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 Post subject: Understand Utility Info
PostPosted: Mon Apr 27, 2009 1:37 am 
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I ask utility company for arc fault information and this is what they sent me.
I am trying to find the best way to uses this information in my calculations.

I have a 2500 kva transformer with a secondary side of 277/ 480.
Actual transformer values: IZ = 5.79%, X = 5.75, X/R = 8.19
Calculated Isca = 51,934

Qty 15 500kcmil cu
20 ft steel conduit
The switchboard is only rated for 50,000 short current.

Calculated fault duties up to the customer's primary terminals, at 13.8 kV system nominal voltage, via normal source feeder.

1.75 CKT Mi = Circuit Distance in miles along the route from substation to fault location in miles. For reference only.

4,090 IF3p = Calculated Fault Duty in Amps, 3-phase, from source and line impedances & System Voltage

3,542 IFpp = Calculated Fault Duty in Amps, phase to phase, from source and line impedances & System Voltage

3,449 IFpn = Calculated Fault Duty in Amps, phase to neutral, from source and line impedances & System Voltage

3.68 X/R 3P = Calculated "X over R" Ratio for 3-phase & Phase-to-Phase faults, from source and line impedances

2.90 X/R SLG = Calculated "X over R" Ratio for phase to neutral (Single Line to Ground or "SLG") faults, from source and line impedances

MEC Standards specify Solidly Grounded Wye-Wye 3-phase padmounted distribution Transformers.

Transformer Impedances for 3-phase distribution transformers 750 kVA and larger, per ANSI/IEEE:

5.75% on Transformer kVA Base +/- 7% of Impedance = Range of 5.35% to 6.15%.

Typical Transformer X/R Ratios:

X/R ratios for transformers depend on loss values and impedance of specific units, typically in range of 7 to 11 for this size range of transformers historically seen at MEC.

Specific impedances and X/R Ratios for the units are unknown. If the specific values were known, MEC would caution against using specific numbers of units in place, as the units can be replaced at any time due to equipment failure or damage physical from outside forces, such as vehicular collisions.

Overcurrent protection of MEC standard padmounted transformer units are Cooper Bayonet style under-oil fuses in the transformer primary circuit. MEC specifies:

2500 kVA, Cooper Fuse, Catalog Number: 4000353C17, or equivalent.

The above fuses completely coordinate with the feeder breaker settings, so the breaker settings are not provided.

Recommendation: Since transformer-specific data is not recommended for calculations, use the impedance ranges and X/R ratios that provide the worst case scenario for Arc Flash calculations from the provided information.


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PostPosted: Mon Apr 27, 2009 1:14 pm 
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Location: Quebec, Canada
Your questions is...?


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PostPosted: Mon Apr 27, 2009 1:25 pm 
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H2Os wrote:
2500 kVA, Cooper Fuse, Catalog Number: 4000353C17, or equivalent.


Strange, Cooper doesn't recommend those fuses for a 2500 kVA transformer feeded at 13.8 kV (http://www.cooperpower.com/Library/pdf/24045.pdf, Table 4).


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PostPosted: Tue Apr 28, 2009 2:25 am 
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I would like to know if I can reduce the fault current by add the utility information in to my calculations.

Right now I am using infinite bus short circuit calculation to determine max short circuit current on the secondary side of transformer.


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PostPosted: Tue Apr 28, 2009 5:19 am 
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Your worst case, which I presume you would like to know, will need calculations for the various scenarios. I think that this will include estimates of minimum & maximum fault contribution from the utility. This side of the pond (England) there is a requirement to provide information but the utility design guys often think that a maximum figure (often estimated upwards) will suffice. In your case you are marginally over the fault rating of the switchboard based on your infinite bus bar calculation but even on this basis the fault level will attenuate from the transformer terminals. I take it that the 20ft of cable is the connecting cable between TX and switchboard in which case if you don't seem to have a fault level violation problem at that equipment. Arc flash is another issue entirely and this is where system modeling software really scores. Its unlikely that you will be able to predict which utility scenario is likely to be worst case to all downstream equipment without doing the calculations. Hope I this helps, Mike


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PostPosted: Tue Apr 28, 2009 11:43 am 
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The plain and simple answer to your question is "Yes" adding in utility impedance will definitely lower your fault current. Here is a good link to an article by Jim Phillips (he is a frequent here so Jim I hope you don't mind me referencing this article on your site.)

http://www.brainfiller.com/documents/TransformerandSourceImpedance_000.pdf


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PostPosted: Tue Apr 28, 2009 4:29 pm 
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How would I get the utility impedance value out of the information they send me.
thanks


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PostPosted: Tue Apr 28, 2009 6:10 pm 
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You have to include the utlitily info. Yes, it will lower your overall fault current. But for purposes of arc flash calcs that is a good thing, as lower fault current, results in lower arcing current, which means longer breaker clearing time. The longer values on the clearing time have a much greater effect on IE than a similar increase in fault current.

The utility should have given you something that said. MVA which could be 'fault MVA, or MVAbf, or it could have given fault current, I-fault, or Ibf. You have to get at least one of these to know or calc fault current.

Not exact but once you include utility impedance you will probably see a reduction in fault current the switchboard of around 10 to 30%.


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PostPosted: Tue Apr 28, 2009 6:17 pm 
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H2Os wrote:
Calculated fault duties up to the customer's primary terminals, at 13.8 kV system nominal voltage, via normal source feeder.

4,090 IF3p = Calculated Fault Duty in Amps, 3-phase, from source and line impedances & System Voltage


Those two lines give you a 97.8 MVA 3 phase bolted fault current. You should now be able to use that instead of infinite bus for the primary of your transformer.

Of course it's only another (closer to truth) approximation, as you probably won't use the X/R ratio.


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PostPosted: Wed Apr 29, 2009 4:23 am 
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Thanks for the help!

Here is what I calculated

4,090 x 13800 x 1.732 = 97,757,544 MVA or ( 97.8 MVA)

( 2500 kva / 97,757 kva ) * 100 = 2.56 % source impedance

SCA secondary = 3007 amps x 100 / (5.79% + 2.56% ) SCA secondary = 36,273


Transformer Calculation
Infinite Bus: SCA secondary = 51,934

With source Impedance SCA secondary = 36012


When I ask the utility for arc fault data, I told them I need MVA and clearing time and line impedances information. That was three months ago.

So when I got this information show above I was little lost on how to use it!


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PostPosted: Thu Apr 30, 2009 12:18 pm 
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Your calculations assume that the X/R ratio of the line equals the X/R ratio of the transformer, which is not the case. To be accurate, you would have to determine both the resistance and reactance of each element and add them separately. This won't make a lot of difference.

Assuming X/R for the transformer = 10:
RLine = 0.67% XLine = 2.47%
RXfmr = 0.58% XXfmr = 5.76%
Ztotal = 8.32%
Ifault = 36142A (1257A on primary)

A Cooper 4000353C17 fuse will clear in about 0.25 seconds at this fault level.

This is maximum fault at the transformer secondary. You have to add in the impedance of the service cable to get the fault at the service. The highest arc energy might be at a lower fault level. You need to check the high end of the transformer impedance range and include the service impedance.


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PostPosted: Tue Jun 02, 2009 5:07 pm 
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Have you considered fusing at a lower rating based on your usage and using current limiting fuses. Our plant is served by a 7500 KVA transformer at 12.5 KV but we dont use anywhere near that under full load so we lowered the fuses in our main disconnect from the utility and used current limiting fuses that way we are not trying to light a 100W light bulb with a 10 MW generator and life is so much easier.


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PostPosted: Mon Jun 08, 2009 10:25 pm 
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Update

After talking with the utility distribution engineering they told me that they can not provide the source impedance value because, the source impedance could change daily as they change feeders around for contingencies.

They recommends: neglect the source impedance and assume that the limiting impedance is the transformer impedance and the clearing device is the bayonet fuse in the transformer.

So it looks like I am back to using infinite bus short circuit calculation again. :confused:


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PostPosted: Tue Jun 09, 2009 8:28 am 
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I do sympathize but I'm rather glad that the problems with utilities are pretty universal. You might think that this is a bit too difficult but I have had some success in going to the utility in person and having a conversation with one of the "sharp end" operational engineers. Just recently in Ireland, a very knowledgeable engineer from the local power company, who happened to be at site in order to arrange a power outage was happy to tell me about their own feeder lengths and configuration plus transformer impedance etc. He also had intimate knowledge about fault and maintenance switching. By modeling this information and also the "official replies" I was able to validate what would be the worst case scenario. I know that none of this is very scientific but it might be worth a try, apologies if you've already been down that road and don't laugh if that all sounds ridiculous in the US.

On a wider issue, is there anybody that can state what the statutory duties are for utilities to provide information in the US? A power company will do a system study but there is a charge for the work, just wonder what the minimum free of charge information is? We all know that the characteristics of supply are crucial in design of electrical systems quite apart from arc flash studies.


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PostPosted: Tue Jun 09, 2009 8:40 am 
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H2Os,

Infinite bus will generally give you less conservative IE values.

Distribution engineering may be made up of engineering techs. I would ask to speak to a degreed electrical engineer.

I generally try to provide the extreme contingencies, giving a range of possible source impedances. There is still the likelihood that a failed transformer will be replaced with one of a different impedance.


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PostPosted: Tue Jun 09, 2009 11:11 am 
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NEMA published a white paper a while back, urging utilities to provide pertinent data for arc flash studies. FWIW

http://www.nema.org/prod/be/lvde/upload/LVDEARCFLASH.pdf

There are also a few posts on this site about estimating a minimum value. I recall someone saying they used interrupting ratings on the utility side as a basis.


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PostPosted: Tue Jun 09, 2009 4:52 pm 
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An unfortunate publication. I doubt there are many utilities that will set up such a notification system. It would need to be invoked every time the system is reconfigured, a daily occurrence.

It is better to let the customers know of the uncertainties involved, so they can adjust their IE values upward accordingly. Providing an "actual" value sounds too precise and permanent, leading to precisely calculated IE values.


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