Complex Imaginary Test 4 Problem 30

[BamaBino]

Any comments?

Agree with the solution?

 

[vdubEE]

Bama,

I think the key here is CI says the autotransformer is rated at 500MVA so you cannot use the two problems from the NCEES practice exam as a basis to solve this problem. I posted two pictures in another thread that I linked below that shows the relationship between Sh/Sx to the Sc (S1=V1*I1 by the autotransformer diagram). It is equation [4.59] below that is the basis for the equation they used to solve the problem. Since I was able to find the formulas they were using and follow the problem using them, I think this is one they didn't mess up.

At first, I was trying to follow the NCEES practice exam examples and was not getting anywhere near the answer shown. Think it all ties back to the autotransformer rating being given and not the standard rating of the single-phase transformer.

 

[Silkworm]

I disagree. I think CI screwed this question up. If the turns ratio of common:series is 5:2, that means N1/N2=7/5=1.4. This is NOT a 230kV to 200kV step down xfmr but rather a 230/164kV step down autotransformer. Once you recognize this, you can apply the same method as NCEES, which is quicker then going through the algebra as described VdubEE.

Could be a trick question to prove you know how to figure out the rating of the common coil with the turns ratio only.

 

[soma]

Silkworm,

Can you please post the question ? I am using new version of CI and the problem is not there !

Thanks,

 

[ecbahr]

The problem is now problem 41 in the newest CI test 4. I agree with Silkworm, I think the voltage ratios are messed up as well. I completely ignored them when doing the problem, but now that you mention it, it does not seem correct.