# question on 3-phase fault analysis



## JayD (Mar 7, 2020)

PU method and MVA method should yield same result?

Please look at the question

A 3-phase fault occurs at a location 1. This point is located downstream from a transformer and
a conductor. The conductor (3-ph/480V) has an impedance of Z = 2.0 ohms/phase and the
transformer has the following properties. 3-phase, 60 Hz, 13.2kV/480V, 10 MVA transformer
with Z = 8%. What is the short circuit current at the fault given only the information above?


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## Dude99 (Mar 7, 2020)

I get the same using

PU or Imped or MVA


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## Orchid PE (Mar 9, 2020)

JayD said:


> PU method and MVA method should yield same result?
> 
> Please look at the question
> 
> ...


Want to show us your work?



Spoiler


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## Dude99 (Mar 9, 2020)

By inspection you can calc the fault

since Z line &gt;&gt;&gt; Z xfmr

277/2 or 480/(sqrt3 x 2) = 138


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## Dude99 (Mar 9, 2020)

Here's mine


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## JayD (Mar 10, 2020)

How can the contribution of an element can be calculated in a fault?

For e.g. what is the contribution of G2 generator?

For a circuit below


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## JayD (Mar 10, 2020)

Dude99 said:


> Here's mine
> 
> View attachment 16674


how come Zbase comes 0.023?

Zbase= Vbase/Ibase=480/12028=.0399


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## JayD (Mar 10, 2020)

Chattaneer PE said:


> Want to show us your work?
> 
> 
> 
> ...


Why can't use 10MVA as MVA base and 480V as Vbase?


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## Orchid PE (Mar 11, 2020)

JayD said:


> Why can't use 10MVA as MVA base and 480V as Vbase?


It can be 10 MVA. I just happened to use 100, because I guess I added an extra zero when I wrote down lol. I use kV for my voltage base in the calculations, so 480V = 0.48kV, since the standard formula for Z_base = kV_base^2 / MVA_base. That makes 0.48^2 / 10 MVA = 0.023 (or 0.0023 on a 100 MVA base). Using 100 MVA as the base, I should have used change of base on the transformer impedance, Z_xfmr = 0.08*(100/10) = 0.8. But, since this number is still so small, it doesn't change the answer.

Corrected PU Method:



Spoiler


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## Orchid PE (Mar 11, 2020)

JayD said:


> How can the contribution of an element can be calculated in a fault?
> 
> For e.g. what is the contribution of G2 generator?


My preference would be to use the MVA method.

I'd first calculate the short-circuit MVA of the generator (MVA / Z%). Then, then calculate the short-circuit MVA of the transformer, and then the line. And if we assume the fault happens somewhere on the bus, I'd just add the short-circuit MVA of the three components I just mentioned, based on adding them in series per the MVA method, and then use the voltage at the bus to calculate the fault current.

Here's an example (no transmission line impedances included):



Spoiler



I put in some example values for the generator and transformer. I'm assuming the transformers are step ups for the generators, and I made them 13.8kV to 115kV.

If you just wanted the _available_ fault current coming from the generator, you would just do: I_G2 = 10 MVA / (sqrt(3) * 13.8 kV * 0.08) = 5230 A. The fault current I calculated below is lower than this number because A) it is on the high voltage side of the transformer, so the current should be less, and B) the transformer limits the amount of current that can be fed to the fault.


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## Dude99 (Mar 11, 2020)

JayD said:


> how come Zbase comes 0.023?
> 
> Zbase= Vbase/Ibase=480/12028=.0399


Zb = V^2 / S (single phase) x sqrt3 = 0.0399

or277/12028 = 0.023


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## smsm_280 (Mar 11, 2020)

Z base should be the same if u used 3phase equation or single phase equation it is 0.023. Zb=V^2 / S (single phase) x sqrt3=270*270(V^2 )/3333333 S (single phase)=0.023


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## JayD (Mar 12, 2020)

Chattaneer PE said:


> It can be 10 MVA. I just happened to use 100, because I guess I added an extra zero when I wrote down lol. I use kV for my voltage base in the calculations, so 480V = 0.48kV, since the standard formula for Z_base = kV_base^2 / MVA_base. That makes 0.48^2 / 10 MVA = 0.023 (or 0.0023 on a 100 MVA base). Using 100 MVA as the base, I should have used change of base on the transformer impedance, Z_xfmr = 0.08*(100/10) = 0.8. But, since this number is still so small, it doesn't change the answer.
> 
> Corrected PU Method:
> 
> ...


But why can't use  Zbase=Vbase/Ibase

why its different from standard formula calculation value?


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## JayD (Mar 12, 2020)

Chattaneer PE said:


> My preference would be to use the MVA method.
> 
> I'd first calculate the short-circuit MVA of the generator (MVA / Z%). Then, then calculate the short-circuit MVA of the transformer, and then the line. And if we assume the fault happens somewhere on the bus, I'd just add the short-circuit MVA of the three components I just mentioned, based on adding them in series per the MVA method, and then use the voltage at the bus to calculate the fault current.
> 
> ...


Thank u so much.

Bt I am sill confused 1.  when in the question they asked you calculate the  fault current contribution by G2 in the fault at the Bus Should we calculate fault current from G2 only or fault current from G2 and transformer both combined? Like you did, you calculate current in both ways.

2. If they ask in the question to calculate MVA fault contribution by G2 for fault at the bus , is it should be

MVA(G2)/Z%=10/0.08=125 MVA?

3. And when they ask you to calculate fault contribution by G2 , we need not to calculate the fault current for the whole system?


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## Orchid PE (Mar 12, 2020)

JayD said:


> But why can't use  Zbase=Vbase/Ibase
> 
> why its different from standard formula calculation value?


In short, you can use Z_base = V_base / I_base, *but only for a single-phase system*.

For a three phase system, we have to use:




Because V_base is based on the L-L value, and I_base is per phase.


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## Orchid PE (Mar 12, 2020)

JayD said:


> Thank u so much.
> 
> Bt I am sill confused 1.  when in the question they asked you calculate the  fault current contribution by G2 in the fault at the Bus Should we calculate fault current from G2 only or fault current from G2 and transformer both combined? Like you did, you calculate current in both ways.
> 
> ...


To answer all three questions:

It depends on the wording of the question.

If the question asks something like "Calculate the fault current contribution of G2 for a fault at the Bus" or "Calculate the available fault current of G2 for a fault at the Bus," use the generator, the transformer, and any line impedances that are given.

If the question asks something like "Calculate the maximum fault current available from G2," use only MVA/Z% (notice I didn't mention anything about a fault at a bus). Most questions I've come across do to ask for this. They usually ask for the contribution at a specific bus.

But always read the question thoroughly to know exactly what you're being asked to calculate.


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