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 Post subject: Lov Voltage Dry Type Transfomer Equipment Type??
PostPosted: Thu Feb 23, 2017 7:45 am 
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When evaluation low voltage dry type transformers [600V and below 300kVA or less] typically what equipment type to you use to establish the “Bus Gap”, in reviewing different studies completed by different individuals / firms for a single campus there appears to not be any consistency and when the question was asked the discussion went quiet….So with that it appears most commercially available software has defaults based on system voltage and equipment type taken from IEEE-1584 table 6, there are four categories Open air 10-40mm, Switchgear 32mm, MCC and Panels 25mm and Cable 13mm each with a different distance “X” factor associated with it [note Open air and Cable are the same “2.000”]. My question would be when completing an arc flash analysis for low voltage transformers what is the preferred approach if there is one, should you measure each transformer terminal pad [typically unlikely as the studies include already energized / existing equipment], look for manufacturer specific data and override the software default with the actual distance, utilize one of the IEEE defaults either open air, switchgear, MCC and panels or Cable as determined by the software?? Just wanted to get the thoughts of everyone on the forum. Thanks for you input….


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 Post subject: Re: Lov Voltage Dry Type Transfomer Equipment Type??
PostPosted: Thu Feb 23, 2017 9:19 am 
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Generally the gaps are quite wide and the enclosures are also quite a bit larger and more open and not well represented by the "large enclosure" (e.g. panelboard) model in IEEE 1584. So your choices are to either measure/eyeball it or else make an assumption. Another fundamental problem is that the IEEE 1584 data set doesn't get to wide enough bus bars in many cases. In that case the best you can do is that EPRI has done some testing and published equations for "medium" size gaps (up to about 12"). They have done test work on much larger gaps but so far it is kind of a work in progress.

You are correct though in that the gaps are really based on how the bus bars get laid out. Since the width of the bus bars is really a function of the size of the core, which is in itself a function of transformer wiring, there is never going to be a standard for this. You can easily measure it with the cover off even if it's energized by using a plastic caliper or ruler and if you are really nervous, wear rubber gloves. But that's taking an awful risk of dropping the cover into the transformer when doing this.

So right or wrong, I usually use the panelboard assumption with the default gap. The exponent (X) is probably closer to 2.0 (open air) and the bus gaps are always less but I've really got nothing else to go on except for the dea that the most likely arcing fault will end up being across the wiring at the end of the wire duct where it enters the transformer housing.


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 Post subject: Re: Lov Voltage Dry Type Transfomer Equipment Type??
PostPosted: Sat Feb 25, 2017 11:44 am 
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However I would have to ask why you are looking for the arc flash on a dry type low voltage transformer. Typically this do not get labels as they are not the type of equipment that would be serviced while energized. The only issue would be if you are removing the covers to do a IR scan which I would think would be risky and probably not worth the risk.

If you are doing this, I would think for the larger 1000-2500kVA dry types this would be risky and you would be better off installing IR viewports during a shutdown.

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 Post subject: Re: Lov Voltage Dry Type Transfomer Equipment Type??
PostPosted: Sun Feb 26, 2017 2:29 pm 
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wbd wrote:
If you are doing this, I would think for the larger 1000-2500kVA dry types this would be risky and you would be better off installing IR viewports during a shutdown.


Actually, this is a great example of bus bar temperature sensors. The ones I'm referring to seem to be from multiple vendors now. They are marketed for switchgear but would work great in a transformer. They have a fiber (plastic?) that transmits light and returns a signal back to a transducer that measures the temperature quite accurately at the sensor which is clipped onto the bus bar. I can't think of a safe way to do this energized other than the IR window and since the cost is about the same, online monitoring would be a better way to go.

The only realistic way I see failures happening within dry transformers is either due to the eventual aging of the insulation, plugging up due to foreign contaminants, overheating from overloading, or fastener failure at the joints. Overloading can (should) be monitored and protected externally. Plugging up is a basic PM requirement (clean out the dust). As long as the pressure is adequate and has been proven to last indefinitely, joint failures shouldn't occur although the only way to test/check this is during installation...by the time it shows up on IR, it's kind of too late. That leaves insulation degrading which is something that is inevitable although I've worked with/around plenty of dry transformers that have been heavil used/abused over the years and they show no signs of impending failure even though some of them are pushing 50 years old and contrary to claims that dry transformers will not outlive oil filled (something I have not found to be true in practice).


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 Post subject: Re: Lov Voltage Dry Type Transfomer Equipment Type??
PostPosted: Mon Feb 27, 2017 8:45 am 
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wbd wrote:
However I would have to ask why you are looking for the arc flash on a dry type low voltage transformer. Typically this do not get labels as they are not the type of equipment that would be serviced while energized.


Not 100% sure for this particular one it may have been an owner driven requirement, however in review of some previous RFP's I noticed [at least in the one's I looked at] they were all over the place but all did require a label on the transformer some even specified what to base the label on [primary vs. secondary] vs. 125kVA and below, not sure what they would “service” while energized as you stated but out of curiosity will ask around of some end users to get their thoughts, however I see the same issue with equipment not specifically “panels”, “switchgear”, etc. for example a transfer switch, they appear to have bus gaps somewhere in between one manufacturer in particular up to 600A 0-600V has a bus gap of 19mm at the lugs the IEEE standard refers to MCC and panels at 25mm and switchgear at 32 so in this case the gap is smaller.


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 Post subject: Re: Lov Voltage Dry Type Transfomer Equipment Type??
PostPosted: Mon Feb 27, 2017 11:31 am 
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On oil filled transformers (yep, not dry), quite often the "padmount" types have the oil drain/sample port and often even the gauges inside the enclosure doors. This is for security reasons because the typical location for a padmount is in a publicly accessible space (think utilities). So especially on the older ones that have live front terminations, obviously an arc flash sticker is extremely appropriate.

On a dry one the only time I've ever accessed one (except accidentally) while live is when inspecting the fans to make sure they are working and even then, it's more of an inspection thing...shut it off, remove the covers, turn it back on and watch what happens.


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