Transformer bolted Fault Current

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lile001

lile001

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I am used to getting utility transformer information in terms of available short circuit in amps, and transformer impedance in ohms. These are also the inputs to the calculations I know how to use.

A utility gave me a specification of 1000 KVA transformer and 13MVA available fault current. How does one convert this into a fault current in amps? :confused: Do I have enough information to accomplish this? Would this generally be at the secondary terminals of the transformer, so that the impedance of the transformer is already taken into account? Secondary voltage is 480 3 phase.
 
Re: Transformer bolted Fault Current

lile001
Utilities talk in MVA, because it is independent of voltage. They can view the low side fault current from either side of the transformer by dividing by either system voltage.

In your case 13,000kva/(480X1.732) = 15,637 amps 3 phase fault current. It would include the transformer imppedance, since it sounds like that is the low side value.
Jim T
 
Re: Transformer bolted Fault Current

Jim,

Would you please explain how impedance is accounted for in your formula. The formula I found for short current divides full load amps by transformer impedance. I'm not challenging your calculation...just want to understand.

Thanks!
 
Re: Transformer bolted Fault Current

I would take the 13.0 Mega Volt Amps and divide it by 480 and the square root of 3 to get phase to phase fault current at the secondary terminals of the transformer.

The simple equivalent circuit would be an infinite source of 480V in series with the transformer impedance. The short would be assumed to be bolted and of insignificant resistance by comparison to the transformer Z.

Divide 480V by the current from this first calculation and you will have Z.

[ January 10, 2005, 12:43 PM: Message edited by: al hildenbrand ]
 
Re: Transformer bolted Fault Current

This is the way I look at it. There are engineering design reference tables commonly available that are real time savers which include available fault current values for standard transformers. I guess you could do the calculations but if may be a better use of your time to use a table which provides these values for you.
A 1000kva has a typical 5.75%z. There are different available fault current values to select from, from 50,000-500,000kva as well as unlimited.
Since 15mva would be considered unlimited on my table, it shows an available fault current to be 20,900rms symmetrical amperes w/o motor contribution. Add 4,800a for motor contribution for a total of 25,700a.
Now, if the worst case scenario of 25,700a you then would consider if doing a study of and calculations for the actual mva, Z, and motor contribution would favorable to take the time to do in your system design, i.e. that it is important to be 25ka or less. As noted motor contributions can add a great deal to the available fault current. But if your available fault current is between 500ka and unlimited there is only meager 500a fault current difference between 500ka and unlimited.
Or, you could use the table as a reference to compare your computations with.
Just a thought.
Dave T
 
Re: Transformer bolted Fault Current

joec
When the utility calculated the 13MVA of fault current, they included impedance in their calculations. By telling us the MVA value they did a calculation that probably included their system impedance as well as the transformer impedance. I say that, because fault currents expressed in MVA are typical from a utility short circuit analysis program.
Jim T
 
Re: Transformer bolted Fault Current

They need to tell you where the 13MVA is calculated. If it is on the primary, then you need to add the impedance of the transformer in question to continue your calc further.
 
Re: Transformer bolted Fault Current

Templdl sez:

>This is the way I look at it. There are engineering design reference tables commonly available that are real time savers which include available fault current values for standard transformers. I guess you could do the calculations but if may be a better use of your time to use a table which provides these values for you.
>A 1000kva has a typical 5.75%z. There are different available fault current values to select from, from 50,000-500,000kva as well as unlimited.

Unfortunately, Templdl, those standard tables are less than useless. :( Utilities are moving to lower and lower impredenaces to improve efficiency. There are many manufacturers of transformers, and all have different standards. I have used those charts, come up with a nice safe looking number, then called the utility to find out their transformer has a quarter of the impedance of the ones in the standard charts! :eek: You really need to ask the utility engineer for transformer impedance, or simple get bolted fault current at the secondary of the transformer.
 
Re: Transformer bolted Fault Current

Thanks for the heads up lile001. My perspective has been limited to from the point of service entrance, transformers that are purchased by industrial customers and not those for utilities. If I'm to understand you correctly then, it is the norm today for industrial customers to opt for transformers with a lower Z like the utilities such as the subject 1000kva transformer. I just wasn't aware that the industrial transformer requirements had changed to be more in line with that of the Utilities which is important to note.
Again, my experience is based upon the available fault current as provided by the utility at the point of service entrance and the fault current that would be available of the secondary side of the common 1000kva transformer which had a 5.75%Z as a norm as would be purchased by the industrial customer.
Dave
 
Re: Transformer bolted Fault Current

lile001 and templdl
I agree with lile that many transformers are becoming more efficient, and in some cases impedance is dropping. Since losses are I**2 x R, efficiency is gained by reducing resistance.
As far as the 5.75% Z, that is an ANSI standard minimum impedance for transformers 501 kva and above. It doesn't apply to lower than 501 kva though.
Jim T
 
Re: Transformer bolted Fault Current

Jim, This is what I appreciate about these discussions, that is having the opportunity to learn, understand and gain an appreciation for the changes that are happening in the industry.
Then it appears as if the 5.75% is a minimum Z for ANSI the standard only allows it to be higher which leads one to conclude that the ANSI standard for impedance iis not relevant anymore.
Yes, as in motors, I tend to believe that transformers designed for higher efficiency and lower losses that is the trend in the industry results in a lower impedance which probably increases the inrush current needed energize the transformer.
This has been an issue for drytype distribution transformers for years going from 150 to 115 to 80degC, K-factors, etc.
Dave
 
Re: Transformer bolted Fault Current

templdl
I believe that the ANSI standard for large transformer impedances is relevant and applicable. It keeps fault currents lower. The impedance affects only a small part of the transformer losses, it is resistance that is the significant variable. You can lower resistance and keep impedance constant and greatly reduce losses.Over 20 years ago, loss evaluation was a part of the purchase equation in all of the top 100 utilities. The cost of losses might be higher today, but the push to evaluate them, at least in utilities, had been won in the 1980's. That is clearly not the same in most commercial and some industrial applications even now.
Jim T
 
Re: Transformer bolted Fault Current

That make sense Jim.
I have always understood that ANSI was a standard that it's intent was to limit fault current but that aspect appeared to have been changed if I read "lile001" reply correctly who said
those standard tables are less than useless. Utilities are moving to lower and lower impedance to improve efficiency
Click to expand...
which seemed to contradict what you are saying and what my understanding was. I was also aware that, as you said, transformer designers are able to reduce losses while still maintaining the impedance.
I agree that reducing losses an large transformers is important as the result is significant savings depending upon how the transformer is loaded which can also benefit smaller transformers depending upon how they are loaded.
But back to the 1000kva in question, then what you are saying is that the 5.75%z or greater would still apply?
From my perspective if one ends up with a boarder line KAIC requirement , such a 25.4ka by using an unlimited available fault current on the primary, where you would need to apply 30kaic devices, and you really want to be at 25ka or less you then could consider if is worth you time to identify what the actual available fault current from the utility is as well as fine tuning the impedance figure.
I would like to think that those who do this every day would be able to identify when they would benefit by doing calculations other than the need for documentation in order to justify the ratings and the devices chosen.
I'm a bit out of the main stream and do my best to keep abreast with the changes that are occurring in the industry. That's what I like about this forum. There is a good cross section of knowledge represented which presents many different perspectives.
Thanks,
Dave T
 
Re: Transformer bolted Fault Current

quoted from jtester:
The impedance affects only a small part of the transformer losses, it is resistance that is the significant variable. You can lower resistance and keep impedance constant and greatly reduce losses.
Click to expand...
jtester, can you explain to me how you can lower resistance and keep impedance constant?

Z = sqrt(R^2 +(XL - Xc)^2

You would have to increase the inductive reactance somehow and I guess that is what my question really is...how do you increase inductive reactance. And in the end, how does this help you?
 
Re: Transformer bolted Fault Current

In transformers, 501KVA and larger, winding resistance (R) is a smaller component of impedance (%Z) than is inductance (Xl). Winding capacitance (Xc)is so small it is usually not included in calculations.

Varying the resistance affects the effciency rating because it is the major source of losses (I^2R).

Transformer inductance can be changed by modifying the physical design (i.e. provide a greater air space between primary and secondary winding).
 
Re: Transformer bolted Fault Current

In addition to Jim's post, there are fundamentally different core designs. Some manufacturers build shell form designs where wire is wound around a core, and others build a core form, where a core is wrapped around coils.
In my experience, core form transformers seem to have more impedance, but equal losses, to the shell form units. I am not sure that is generally accurate, but was in my observations.
Then when you get to three phase units you have 3 legged designs, 5 legged designs, etc.
Jim T
 
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