Turns Ratio

Dammit I should have this down by now. But I was thrown by 138 turns ratio where they 28kv/sqrt3 to 115Kv whereas in 518 they dont. Looking at Grainger and another machines book I dont get why 138 was done that way, unless its the three single phase xfmrs, but its represent as 3 phase.

John

John,

I'll take a stab at this one. Hopefully it will boost my confidence if I am right. But I caution you to not take my explanation as correct until we have received the blessings of Luis, Jim, Benbo, etc.

For NCEES 138, look at the equation you're setting up:

V1=115kV

V2=24kV

V1/V2 = N1/N2 ==> V1*N2 = V2*N1

So, since the line and phase voltage is the same for delta, the V1*N2 term requires no sqrt3 and we'll do phase calcs instead of line calcs.

And, since V2 is wye, there will be a sqrt3 needed in the V2*N1 term to make sure it jives with the phase calcs on the V1*N2 term.

Phew, I know thats wordy. Hope I didn't confuse you.

Now, for NCEES 518, once again look at the equation you're setting up:

Vg=13.2kV

Vl=132kV

Il=75.93A

Vg/Vl = Il/Ig ==> Vg*Ig = Vl*Nl

So, it doesn't matter what's wye or delta configured because the V*I terms are corresponding to the same configurations when we cross multiplies the turns ratio. Hence, no sqrt3 needed.

chicago said:

John,
I'll take a stab at this one. Hopefully it will boost my confidence if I am right. But I caution you to not take my explanation as correct until we have received the blessings of Luis, Jim, Benbo, etc.

For NCEES 138, look at the equation you're setting up:

V1=115kV

V2=24kV

V1/V2 = N1/N2 ==> V1*N2 = V2*N1

So, since the line and phase voltage is the same for delta, the V1*N2 term requires no sqrt3 and we'll do phase calcs instead of line calcs.

And, since V2 is wye, there will be a sqrt3 needed in the V2*N1 term to make sure it jives with the phase calcs on the V1*N2 term.

Phew, I know thats wordy. Hope I didn't confuse you.

Now, for NCEES 518, once again look at the equation you're setting up:

Vg=13.2kV

Vl=132kV

Il=75.93A

Vg/Vl = Il/Ig ==> Vg*Ig = Vl*Nl

So, it doesn't matter what's wye or delta configured because the V*I terms are corresponding to the same configurations when we cross multiplies the turns ratio. Hence, no sqrt3 needed.

Click to expand...

I got it and Im sure thats it well done, I need to stick to the ratio relationships given in EERM eq 28.12. I keep wanting to use "a" and rearrange and I get burned. Youre going to do fine!

Thanks

You are doing great guys. The good work is and will pay off.

I'm sure it's right. Where power is concerned I leave it to you guys.

Sorry guys, I'm at a loss because I don't have the NCEES book. I never bought one because I borrowed it from the library at work. I just went to look into buying one and I see that a new version is coming out next month so I guess I'll wait.

jdd18vm said:

unless its the three single phase xfmrs, but its represent as 3 phase.

Click to expand...

Does one problem have single-phase transformers and the other have a three-phase transformer bank? If so that may be part of the confusion. In a three-phase bank, the delta/wye connections are internal to the bank, and are already accounted for in the voltage ratings (i.e. the sqrt(3) is built into the nameplate data on the bank). If you are hooking up three single-phase transformers you have to take the delta/wye (and sqrt(3)) connections into account yourself. In either case, if rated voltage is applied across one set of terminals of the transformer, it is going to produce rated voltage on the other set of terminals. In the case of a single-phase bank, you have to be careful to determine if line-line or line-neutral voltage is being applied on each set of terminals.

This is how I've always looked at transformers, but you may find it confusing. Chicago has a different approach (and Art and IFR had different approaches in the 518 thread). They are all correct, and since we all think in different ways you should pick whichever one makes the most sense to you.

mudpuppy said:

Sorry guys, I'm at a loss because I don't have the NCEES book. I never bought one because I borrowed it from the library at work. I just went to look into buying one and I see that a new version is coming out next month so I guess I'll wait.Does one problem have single-phase transformers and the other have a three-phase transformer bank? If so that may be part of the confusion. In a three-phase bank, the delta/wye connections are internal to the bank, and are already accounted for in the voltage ratings (i.e. the sqrt(3) is built into the nameplate data on the bank). If you are hooking up three single-phase transformers you have to take the delta/wye (and sqrt(3)) connections into account yourself. In either case, if rated voltage is applied across one set of terminals of the transformer, it is going to produce rated voltage on the other set of terminals. In the case of a single-phase bank, you have to be careful to determine if line-line or line-neutral voltage is being applied on each set of terminals.

This is how I've always looked at transformers, but you may find it confusing. Chicago has a different approach (and Art and IFR had different approaches in the 518 thread). They are all correct, and since we all think in different ways you should pick whichever one makes the most sense to you.

Click to expand...

Yes it clearly states 3 single phase, that has to be it, and made sense to me. So that is why the turns ratio in 518 is 10 132 kV L-L on the Delta 13.2kV on the Wye?

John