Fault Analysis Help
Fault Analysis Help
(OP)
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.
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.






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There is nothing in the attachment you posted.
desertfox
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desertfox
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As for the 2000 A in the 400 A ngr, I have to think about it.
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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.
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Let me know if you are having trouble unzipping them and I'll post individually.
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David Castor
www.cvoes.com
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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?
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
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Can you please provide it if it did - this will help substantiate the 2000A claim.
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Always assume the instruments are lying to you until you prove otherwise.
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If we learn from our mistakes I'm getting a great education!
RE: Fault Analysis Help
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.
RE: Fault Analysis Help
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
www.cvoes.com
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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."
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Thanks,
Dave
David Castor
www.cvoes.com
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I totally agree with the 'trust the relays' philosophy when discussing / arguing with Operations and/or Management who would rather just re-energise the equipment and risk damaging it. I've fought that battle on many occasions, with some wins and some losses.
In this instance though if a relay says 2kA flowed through a 400A NGR then my initial thoughts would be that the relay is not reporting correctly - usually because of human error - rather than trying to find an explanation for 2kA flowing in the NGR. The data the relay reports has to be credible.
eleceng01,
I like the "In God we trust..." line.
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If we learn from our mistakes I'm getting a great education!
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You can see during the event that all three phases had a high magnitude current (upwards of 20kA). Because of the scaling its hard to see the ground fault current which I'm not sure is either Ig or Isg. I have moved the cursor through all points on the ground current as well as looked at them in a raw data spreadsheet that was produced, and do not see any ground current over about 29A or so. This is contradictory with what the utility relays saw. This multilin 750 did trip on an Neutral OC which I believe is set up as the ground sensing element. I need to check settings.
The strange thing in this plot is that the Vbc voltage appears to be zero. I would think that this would be an error since on the relay during normal operation it shows this voltage. PT's are in an open delta arrangement. What do you make of this voltage?
I will try to get better polts and information from this relay.
Thanks again for your input on this one guys!
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I had posted the utiltiy relay scheme in addition to the one-lines I had posted on 25 Jan 10 9:51. This scheme shows both the 387E relay and 501-2 relay and the CT's which they are looking at. Please let me know if you want additional information and I will find it and provide it.
Eleceng01
Thanks for posting the plots of the two relays, its alot easier to understand with a visual. I think you may be misinterperting the two gronund CT's however and which relay they corrospond to.
Looking at the arrangement, it appears that the 387E is looking at the neutal CT on the neutral bushing of the transformer as shown as element z15 and z16 on the 387E on the drawings. The 501-2 relay is looking at a CT on element z110 and z109 that is looking at a connection between the trasformer tank casing and ground. There is a copper wire bonding the transformer tank to the ground grid, and there is a CT on this bonding wire.
I also notice that in your plots you are looking at the sequence currents on the high side of the transformer. Is looking at the sequence components on the high side of a transformer a usefull tool in analyzing faults, as opposed to maybe looking at the low side?
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Here are the SEL-387E LV side wiggley-squiggleys (as my old boss used to say). I have also plotted the neutral current and it does reach around 2000A after all.
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The 3-line shows the 387E measuring the transformer neutral current, but the 1-line shows this measured by a 501.
The 1-line shows two 501's, one on the high side and one on the low side, but the 3-line shows only one.
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We can stare at the event reports until our eyes cross, but it's all for naught if the basics of the systems are not identified correctly.
Something is fundamentally wrong if you can see 2000 A through a 200 A resistor.
David Castor
www.cvoes.com
RE: Fault Analysis Help
The utility relaying shown on the one-line was a proposed scheme by the plant distriution system designers, however the utility changed the scheme slightly to what is shown in the 3-line drawing. The 3-line is what is currently in the field.
Thanks for posting the 387E secondary current Eleceng01. I am now puzzled as to why we would see 2000A on the neutral if this system is resistance grounded to 400A? Does the resistor heating during a fault change the resisance? I would think if anything the resistor would heat up, and thus be a greater resistance. Is it possible that somehow this resistor is shunted?
I have always been confused about the transformer tank to ground connection. Why would you want to monitor this current if all current would flow from ground to the neutral? Is this CT usually only to look at current in the event that there was an internal transformer fault between the tank and the neutral?
If the utility relay saw this 2000A fault current, then why do you think that this does not appear on the Multilin 750 relay on my main breaker? As I mentioned, scrolling through the 750 event report, I only see about 29A max ground current.
Does anyone find the fact that the Vbc voltage wavform on the 750 shows 0V? would it appear that there is something wrong with this measurement?
I know that we have a 400A LRG system to limit ground to keep ground current to apropriate levels. However if something is indeed wrong with this grounding system, then can the fact that the ground fault magnitude was high play a factor in making this L-L fault worse? Can you tell from the event repors what happened first, wheather a L-L fault turned into a ground fault or vise-versa with a ground fault turning into a L-L fault?
I am going to place a call to the utility to to come out and verify all relay connections.