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 Post subject: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Mon Aug 29, 2016 4:55 am 
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Hi All.

I know I've seen a picture or two of your typical oil filled pad mounted transformer that had fuses or a breaker installed right off the paddles on the secondary compartment. I tried googling it and could not find any.

I've asked our ABB and Cooper reps if they offer and factory installed breakers or fuses and they both said "no".

Has anyone come across anything like this? I realize on the bigger sizes it may be very difficult to do and be able to get several sets of parallel 500's landed due to limited room but there would be some instances where it could be an advantage to "move the problem" from the line side of the switchboard main out to the transformer.

Thanks.

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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Mon Aug 29, 2016 5:03 am 
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Maybe this is what I saw?

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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Mon Aug 29, 2016 8:00 am 
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I have not seen the big XFMR companies do this but a local re-wind shop had done this for a wind farm. The fuses were bolted directly to the stabs and the other end was bolted to cables that were strapped down to strut. The strut went from side to side in the LV cubical.

No analysis was done to study the movement during a fault but the XFMR company felt comfortable in this configuration. I do not recall if there was any insulating board installed.


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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Tue Aug 30, 2016 6:57 am 
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I saw this at a wind farm on a 1750kVA, 12.47Y/575 txf.


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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Tue Aug 30, 2016 7:34 pm 
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What I usually see is that you can get fuses and/or load break disconnects in the oil filled compartment (fuses are typically bayonet style). With oil surrounding them, arc flash is pretty much impossible but obviously whenever you do quench an arc, you get carbonization of some oil as well as a small bubble of hydrogen that does not ignite except when someone immediately cycles the switch again without waiting for the hydrogen to dissipate. Granted this is all kind of old school technology but old school doesn't necessarily mean it's wrong.

Second with elbow connectors such as what you have in one of the photos shown above, there are two types. In the 200 A types, there is a load break and a non-load break type. The load break type is quite literally a miniaturized single phase switch that is operated with a hot stick from a distance. And in addition to that Cooper (and probably others) also offers a version of these types of devices that includes a fuse within the housing so that you effectively get a fused load break switch built into an elbow connector that is operated remotely (with a hot stick).


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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Wed Aug 31, 2016 3:38 am 
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wbd wrote:
I saw this at a wind farm on a 1750kVA, 12.47Y/575 txf.


That looks like a very good install. Thanks for sharing.

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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Wed Aug 31, 2016 4:07 am 
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PaulEngr wrote:
What I usually see is that you can get fuses and/or load break disconnects in the oil filled compartment (fuses are typically bayonet style). With oil surrounding them, arc flash is pretty much impossible but obviously whenever you do quench an arc, you get carbonization of some oil as well as a small bubble of hydrogen that does not ignite except when someone immediately cycles the switch again without waiting for the hydrogen to dissipate. Granted this is all kind of old school technology but old school doesn't necessarily mean it's wrong.

Second with elbow connectors such as what you have in one of the photos shown above, there are two types. In the 200 A types, there is a load break and a non-load break type. The load break type is quite literally a miniaturized single phase switch that is operated with a hot stick from a distance. And in addition to that Cooper (and probably others) also offers a version of these types of devices that includes a fuse within the housing so that you effectively get a fused load break switch built into an elbow connector that is operated remotely (with a hot stick).


Thanks Paul.

Yes, I'm familiar with all of the above types of primary side fused protection.

The reason I am asking in the first place is that last week we energized a new 500 kva transformer with primary side bay-o-net fuses AND a programmable VFI on the feeder to the transformer. One of the secondary feeder conductors was nicked and in contact with the secondary cabinet so fireworks ensued upon closing the switch. The primary fuses did not blow nor did the VFI trip. It took a good 5 seconds to re-open the switch. This is a remotely operated SF 6 switch. If we had a secondary side breaker it could have had settings making it more likely to trip.

This is in a vault. I closed the vault door and stood outside prior to energizing. I could hear multiple explosions and see multiple flashes under the door during the event. I expect the actual amount of fault current was fairly low as I can see the fault path through the cabinet at each bolt (the cabinet is bolted on to the transformer tank.) I will upload some pics. NOTE: There was rubber or vinyl trim molding around the cutout in the side of this transformer cabinet and the wireway that was bolted to it. Not sure what happened. It must of come off to some extent while pulling in the secondary conductors.

Attachment:
6 way SF6 switch.jpg


Attachment:
transformer setup.jpg


Attachment:
cables.jpg


Attachment:
cable fault.jpg


Attachment:
cabinet damage.jpg


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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Wed Aug 31, 2016 4:17 am 
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Oh, we have a few fused elbows around here too. Plus some old Joslyn fuses you plug on to the 200 bushings then plug a regular 200A loadbreak elbow on to. They don't make that type anymore but we still have several in service. Fortunately we have a good stock of spare on hand. See pics of both types I just happen to have on hand.

Attachment:
fused elbow.jpg


Attachment:
fused elbow paperwork.jpg


Attachment:
elbow fuse.jpg


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plug on fuse.jpg


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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Wed Aug 31, 2016 4:58 am 
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bbaumer wrote:
One of the secondary feeder conductors was nicked and in contact with the secondary cabinet so fireworks ensued upon closing the switch.


Were the secondary cables meggered prior to final connection? That may have spotted a problem prior to energizing.

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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Wed Aug 31, 2016 5:06 am 
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wbd wrote:
bbaumer wrote:
One of the secondary feeder conductors was nicked and in contact with the secondary cabinet so fireworks ensued upon closing the switch.


Were the secondary cables meggered prior to final connection? That may have spotted a problem prior to energizing.


No, they were not.

This was a contractor installed job that I drew. The week prior when they were pulling in the secondaries I was standing there in front of the transformer with the the foreman (on site every day) and his superintendent (stops in once in awhile to check on the job). I said to the foreman "make sure you ring those out" and the super immediately chimed in before I could get another word out "no, I want you to the meg 'em".

I didn't think of it again as this contractor has done a lot of good work for us over several years and visual inspection looked good.

After the incident I the pair if they had tested the cables. The foreman admitted he had not............

None of us will be making that mistake again. I can tell you that.

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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Wed Aug 31, 2016 6:56 am 
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Instead of a physical breaker in the TX compartment, you might be able to employ a Virtual Main scheme instead.
Install CTs directly on the secondary spades, then have a protective relay trip the primary side VFI.


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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Wed Aug 31, 2016 7:08 am 
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JBD wrote:
Instead of a physical breaker in the TX compartment, you might be able to employ a Virtual Main scheme instead.
Install CTs directly on the secondary spades, then have a protective relay trip the primary side VFI.


That's a good suggestion and very do-able in this installation. The primary switch is in the vault as well. It would be easy to get wiring between the transformer and the VFI. I had not thought of this.

A vendor proposed this option to me a long time ago just in general but we don't have VFI's everywhere (see comments about fuses above) and often where we do we are 100' or more between the VFI switch and the transformer and getting the wiring between the two had challenges so I didn't consider it.

In this case though, it is quite feasible. Thanks!

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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Wed Aug 31, 2016 8:33 am 
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Field modification? Looks like the semi-circular hole adjoining the square one was an afterthought that left a sharp edge.


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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Wed Aug 31, 2016 9:09 am 
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stevenal wrote:
Field modification? Looks like the semi-circular hole adjoining the square one was an afterthought that left a sharp edge.


That semi-circular hole was created by the fault, so yes, I guess you could consider it a field modification. Ha!.

Seriously though, although this is a pad mounted transformer it is installed in an existing below grade vault. It would be very costly to bust up the slab to get conduits in from below grade so we fed it from/to overhead. The primary is in cable tray over and down through a conduit. The secondary is in wireway. This pic gives you an idea.


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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Wed Aug 31, 2016 11:16 am 

Joined: Wed Jan 05, 2011 5:00 pm
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It looks like you have a clearance issue with your secondary gear, or is it just perspective?


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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Wed Aug 31, 2016 11:24 am 
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dkidd wrote:
It looks like you have a clearance issue with your secondary gear, or is it just perspective?


Perspective. We have 1" to spare!

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 Post subject: Re: Adding fuses or a CB in a pad mount xfmr secondary
PostPosted: Wed Aug 31, 2016 1:27 pm 
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The only problem with the "virtual main" concept is that either you have to wire up both trip relays in parallel, feed one into the other, or typically to get essentially 6+ CT's worth on input functions in microprocessor trip relays, it forces you to buy the relay that is really intended as a recloser relay and then turn off all the extra stuff that you don't want/need stripping it down to just a basic 50/51 relay and deal with the strange looking stuff from there. I've done this with an SEL 651 but it's not the most obvious thing to be doing, and I kind of like the lower level of confusion that having two 50/51's wired in parallel causes when operators and linemen these days are so used to seeing a single multifunctional micrprocessor "swiss army knife" relay that having multiple relays is actually confusing compared to the older equipment where we might have a 50, 51, lockout relay, synchronizing relay, etc., etc., all as separate units.


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