attachment in your original post.

desertfox

 

[rockman7892]

I'll try again.

 

[rockman7892]

Still didn't work. I'll try once more. Its a picture so maybe its too large.

 

[unclebob]

At the fault point, heat is generated and soon copper gas is produced. It's possible that a ground fault appeared first and subsisted just long enough to produce a L-L fault with the conductor nearby.

As for the 2000 A in the 400 A ngr, I have to think about it.

 

[Eleceng01]

rockman7892, did you post the SEL-387E event report or am I missing it? Also, can you provide a simple one-line showing the power system?

 

[Eleceng01]

We also need the DAT file that goes with the CFG file for the SR750 relay. Thanks.

 

[jghrist]

say a L-L fault because looking at my upstream relays it shows what appears to be a phase fault of aprox 22kA. Our upstream breaker actually tripped on a ground fault but the phase overcurrent element picked up when the 22kA or so was seen. I guess somehow this L-L fault produced some ground current as well.

Do the relay records indicate fault current in more than one phase? It would not be unusual for the phase overcurrent element to pickup for a single-phase-to-ground fault. There is current in the phase as well as the ground, and if it is above the phase element pickup, it will pick up.

 

[rockman7892]

Eleceng01

Attached is the one-line showing the the fault location, as well as the feeder and main breaker. Both main and feeder breaker were tripped. Feeder breaker is a Multilin SR735 and does not offer much information, and only shows that it tripped on phase overcurrent and shows the same 20kA plus fault current. I'm wondering how this feeder breaker and main breaker tripped at the same time? Must be a coordination issue that needs to be looked at.

I have also attached the relaying scheme for the utility substation. This scheme is somewhat different than is shown on the one-line, but is indeed what is actually in the field. It shows the 387E and 501 relays which I will attach event files for.

 

[rockman7892]

Attached are all of the relay files from both the Multilin 750 and Schweitzer 387E and 501 relays.

Let me know if you are having trouble unzipping them and I'll post individually.

 

[dpc]

So was there a question about something in this avalanche of data? I've lost track?



David Castor

http://www.cvoes.com

 

[TurbineGen]

You said you have a high resistance ground system correct? How is it grounded? Through a zig-zag transformer perhaps?

I unfortunately cannot open the relay files, but from how it sounds, it would be my belief that you had a L-L fault. Where the ground fault came from (and if it in fact had a magnitude of 2000A) might be more visible if we had more details about your grounding scheme. It could have flashed to ground very easily, but with a HRG system limited to 400 A, I don't know whether or not believe the 2000A ground current.

What phases did you see the fault? What were the voltages at this time?

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[Eleceng01]

Did the SEL-501 on the transformer ground trip? Did it capture an event file?

Can you please provide it if it did - this will help substantiate the 2000A claim.

 

[ScottyUK]

It will substantiate that the relay saw however many secondary amps it recorded.

Always assume the instruments are lying to you until you prove otherwise.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[rockman7892]

Eleceng01

Yes the 501 relay did capture an event, and I believe is what saw the 2000A ground current. I have attached this event file in the last zipped file and it can be found in the folder named HL2 OC Fault After Breaker. I'm not sure what exactly tripped when, since both my breakers opened and the utility circuit switcher opened. The utility circuit switcher is programmed to open with our main breaker opening so I'm not sure if the realy tripped it or it tripped with a signal from our breaker. Thats what I was hoping to look at the event files to determine.

dpc

I do not have a particular question per say, I'm just trying to understand exactly what the sequence of events were to better understand for myself. Unfortunately I am not able to view these event files, so I thought that anyone who was able to veiw them might be interested in offering their opinion as to what they see.

One thing I am trying to understand, is why there was ground fault of the magnitude of about 2000A.

My relays showed fault current on all three phases in the neighborhood of 20kA plus amps.

 

[dpc]

IIRC, the SEL-501 determines ground fault current via a summation of the three phase currents. It does not actual measure the current flowing in the neutral through the resistor. If the three phase CTs are not well matched, or saturate, the computed ground current may not be correct.

The best way to determine the magnitude of ground fault current is to look at the the CT on the transformer neutral bushing. For a ground fault, all current must end up flowing back to the transformer neutral.

If this CT shows current significantly greater than 200 A then one (or more) of the following is the case:

The resistor is not actually 200A at the applied voltage.
The CT ratio is programmed incorrectly.
The resistor failed and shorted to ground.
The transformer failed internally to ground.

Maybe there are other possibilities, but those come to mind quickly.

(Scotty - your comments about not trusting the instruments is interesting because I generally tell my clients exactly the opposite - trust the relays unless you are sure they are wrong. I like to avoid the unwritten law that if a relay trips, the next step is to immediately try to close the breaker in again to see what happens. During testing and commissioning, I would agree with you - assume everything is fubar. But once things are tested, trust your relays.)



David Castor

http://www.cvoes.com

 

[Eleceng01]

rockman7892, I missed the SEL-501 files earlier so thanks.

I might be missing something, but I have attached plots so that you can at least see the relay operations. The SEL-501 that is on the Y-ground recorded approximately 200A of ground fault current and was tripped by the 51P element (which is connected to the ground CT).

Also, if you look at the SEL-387E plot you will notice that there is little 3I2 or 3I0 on the HV side.

I would ask the utility to verify the 2000A ground fault current if possible.

ScottyUK, I agree - but I have assumed that the relays recorded correclty here (sometimes a mistake). As a former colleague used to say "In God we trust, all others bring data."