I realize that each of these problems is extremely fundamental (as are all of the sample problems when one recognizes which one or two basic concepts is in play) and I apologize for not recognizing the difference here. NCEES sample problem #132 is identical to EESE (Camera) power #69. NCEES #513 uses L-L voltage in the solution (12kV) while NCEES #132 and EESE #69 each uses a L-N voltage (60/sqrt(3)=34.6kV and 13.2kV/sqrt(3)=7.62kV, respectively). In response to this issue, Flyer wrote “The standard practice is to do the math on a phase voltage basis. The same result can be determined using line voltage but you wind up adjusting the current by a factor of sqrt(3) rather than the voltage. It still works out the same but the math isn't nearly as simple”. NCEES #132 and EESE #69 don’t mention whether the respective system connections are wye or delta, therefore to use phase voltages scaled by the sqrt(3) one needs to assume wye connections for each system. Assuming a given system connection without good reason seems counter to good engineering judgment. These problems should be automatic, almost trivial. Please advise regarding any suggestions and if I’m completely missing some very basic concept in play here.
NCEES #513/#132
- Thread starter Bluekayak
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Flyer_PE
Jr. PE / Sr. Company Pilot
Take a look at NCEES problem #111. The system is delta, yet they determine a phase voltage (Van) by dividing the line voltage by the square root of 3. They determine the voltage drop across the cable on a single phase equivalent basis. They then convert the voltage arrived at back to a line value.
You can get the same result on a line-line basis but there is a sqrt3 factor that has to be accounted for. I did the math that way once, it's not a lot of fun.
When faced with a black box 3-phase source, it really doesn't matter if it is actually delta or wye. The math works out the same.
You can get the same result on a line-line basis but there is a sqrt3 factor that has to be accounted for. I did the math that way once, it's not a lot of fun.
When faced with a black box 3-phase source, it really doesn't matter if it is actually delta or wye. The math works out the same.
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So regardless of the system connection, the impedance is always in reference to a phase value, correct? This takes us back to the basics of per unit analysis and logically follows that the base impedance can be calculated as either
1) Z(base) = (Base kV L-N) / (Base kA)
2) Z(base) = (Base kV L-L)^2 / (base MVA)
