NCEES #108

[mhelms_2]

We are given VAB as 13.2 kV for a 3 phase, 4 wire, wye connected utility line. Two loads are placed between line A to N and B to N. So, we are going to need to convert the line voltage VAB to phase voltages. I understand the sqrt 3, but why would VAN be at an angle of -30 degrees, and VBN be at -150?

 

[knight1fox3]

Try this thread mhelms_2 and see if that helps.

 

[mhelms_2]

I understand the use or non use of a negative, but I'm wondering why the angle didn't shift 30 degrees going from VAB to VAN

 

[gte636i]

I don't have the sample exam book in front of me.

But, you get to pick one parameter as your reference (0 degree angle), it looks like they used VAB as their reference in which case it would be at an angle of 0. VLL leads VLN in the positive sequence by 30 degrees, so when they converted VAB to VAN, VAN is lagging by 30 degrees or in other words is located at -30 in this case, since this is positive sequence (assumed if it doesn't state otherwise) VBN would be 120 degrees in the clockwise position from VAN.

 

[Wildsoldier PE]

Make sure you understand this...try an electrical ckt book.. But V line to line and V line to neutral lags or leads by 30 degrees..since i stop studying a long time ago i dont remember if it lags or leads. I think this is explined on Nilsson electrical circuit book

(null)

 

[mhelms_2]

I see I misquoted in my first statement. My question should have read "why WOULDN'T it be at angle of -30 degrees. The answer in the back of the book shows Van to be 7.62 at angle 0. After converting from line to phase, I thought it should have been 7/62 at angle -30.

 

[cconnawa]

I see I misquoted in my first statement. My question should have read "why WOULDN'T it be at angle of -30 degrees. The answer in the back of the book shows Van to be 7.62 at angle 0. After converting from line to phase, I thought it should have been 7/62 at angle -30.

The magnitude is the same no matter what is the initial reference angle, so long as Phase A is 120 degrees in front of Phase B. It doesn't matter if Phase A is 0, -30, or -10, for that matter... as long as Phase B is 120 degrees behind it, i.e., balanced. Do the math and try it. Plug in different #'s for the reference Phase A angle, and see that you'll get 29.3 everytime...