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So if they had asked for the turns ratio of the three phase transformer as a whole, it would have been 13/138? (Just like NCEES #506 would have been 115/24?)

Sorry, I do not have access to a scanner.

So one of my books says that the turns ratio of a three phase transformer bank of a 13 kV delta - 138 kV wye transformer is 13/ (138/sqrt 3). Is that the single phase transformer ratio of one of the three transformer making up the bank, or is that the total three phase turns ratio?

So i'm throwing this out there, wanting to see if anyone has other strategies. My main reference material to go to will be: 1. My notes 2. Grainger's book 3. Chelapati book 4. Chapman's book 5. Handbook of electric power calculations (beaty) 6. Wildi's motors and drives 7...

If you assume a 1 MVA base (per the transformer rating), then: 1. Zpu of system is (12.47^2/40)/ (12.47^2/1) = 0.025 2. Ztotal = 0.025 + 0.04 = 0.065 3. 1pu volts/ 0.065 = 15.38 pu amps 4. Ibase = 1,000 kVA/ (sqrt 3 * 0.480) = 1202.8 amps 5. Isc = 15.38pu * 1202.8 amps = 18500 Amps

Never mind my above reply. I didn't think about the diameter being a function of a sqrt., which makes it not directly proportional. I coulda sworn that I keep reading in places where it says "...........is directly proportional by the square of (or square root)....." Maybe they are missusing...

I've put off trying to study any kind of electronics. I gave it a shot, but seems like none of my reference material is leading to any answers. So hopefully I guess right on these, because I stopped giving any kind of effort into comprehension of electronics a few weeks back.

Wow so this is pretty much just a V = IR question (seems like a lot of these problems are when you break em down to p.u....just comes to knowing your source pu values, and that's where the tricky part for me is, to know which one is the 1 p.u. item...sometimes seems like it's the source...

I guess you have to leave out the angle difference when multiplying by 400? When I include the 0.8 pf lag in the 400 amps and carry out the calcs, I get the 9V angle (-36.8), but then when I subtract from 480 I get 472.8. So do you leave the angle out of the VD calc method or is my calculator...

Still got one more day to study. Let's keep those questions coming, every little bit could end up helping (if you haven't noticed, I've been trying to post every question that comes to mind, no matter how fundamental :) )

Is anyone bringing any of the IEEE books with you to the exam? I have all of them in a pdf format so I'm not sure I want to go spend all this $$ to print them out, especially since mostly all of them are hundreds of pages...and that'd be a lot log lug around in addition to all the other books...

All those extra lines show is how they got the impedances on their selected bases, but they go right into I = I/Z after that, which doesn't answer my question yet

This is an easy points question that might trip me up on the test if I think about it too much. I got it right, but when I start thinking about it, I get confused. So they are asking for the relay tap setting, which in an 800:5 CT, that 5A is the tap setting, isn't it? Or is the questions...

Since when is dividing amps by impedance give you amps? The way I did it was to take 1pu (volts) / total Z to get the Ipu. Then multiply that by the Ibase. I just assumed a 1pu volts, I'm not sure that was even correct. But nonetheless, I still don't understand how they got real amps by...

Ya I did...I still come up with 472.7V, which is still closer to 475 than to 470. Anyone done it using the effective Z method and have step by step they want to share?

But when they convert the line-line voltage to a phase voltage, in delta those two have the same value. I took it as if the line current given already had the sqrt 3 built in, meaning the phase current was (70 angle -20)/sqrt3. Still not getting it. How can the 12.5 kV in that delta config be...

If all we are looking for is the turns ratio, then are these the basic rules: 1. For 3 phase transformers, use line to line ratios. 2. For 1 phase transformers, use line to neutral ratios. This question comes in reference to NCEES #506, where they use the line to neutral ratios. But in...

How can you use the effective Z to find the voltage drop and still get the same answer? it specifically says that it is an approximation of the answer, but when I do it (400 amps with angle corresponding to 0.8, times the effective Z), I get 473.xxx, which is closer to the 475 answer than the 471.

Why do they divide 12.5 by sqrt 3 for doing the single phase equivalent of Van? It is a delta load, where there isn't even a neutral to consider. I know I am wrong, just don't get why at all.

For NEC Table 9 for impedance values, why do they give the ohms/neutral? what does the neutral have to do with the impedance of a phase line? And what if it is a 3 wire system with no neutral? Also, to use the table for voltage drop calcs, what pf do you use to calculate the effective Z, is...