Transformer Primary Fuses Blown 2

[cslater]

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?

 

[Jay27]

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

 

[stevengbvr]

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.

 

[waross]

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

 

[oldfieldguy]

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

 

[cslater]

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.

 

[cslater]

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?

 

[cslater]

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!

 

[jraef]

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

 

[jraef]

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.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Zogzog]

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