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1. Junior Member
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Originally Posted by Ingenieur
not the accepted way, in 30+ yrs can't remember ever seeing it done

line reactors are used, cheaper and can do better
Right I agree there may be a cheaper way, but this may be just one of the pros used to install it. I'm not saying to do it financially, but just asking about the theory and calculations.

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Originally Posted by hello21
Right I agree there may be a cheaper way, but this may be just one of the pros used to install it. I'm not saying to do it financially, but just asking about the theory and calculations.
it isn't a reason to install it, it's a consequence, just like a long wire run
reactors are cheaper and more effective
i limiting fuses
proper protection sizing
many better methods

I would not focus on this
you need basics based on your question as to whether sec faults affect the prim

but yes, ANY Z in the fault path reduces its magnitude

3. gar
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hello21:

Your post #1 seemed to imply that you did not understand that any secondary current was reflected to the primary.

In post #7 you now introduce the question of whether an isolation transformer, as compared to no transformer, limits fault current. Others answered this for you. At this point I do not know whether you understand that secondary current reflects to the primary.

I question whether you have a good background in electrical circuit theory. You need to study basic electrical circuit theory, and then try to do an analysis by yourself. If you did that I don't think you would have needed to start this thread. Start from basics.

In post #9 you seem to understand that a transformer vs no transformer can reduce fault current. This results from the leakage inductance of the transformer providing series impedance in the circuit vs no transformer. The greater the transformer leakage inductance the greater is the series impedance and thus lower fault current.

The following is a useful discussion on a transformer equivalent circuit, was simply the first one Google brought up.
https://www.electricaleasy.com/2014/...ansformer.html

Don't rely on hearsay information. Possibly use it to go study the subject. It may be valid, or totally incorrect.

Just because you can plug values in an equation and get some answer does not mean the result is of any value.

.

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5. gar
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180712-0644 EDT

hello21:

You should not be sorry for asking.

What I am seeing is that you are picking up smatterings of information from various sources, quite possibly from persons without a good understanding of basics, and that you have not been taught, or learned on your own, basic electrical circuit theory.

Two hundred years ago little was understood about electricity. Gradually thru the 1800s various experimenters and theoreticians developed concepts on the basic operation of electrical devices, and circuits.

Ohm studied the relationship of voltage, current, and conductivity. The results later became known as Ohm's law. But Ohm did not study power, and therefore the power law is not part of Ohm's law. Joule is the person that studied power and its relation to electricity. Ohm invented a way to produce a continuously variable voltage for his experiments. This was long before we had any electronic or mechanical means to do this.

In developing an efficient DC generator (dynamo) Edison discovered magnetic saturation of an iron core.

Edison while working on the development of his incandescent light bulb discovered electron flow, but did not know it. Circa 1880. See http://ethw.org/Edison_Effect . The electron was not discovered until 1897 by J. J. Thomson. See https://www.nobelprize.org/education...eriment-1.html .

The early study of transformers is discussed at https://en.wikipedia.org/wiki/Transformer .

I don't know how your original post could have been presented in some different way that would have more clearly defined your question. Often times on any problem the major problem is to figure out what are the correct questions to ask.

Don't hesitate to come back and ask more questions.

.

.
Last edited by gar; 07-12-18 at 09:11 AM.

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Originally Posted by hello21

no need
but do not get upset when people disagree with your assertions, this place has literally 1000's of years of experience...for free
learn from it

research 'current limiting reactors' to see their advantages over an iso xfmr

7. Originally Posted by hello21
Don't be sorry. You are actually right. Transformers are certainly used to limit fault current- and in terms of blocking zero sequence current (delta-wye), they are the best option. Even a transformer with a low impedance will limit the fault current if its kva is small relative to the source supplying it.

FWIW in data centers there has been talk of going to 240/416Y instead of stepping 480 down to 120/208 at various locations, but the issue of increased fault current has made it a challenge.

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Originally Posted by mbrooke
Don't be sorry. You are actually right. Transformers are certainly used to limit fault current- and in terms of blocking zero sequence current (delta-wye), they are the best option. Even a transformer with a low impedance will limit the fault current if its kva is small relative to the source supplying it.

FWIW in data centers there has been talk of going to 240/416Y instead of stepping 480 down to 120/208 at various locations, but the issue of increased fault current has made it a challenge.
transformers are not used to limit fault current
transformers are used to transform voltage/current levels, or for electrical isolation
ANY Z will limit fault current

on a delta:wye it doesn't 'block' 0 seq I, a gf can't exist in an ungrounded delta
but its phase current will rise on the grounded wye leg, although it will remain pos seq balanced (ie, a current will circulate in the delta)
Last edited by Ingenieur; 07-12-18 at 11:51 AM.

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starting on 174 http://www.zmuda.ece.ufl.edu/Fall_20...Components.pdf
good explanation

page 193 shows yg:d

a ground fault on the y will cause a current to circulate in the delta, in essence, that is the 'ground fault current'

10. Originally Posted by Ingenieur
transformers are not used to limit fault current
transformers are used to transform voltage/current levels, or for electrical isolation
ANY Z will limit fault current

And a transformer is Z- which means it can limit fault current. A transformer is one more thing on the list that can do so.

on a delta:wye it doesn't 'block' 0 seq I, a gf can't exist in an ungrounded delta
but its phase current will rise on the grounded wye leg, although it will remain pos seq balanced (ie, a current will circulate in the delta)

Explain.

A line to ground fault on the secondary will not generate any ground or neutral current on the primary. As such zero sequence can not pass- draw the equivalent circuit out, there is no path.

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