ElecPwrPEOct11
Well-known member
Hi all. I'm having a little trouble with #111 and trying to figure out if I'm approaching the problem wrong or what. It's a fairly basic balanced 3-phase delta load with identical source to line impedances on all phases. We're given a load line voltage (VAB), the impedance on a line, and the current on line A to the corner of the delta (IaA). You're asked the magnitude of the source line voltage Vab.
Solving this problem I used basic KVL around the loop. Vab = VaA + VAB + VBb = IaA * Z + VAB + IbB * Z. You can find all of these values from the given #s and I was careful to keep the two currents 120 degrees out of phase from each other. After plugging & chugging I get Vab = 13,252 @ -0.97 degrees. This is close to the book answer of 12.95kV, but seems off further than rounding would cause. The NCEES answer solves the problem quite differently by finding Van, even though there is no neutral conductor going to the load. While I think you can technically do this it seems less intuitive. The 1st line of the NCEES solution also gives the source voltage @ -30 degrees. Can anyone explain where this angle came from?
Ultimately I'm trying to figure out if I did something fundamentally wrong and am solving this the wrong way. Did I get this right by luck or am doing it right? KVL makes more sense to me than inserting a neutral conductor. Any help/explanation/guidance is very appreciated. Thanks!
Solving this problem I used basic KVL around the loop. Vab = VaA + VAB + VBb = IaA * Z + VAB + IbB * Z. You can find all of these values from the given #s and I was careful to keep the two currents 120 degrees out of phase from each other. After plugging & chugging I get Vab = 13,252 @ -0.97 degrees. This is close to the book answer of 12.95kV, but seems off further than rounding would cause. The NCEES answer solves the problem quite differently by finding Van, even though there is no neutral conductor going to the load. While I think you can technically do this it seems less intuitive. The 1st line of the NCEES solution also gives the source voltage @ -30 degrees. Can anyone explain where this angle came from?
Ultimately I'm trying to figure out if I did something fundamentally wrong and am solving this the wrong way. Did I get this right by luck or am doing it right? KVL makes more sense to me than inserting a neutral conductor. Any help/explanation/guidance is very appreciated. Thanks!
