Door Blowing Open when PMT Fails

[FEinTX]

Question - Is it normal for a Pad-Mount Transformer to have doors blow open when there is an internal arc flash?

Background

+ Equipment = 1.85 MVA transformer at 34.5 KV at a wind farm
+ Initial Electrical Failures - Elbow failure or Bushing failure
+ Typical Fault current 6-20 KA
+ Door Failure - the low-side door only. The high-side door which is bolted down with 2 bolts sometimes bows outward, but has not opened in any of our failures. The low-side door has rods going to the top and bottom and a plate that rotates into a slot when in the closed position.

The manufacturer (a LARGE company) touts that the enclosure has been built and tested to IEEE Std C57.12.28™-2005 (IEEE Standard for Pad-Mounted Equipment—Enclosure Integrity). I've read this standard, and it seems to mainly focus on ensuring the enclosure is built such that it is difficult to break into (penta-bolts, difficult to pry open, etc...) I did not see anything addressing what type of failure the enclosure should withstand. Regardless, I'm sure the IEEE has an opinion on doors flying open and potential hurting people when they do. Is there another standard I should be referencing? All help is greatly appreciated.

 

[BJC]

NO - not just No but Hell NO

 

[dpc]

I don't think there are any specific requirements for performance under arcing fault conditions for pad-mounted transformer enclosures. This is generally true for all electrical equipment that are not specifically tested and rated for arcing faults.

I am not surprised that the doors blew off. This happens also with motor control centers and 480 V switchboards.

A closed door offers some protection, but they are not designed to withstand the enormous blast pressures that can occur with arcing faults.

Do you have pictures?

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[FEinTX]

DPC,

One difference between this and a 480V switchboard, PMT's are typically not behind security fences, or in mechanical rooms, or off-limits in some other way. There are smaller, lower voltage PMT's sitting in people's front yards as part of underground distribution systems. I've often seen kids sitting on the top of PMT's in my own neighborhood. I would think the cabinet would have to be designed to contain the blast pressures or channel the energy safely away somehow.

I've attached pics of the latching mechanisms for the low-side (600V) doors. The rods at top and bottom are bent back during a failure. No damage has been observed on the center plate. Hopefully the attachment is there--I'm new to this.

Thanks!

 

[FEinTX]

2nd Pic

 

[rbulsara]

Where is the blown out door?

Arcing fault can quickly lead to flash over and very dangerous explosion, that is what all the fuss is about the arc flash analysis. So yes, a door can blow out but would be rare.

What would worry me more is why are there "four" failures? THAT is not normal, if in a short period of time at the same site. Are they?

When mfrs. say equipment are built to meet certain criteria, it is also assumed that appropriate overcurrent or fault protection is in place.


Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

In the first photo, the lock system looks like it would be mechanically stronger if the bracket was welded to the door considerably closer to the frame member the rod engages with - it would then be much harder to bend the rod to burst the door. Poorly thought out design, or maybe a reasonable design with indifferent fabrication?


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

 

[dpc]

I understand your concerns about safety implications, but that still does not change the facts of the situation.

You might Google up some photos of arc-resistant switchgear and take note of the huge duct in place to vent the gas created during the arcing fault, as well as the construction of the doors and inter-cell barriers.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[FEinTX]

Rbulsara - I do not have a picture of a blown out door, but nothing very dramatic. Often, no damage at all. Worst one was bowed and had to be sent back for repairs.

We have had 6 failures (installed base is 180) where the bushing arced to the retainer plate (GND) on the high side, and yes, this is unusual. The manufacturer is working on root cause--may be bushing, overtorque-ing, or insufficient retainer plate (bends w/ only 125 in*lb torque). They are dismissing the blown open doors. The high-side door is bolted into the center panel that divides high and low side. These sometimes bow outward, but have never blown open. The low-side door is blown open more often than not.

ScottyUK - I have the same concern. They are providing an unnecessarilly large lever.

 

[dpc]

The photos don't show quite the devastation I was expecting. What's the secondary voltage - 600 V?

Regardless of the design details, the big problem is that ANSI does not have a test requirement for withstanding arcing faults. Although your transformers are oddballs, in general pad-mount transformers are built to meet ANSI standards as cheaply as possible since there is tremendous price competition for the high-volume common sizes. If the only requirements are to meet ANSI C57 pad-mounted transformer standards, you will probably not be getting a high-end product.

If this is something you have any control over, you may want to review whatever specifications being used to purchase these and try to beef up the specs a little. If you're buying a lot of them, you may be able to get a decent upgrade in quality - if the powers that be are willing to pay.

Dave

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[FEinTX]

Dave,

The pics were of a new unit--just showing how the mechanism is designed. The failed units just have the rods bent back a couple inches though.

If interested, I could post a pic of the bushing failure. These are a lot more interesting, but not really the topic I was chasing here...

 

[dpc]

The latch looks pretty typical, unfortunately. Honestly, large 480 V pad-mounted xfmrs are some the scariest things on a distribution system. Especially with the low voltage door open.

With modern dead-front primary connections, the low-voltage side of these is much more hazardous than the high-voltage side. But according to ANSI standards, the low-voltage doors have to be open before the high-voltage side can be opened.

But that's another topic.



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[rbulsara]

FEinTX:

The deformation you describe would not surprise me, but they do indicate that you have had high energy faults. Also totally enclosed enclosure are more prone to get deformed as the expanded gas has to go somewhere or something will explode.

In some sense the deformation may allow gases to escape, intended or not, and save more damaging explosion.

I would concentrate on finding the cause of the failures than its effects, at this time. Include installer's skills in the review.

Rafiq Bulsara

http://www.srengineersct.com