Calculating Per Unit Fault Impedance: Finding Errors in Given Relationships

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Discussion Overview

The discussion revolves around the relationships and equations used to calculate per unit fault impedance in electrical engineering. Participants are examining the validity of given equations and their derivations, particularly focusing on the equation for Xohms and its role in the overall calculations.

Discussion Character

  • Homework-related
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion about the relationships provided and suggests that one of them may be incorrect, seeking confirmation from others.
  • Another participant questions the validity of the equation "Xohms = [ V / sqrt(3) ] * Ifault," asserting that resistance should be defined as voltage divided by current.
  • A participant claims to have solved part of the derivation but struggles to see how the equation for Xohms fits into the derivation of another equation, indicating potential issues with the given relationships.
  • There is a suggestion that the equation for Xohms may contain a typo, proposing an alternative formulation that changes the relationship between voltage and current.
  • Further clarification is provided regarding the proposed typo, reiterating the alternative formulation for Xohms and suggesting another equivalent expression.

Areas of Agreement / Disagreement

Participants do not appear to reach consensus on the correctness of the given equations, with multiple views on the validity of the equation for Xohms and its implications for the derivation process.

Contextual Notes

Participants express uncertainty regarding the relationships and their derivations, particularly the equation for Xohms, which may depend on specific assumptions or definitions not fully articulated in the discussion.

acw260
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Homework Statement


Given the relationships:

Xpu = MVAbase / MVAfault

Xpu = ( MVAbase / MVArated ) * ( X% / 100 )

MVAfault = sqrt(3) * V * Ifault

Xohms = [ V / sqrt(3) ] * Ifault

Homework Equations



Show that:

Xpu = [ MVAbase / V^2 ] * Xohms

The Attempt at a Solution



No matter how I approach this problem I keep finding that one of the given relationships must be incorrect. Could someone with better algebra skills then me please take a look and confirm whether (a) there is an error in the set problem or (b) I'm just being dense?

Thanks
 
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I have no idea of what you are trying to ask - but "Xohms = [ V / sqrt(3) ] * Ifault" seems very wrong. Resistance (ohms) is always Voltage divided by current.
 
Please see the attachment for the problem in its original format, which should make things clearer.

I have solved the derivation of equation 5(a).

I can solve the derivation of equation 5(b) no problem using "V=IX" (ignoring the resistance of the cables for the purposes of the fault calculation, as instructed in the problem).

What I can't see is how the given equation "Xohms = [ V / sqrt(3) ] * Ifault" fits into the derivation of equation 5(b). This equation seems wrong.
 

Attachments

acw260 said:
What I can't see is how the given equation "Xohms = [ V / sqrt(3) ] * Ifault" fits into the derivation of equation 5(b). This equation seems wrong.
A typo. It should be "Xohms = V /[sqrt(3) * Ifault]".
 
acw260 said:
Xohms = [ V / sqrt(3) ] * Ifault

Svein said:
A typo. It should be "Xohms = V /[sqrt(3) * Ifault]".

Or, equivalently, Xohms = [ V / sqrt(3) ] / Ifault
 

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