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 Post subject: utility information - incorrect ?
PostPosted: Tue May 24, 2011 11:49 am 

Joined: Tue Apr 12, 2011 6:11 am
Posts: 17
Location: Oregon, USA
I am still at the beginning of an arc flash study, and am focusing on the short circuit analysis at the electrical utility service. I have already encountered a problem, with a seeming contradiction in the information provided to me by the electrical utility representative. Essentially, it appears to me the initial information I have been given is in error. Before going back to the utility rep. again with my quandary, I would first like to post the issue here to see if anyone agrees with my assessment, or else can tell me what I may be overlooking.

At this point, it all seems like it should be elementary enough :

The utility delivers power to the site at 20.8 kV, 3Φ, via overhead lines at the street. 100 amp fused taps drop down from the lines to the padmount 2500 kVA service transformer on the premises.

The utility rep informs me the maximum available short circuit current at the primary ‘bushings’ (terminals, I suppose) of the transformer is 3000.61 amps.

He also tells me the “impedance to the source”, which he later more specifically defined to me as, “the impedance seen from the transformer primary bushings back through (the utility’s ) line to the source.” is :

R= 0.08 and X= 0.22

from which I calculate --> Zsource = sqrt ( R2 + X2 ) = 0.234 Ω

The problem I seem to encounter thus arises :

Ib = VL-N / Z --> Ib = 12kV / 0.234 Ω = 51,282 amperes

Which is a far greater magnitude than the 3000.61 amperes given as the maximum available fault current. While I am not directly concerned with any fault current on the primary side of the transformer, my concern with this discrepancy is simply that it leaves me with uncertainty about the true system impedance.

The only explanation I could think of was that the lesser value of 3000.61 amps may represent the effect of the tap fuses if they are current limiting. However, the manufacture’s time-current data table for those specific fuse links, which are S&C, T-100, shows up to 10,000 amps for 0.02 seconds – greater than one cycle. The same data table shows these fuses clearing 3000 amps in approximately 0.1 second.

Could anyone here more familiar with information from utilities either confirm my conclusion that the utility information is erroneous, or else clarify to me anything I may be unaware of or mistaken in ?
Any additional suggestions are also welcome.


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PostPosted: Tue May 24, 2011 12:52 pm 
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Something certainly seems amiss. I looked at this from a few directions.

OHMS
Using 3000.61 amps at 20.8 kV would give a source impedance in Ohms (at 20.8 kV) of:

(Kv^2)/ MVA
MVA = [3000.61 x 20.8 x sqrt(3)] / 1000 = 108 MVA
20.8^2 / 108 = 4 Ohms.

PER UNIT
Calculating in per unit assuming a 100 MVA base:

Zbase = kV^2 / MVA = 20.8^2 / 100 = 4.3264 Ohms
Zp.u. = Zactual / Zbase = 4 / 4.3264 = 0.925 p.u.

TRANSFORMER MVA
Using the transformer MVA as the base:
%Z = (MVAtransformer / MVAsource) x 100 = (2.5 / 108) x 100 = 2.315% (0.02315 p.u.)
(close in digits but off by factor of 10)

None of these even come close. I think you are on solid ground to go back and question the information. What is the secondary voltage?

We would be interested in hearing what you find out.

Good Luck!

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PostPosted: Thu May 26, 2011 3:29 pm 

Joined: Tue Apr 12, 2011 6:11 am
Posts: 17
Location: Oregon, USA
Thank-you Jim for this helpful response.

[ And for this excellent and very helpful Arc Flash Forum, as well ! ]

The secondary voltage on the transformer is 480V. A pretty good step-down factor, 43.3 : 1.

I suspect data contradictions like this provided by utility company representatives may not be an entirely rare occurrence out there, and therefore may be a potential pit fall to be on guard for. This would be especially so when one has the impression the particular utility representative one deals with may be either not very conversant with, or does not fully appreciate, the extra particulars of an arc flash study as compared with the simpler requirements of a ‘worst case’ short circuit analysis alone. The latter of which, I would suppose, they are occupationally likely to be far more familiar with.

I am sure others beside myself either have or will encounter problems similar to this. I will gladly post back on how my experience goes from here.


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PostPosted: Fri May 27, 2011 9:23 am 
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Thanks for your support! As I am sure you realized, there are many people with a lot of good experience on the forum and everyone seems quite willing to help out.

Best Wishes and let us now how it turns out.

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PostPosted: Thu Jun 23, 2011 10:58 pm 

Joined: Tue Apr 12, 2011 6:11 am
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Location: Oregon, USA
I’ve been away on vacation form work, computers, and arc flash studies for the past 2 weeks or so. Now that I am back, I am picking up again where I left off, including follow-up on this thread.

I have re-contacted the electrical utility engineer, and explained the apparent problem in his data. He listened to my explanation, and then said he would get back to me. The next day I received an email from him containing only this :

“Here is the positive sequence driving point Impedance values.
R: 0.452218
X: 3.97354 “



COMPARISON:
The former data for the electrical utility source system, initially referred to as “Impedance to Source in ohms”, and defined as: “the impedance seen from the transformer primary bushings back through the utility line to the source”, was given :

R= 0.08 and X= 0.22, --> Z = sqrt ( R2 + X2 ) = 0.234 Ω


The new data is now referred to as the “positive sequence driving point Impedance”, and is given as :

R = 0.452218 and X = 3.97354,  Z = sqrt ( R2 + X2 ) = 3.999 Ω


Obviously, these are completely different values.

Putting the new value into the bolted fault current equation :

 Ib = VL-N / Z --> Ib = 12kV / 3.999 Ω = 3000.75 amperes

This now squares closely with the given maximum short circuit current at the service transformer primary terminals of 3000.61 amperes.

One suspicious aspect of the initial impedance values was that the X/R ratio seemed very low : 0.22 / 0.08 = 2.75.
However, looking more closely, did that not also seem a rather ‘crisp’ number, being exactly 2.75 ?

Now, the X/R ratio of the new data is : 3.97354 / 0.452218 = 8.78677.

At least this looks more believable. Perhaps I can take hope this new data value is a more scrupulous reflection of the system parameters.


EVALUATION :
The utility engineer made no explanation as to the error in the initial data. I am somewhat dismayed as to that, because the accuracy of the service data must affect the accuracy of the entire study of the downstream power system. The new impedance value given is radically different from the initial value, so I am not sure how such an error would be accounted for. Furthermore, I’m not sure what to make of the change in terminology for the impedance data. From the initial “impedance to source in ohms”, to what is now called the “positive sequence driving point impedance”.

SUMMARY :
While I appreciate the utility engineer’s cooperation in providing me with this ‘correction’, I still feel some discontent as to why the error occurred in the first place. I am left with a definite uncertainty as to the utility company’s competence toward realizing the greater data integrity needs for the purposes of an arc flash study over the less exacting needs of a ‘worst-case’ short circuit analysis alone, with which they are probably much more familiar. This may only stand to reason, considering that OSHA and NFPA 70E have no direct applicability to electrical utilities. Therefore, the utility companies and engineers may be slow in picking up on the additional nuances of arc flash studies, and on their responsibilities in this process.


I truly appreciate the many invaluable discussions and helps I have found here on the Arc Flash Forum.
I also welcome any additional observations or comments from anyone on the issue of this thread.

Thank-you.


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PostPosted: Fri Jun 24, 2011 2:47 pm 
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Nice to hear the utility engineer is cooperative. I am sure it does leave you a bit puzzled about what happendc but I'm glad to hear you were able to straighten it out.

Stop by if you have any other questions. A good group of people visit the arc flash forum and are always willing to kick in their 2 cents.

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