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Transformer Primary Fuses Blown
2

Transformer Primary Fuses Blown

Transformer Primary Fuses Blown

(OP)
A utility transformer at our site was brought online a few days ago and blew all three line side fuses after about 10 minutes.  The transformer is a 67kV/12kV 10000 kVA unit.  It was brought online unloaded and none of the secondary relays tripped.

The working theory is that either water got into the transformer, or there is some other short inside it.  

I'm wondering if its more likely that the fuses weren't sized for inrush.  I haven't heard yet how large the fuses are or if they're slow-blow.  

Is it possible that the inrush could weaken them so that they would blow after 10 minutes unloaded?  I've seen some mention of similar things on this forum but am not familiar enough with fuses to know if that applies here.

Are there other causes that we're not considering?

RE: Transformer Primary Fuses Blown

If the fuses lasted even seconds, let alone minutes, it wasn't inrush.  Inrush is measured in cycles.

10MVA is rather large for fuses, a relay and a circuit switcher would have been a far better design, and the relay event report would provide all the information necessary to know what happened, but save a few penneys first cost and pay lots of dollars dealing with the "savings".

RE: Transformer Primary Fuses Blown

Is this a new transformer installation or it is an existing transformer which was taken out of service for maintenance or a power shutdown?

If the unit was a new installation, was electrical testing, oil sample, inspection, and test of all auxiliary devices performed?  Stardard commissioning procedures done by qualified persons followed to NETA Acceptance Testing Specification is recommended.  

I have performed maintenance and commissioning on similar sized transformers protected by fuses at some of our customer sites.  Test results are normally compared to factory test results or previous maintenance test results.  Oil samples are sent to a lab for DGA and stanardard oil screen.  

Testing after the blown fuse event will have to be completed to give some clue to the cause of the fault.  How is the secondary side of the transformer connected; by cable or overhead line?  Where is the secondary main feeder protetion located?        

RE: Transformer Primary Fuses Blown

(OP)
This is something that I'm not directly involved in, so my knowledge is spotty in places.  Apologize for that.

I believe it's a new transformer.  A reputable testing company came and tested it thoroughly before it was energized - everything passed.  Oil is currently being sent for testing.  

As far as I know, the secondary comes out underground and goes to a switchgear lineup with protective relays before going out to overhead 12kV lines.

RE: Transformer Primary Fuses Blown

If it was tested correctly my guess would be either that cable is bad or the primary fuses were incorrectly sized. Do you know what testing was done on the cable? More than a DC hipot I hope.  

RE: Transformer Primary Fuses Blown

I'm thinking that if the time from energizing to fuse blowing was seconds, examine closely the cables and equipment connected to your secondary.

It sounds like a hard secondary fault.  Current on these faults is limited by the transformer impedance, so it takes a bit of time for high side protection to operate.  The most common failure is having crossed cables in multiple conductor per phase installations.

Ditto the DGA.  If you have a good electrical testing company available have them do an immediate TCG (Total Combustible Gases)if the transformer has a nitrogen pad.  If there was an internal fault, the TCG should show it immediately.

old field guy

RE: Transformer Primary Fuses Blown

(OP)
It took approximately 10 minutes to blow the fuses.  Also, there was no load on the secondary side - but I suppose the fault could have been between the transformer and the switchgear.

Can you point me to more information on what a TCG test is?  I'm not familiar with that.

Thanks very much - everything so far has been very helpful!

RE: Transformer Primary Fuses Blown

If all three phases blew, then a secondary cable fault is unlikely, unless it's something easily seen like grounding straps left on all three phases of the terminations.
 

RE: Transformer Primary Fuses Blown

(OP)
Could water in the oil cause all three fuses to go at once?  It seems like the failure would be more likely across one coil which should only blow 1 or 2 of the fuses, right?

RE: Transformer Primary Fuses Blown

There are two ways to get all three fuses "at once".

One is a hard three phase fault; fault to fuse clearing in less than a cycle or two.  The other is that there was enough leakage to ground from all three phases that each fuse saw and cleared the fault independently around the same time.  If the fuses really took 10 minutes to clear, you would had to have had fantastically well matched fuses, otherwise you would not have gotten more than two of them.

RE: Transformer Primary Fuses Blown

(OP)
Just heard that the oil tests came back and there's no sign of anything in there.

I got a little more information that I hadn't heard before.  This transformer was actually put in place a year ago and has run fine for that whole time.  A few weeks ago a raccoon got into the substation and caused a fault, which blew out a bushing.

The bushing was replaced, along with a lightning arrestor on the secondary side.

The transformer was re-energized, with no load and - as stated before - all three primary fuses blew after 5-10 minutes.

This is not a transformer that I'm responsible for, but as an engineer it has me really curious.

RE: Transformer Primary Fuses Blown

Probably a transformer problem. You can rule out the secondary side as previously stated unless a fault occurred across the switchgear(highly unlikely). If there is no load, the fuses would have no purpose if there is no thru put(unless the previously stated scenario had occurred). Although I would agree it could be a 3-phase fault as previously stated. Probably a winding fault in the transformer, or  a fault somewhere along the line between the transformer and the switch gear.  

RE: Transformer Primary Fuses Blown

Or it may have been Charlie Raccoon looking for his friend Rocky.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer Primary Fuses Blown

Like davidbeach said, three fuses clearing all at once after 10 minutes is a bit of a freak occurrence. A hard fault seems more likely. But it would have to be something that 'cooked' for 10 minutes before failing. I'd take a look at the secondary lightning arresters. Particularly the one that was replaced.

RE: Transformer Primary Fuses Blown

Do you know what fuses blew from the initial racoon?

Alan

RE: Transformer Primary Fuses Blown

(OP)
racobb - That's a good question.  I believe all three fuses were replaced, but I would have to confirm that.

PHovnanian - I wondered about the lightning arresters too.  But if only one lightning arrester was replaced, how would that kill the other phases?  Is there some way for a ground fault to put a high enough voltage on the ground buss and somehow transfer that upstream to the other phases?

willhammett - The current working theory is a winding fault in the transformer that didn't short until everything had warmed up a bit.
 

RE: Transformer Primary Fuses Blown

cslater-

"TCG" is Total Combustible Gases", typically performed in the field with a portable test set.  It takes a sample from the transformer's gas space and checks it for combustible gases.  Typical "good" transformers read less than 0.5% (IIRC, It's been a while) and transformers with internal problems will practically peg the meter.

In my field service days, the first test I did when I got called about a transformer coming offline was a TCG test.  It's fast, almost instantaneous, and 24 hours faster than a 'rush' oil sample.

I missed the ten minute time you referenced.  That colors my initial assessment, but since your oil tests (I'm assuming that includes dissolved gas analysis) show good, we're still looking at something on the secondary side of the transformer.  The transformer impedance limits current and the fuses may have long melt times at low currents.

old field guy

RE: Transformer Primary Fuses Blown

Another animal exploring the bushings could take out all three fuses. You have prior history!

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer Primary Fuses Blown

Quote:

But if only one lightning arrester was replaced, how would that kill the other phases?

The initial single phase to ground fault (one bad arrester) could cause a voltage imbalance that would cause a second marginal arrester to trigger. Or for a L-G fault on the outside of an arrester or bushing, the arc could propogate to involve another phase or two.

RE: Transformer Primary Fuses Blown

Quote:

Another animal exploring the bushings could take out all three fuses.

(Sniff, sniff.) Fried raccoon. Mmmmmmm!

RE: Transformer Primary Fuses Blown

DGA on the transformer.  It will tell you if it was an internal fault or something else. Once you've narrowed it down, investigate more specific causes.

you can't fix stupid

RE: Transformer Primary Fuses Blown

I have been told that the transorfmer documentation indicates that bad tap changers can cause the fuses to blow.  The tap changers appear to be inoperable. Any thoughts on this?  Thanks.

RE: Transformer Primary Fuses Blown

In some configurations different tap settings on different windings may cause circulating currents that may blow fuses and/or damage the transformer. However typically only one or two fuses blow. However, in extreme cases the circulating currents may lead to transformer failure which can blow all three phases.
But, "The tap changers appear to be inoperable."
you may have had a fault in the tap changer compartment.
Also "A reputable testing company came and tested it thoroughly before it was energized - everything passed."
These statements seem somewhat contradictory.
I understand your desire to more fully understand the possible causes of an incident that is impacting your project or work site. No problem.
The short answer is, most problems don't take out three fuses, but when one or two fuses blow it is often good practice to replace all three.
Was your information that three fuses blew or that three fuses were replaced?
Usually it takes a fault that involves all three phases to take out all three fuses.
However, we are making guesses based on second hand information and assumptions.
At this point the tap changer is suspect. It may have been the cause or the victim. I am assuming an on-load-tap-changer.
Another guess based on another assumption!

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer Primary Fuses Blown

Sadly, even a "reputable testing company" can provide horribly incorrect results.

I sympathize with "A reputable testing company came and tested it thoroughly before it was energized - everything passed." but I know of many incidents where clients have been severely misled by that.

In a recent incident, a "reputable testing company" supposedly tested and signed off on a substation for my company.  On first energization, an autotransformer that was part of the starting circuit for a large motor tripped immediately.

Investigation found that the CT inputs to the relay were incorrectly landed.  The relay could NEVER have worked nor could it have been successfully tested unless the technician lifted the terminations from the rear of the relay case.  We had in our possession an official test report on this relay and others.

In a transformer-related incident, a competitor's technician condemned a generator step-up transformer because he did not understand that the GSU was wye-delta and shorted the H0 bushing to the X1-2-3 bushings for power factor tests.  His ratio test results were equally 'interesting'.  Had the client taken those results alone, they'd have spent a couple of million bucks, but they called my bunch in for a second opinion.

Having been a manager for a third-party testing company, I can tell you horror stories of trying to find qualified test technicians and to differentiate between those who will actually do a good job and those who will go through the motions and hope everything holds together after they leave.

In the case of the relays I spoke of above, we employed yet another technician who, according to the employer, could do the job.  He couldn't.  But he did bring the proper test equipment and between me and a young engineer, we got the relays properly tested and put in service, and subsequent test runs verified our work.

My advice is to be very careful about accepting test results at face value, and build a strong relationship with people who are knowledgeable in interpreting test data.

 

old field guy

RE: Transformer Primary Fuses Blown

(OP)
I've learned some additional information.

The tap changer is operational, but has had some issues where it won't always operate automatically.

Oil tests were done on the tap changer and - as with the transformer - there is no sign of water.  They are also saying that the oil tests indicate that there has not been any arcing in the transformer or the tap changer.

So here's something odd...

I was looking over the tests, and they list the X1-X3 arrestors as being Type EXLIM Q, rated at 8.4 kV.  The H1-H3 arrestors are type XPS rated at 48 kV.  

Looking at the A-BB site, it looks to me like EXLIM Q arrestors are supposed to be for over 44 kV.  I can't find information on XPS.

Is it possible that they swapped the arrestors?  Could this cause the type of failure we saw?

By the way - thank you all very much for your input thus far.  I have learned a great deal through your input and I very much appreciate it.  

RE: Transformer Primary Fuses Blown

(OP)
Another thing I'm thinking is that the surge arrestors may be under sized.  According to the part number the primary arrestors are catalog number Q060XA048A.  If I understand ABB's numbering right, that is a Ur of 60kV and a Um of 48kV.  But Um is the system voltage, which is 67kV.

If a suppressor is under sized, couldn't you wind up somewhere out on the TOV curve and have too little resistance to ground?

It seems like that might take a few minutes to blow the fuses...

Am I completely out in left field?  

RE: Transformer Primary Fuses Blown

(OP)
I wanted to follow up not that this has been figured out.

Turns out there was a short in the CT section of the secondary switchgear.  What was very helpful was the assertion from several of you that it sounded very much like a hard secondary fault.

Thanks again for all of your help on this!

RE: Transformer Primary Fuses Blown

I liked the secondary "critter sniffing around for BBQ" theory though.

RE: Transformer Primary Fuses Blown

Damn, didn't mean to post yet...

I had an incident where a pair of crows decided the warm top of a pole mounted XFMR was a good nest site, but one of them completed a circuit and fried. That's when I learned that crows mate for life and the mate stayed around for weeks even though we removed the nest. Eventually it too completed the circuit and joined its mate, but I had a difficult time convincing the powers-that-be of the situation, they wanted to blame my contractor for a defective installation after the first failure. I told them I had buried both crows near by and offered to dig them up for them, they declined... thank goodness, because I was fibbing.

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RE: Transformer Primary Fuses Blown

Thank you for the update, so nice (and rare) to hear how things turned out.  

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