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Dont know why this hasn't come to me till now, maybe a cold beer on a Friday evening does the trick, but looking from the output back makes sense.  To evaluate any equipment, you need to know how it responds to a certain condition.  The only way to do this is to expose it to said condition and...

Just looked it up.... your way seems to be the consensus.  

I see what you're saying, and I could be wrong (been a while since I've looked at this), but, to me, it is not intuitive if solved in that way.  It makes sense, and I could see why, but it still ain't intuitive.   The answers are very close to the same, but it's not the same.  

And the output is the rated input minus losses, not added.  So, it's out/in with in at 25... you can see the rest.

I figured you were rattling my cage, just keeping the discussion going is all.

I understand that, but he's referring to shaft power.  A XFMR doesn't have a shaft  :thumbs: ... So no need to take that into account.  Only electrical power at this point.

We're working with a XFMR here, not a motor.

You'd be right.  I noticed that error as well, that he used a higher "input" than what was actually there.  

Don't really have the energy at the moment to comb through "what" they did, but kW is NOT demand!!!  The only time "demand" is equal to kW is at unity.  This problem is not dealing with unity, so I'll have to come back to this later. Furthermore, highly unlikely you'd see a scenario this...

Unless otherwise told, you always hold kW constant when performing PF correction.  Did you mean KVA or KVAR, because each changes with kW held constant?  Likewise, all quantities change if kW is changed and you're adjusting PF with added reactance. 

Please post the problem @cos90

I don't have that material, but a description or simply posting an img would be good.  From there, I can try to help you in understanding ratings and analyses methods.   I have always worked with KVAR, kW and KVA from the power triangle, unless other methods are necessary to solve the problem...

You're not necessarily taking bases "across" a XFMR, per se. Think of it more along the lines of a zone.  A generator zone up to a XFMR, say.  From the secondary "base" of the XFMR, traveling down the transmission line to the next XFMR... Secondary base of this XFMR down line to the load...

@rg1 That is the only approach I've ever used when working with pu.  Not that you're approach is "wrong," but it is convoluted and more difficult than the pu approach.  You can do everything necessary with a given power base and a given voltage base.  From this, move from the generator down...

@rg1 No, that is not what's being asked.  Per unit analysis does not involve reflecting any impedances through a XFMR, regardless of side.  You practically develop an equivalent circuit from a system power base and voltage base for each respective zone.  You may be overthinking this.  Moving...

You don't reflect across a XFMR when completing this analysis.  It is taken care of in the system power base and appropriate voltage base to that section of the one line.  Also, %Z of a XFMR is the same looking from either side, so no worry there. As rg1 said, please post the rest of the...

PU analysis can look like a sleight of hand, voodoo black magic show... But... Z=Vline^2/Sbase is correct But why?  It's pretty intuitive once you see it.  

That is true, but most commonly, you're given power and voltage when doing pu analysis.  And Vline is the "base" that you use... That's my question for the board, why?  I'm askinng this since it sort of relates to the original post.

A scenario to consider: Why do you use Vline^2/Sbase when calculating base impedance in per unit, 3-phase analysis?  Why not phase voltage, if practical and other necessary conversions are performed?

Q=V^2/X=wCV^2 but You're looking for C, which yields: C=Q/(wV^2) From the parameters given, and the adjustments @rg1 recommended, this is a rather easy problem.