NCEES Question 118 and 506

[Iheartyou]

I have some confusion on the ratio

On Q118, the delta - wye ratio did not take the sqrt 3 into the consideration on the ratio.

13.2/132

But on the Q506, delta-wye ratio, sqrt 3 is taken into consideration

why?

 

[Iheartyou]

any thought on it? I could be crazy

 

[jnhbp3]

I am wondering the exact same thing! Can someone please clarify this for us?

 

[jnhbp3]

I think I know why, in question #118 I guess you assume it is a 3-phase transformer and in question #506 it is a bank of 3-single-phase transformers and you are looking for the turns ratio of each individual single-phase transformer.

 

[Iheartyou]

if we using single phase then we won't need sqrt 3...right?

I think I know why, in question #118 I guess you assume it is a 3-phase transformer and in question #506 it is a bank of 3-single-phase transformers and you are looking for the turns ratio of each individual single-phase transformer.

 

[jnhbp3]

They give you line-line voltage on both sides of the transformer. A single-phase transformer usually has its high-side terminals connected to 2 phases and its low-side terminals connected to one phase and a neutral. So you have to use line-line voltage for the high-side and line-neutral voltage for the low-side to get the transformer's turns ratio.

 

[Swiftman25]

I got the correct answer to 118 but in a different way than the book.

This is how I looked at this problem: generator side as primary and line side as secondary. a = 13.2/(132/root(3)) = 13.2/76.21 = 0.17321.

Know equation: Is/Ip = a; therefore Ip=Is/a = 75.93/0.17321 = 438.38A. HOWEVER, this is amperage in the phases of the delta. LINE current into the delta is root(3) larger,

Therefore Igenerator=root(3)*438.38 = 759.3A. This is close to 760 or answer (A).