NCEES Questions 508, 509, 510

[DMB5mil]

After taking a few days off of studying, I can't recall where, if anywhere at all, I have gotten practice with three phase circuits where loads are not balanced, or are connected between wye and delta systems.

I know this sounds silly, but I don't recall touching on the types of questions from the NCEES test Nos. 508, 509, 510 in any of my books (EERM, Basic Circuit Analysis, Elements of Power Systems Analysis, etc...)

Anybody have a good reference for such problems, or know where good practice problems reside? I can work easily enough with three phase when it comes to balanced loads, power triangle work (finding the needed kVAR...) but these three questions have me stumped...

Thanks,

Brian

I suppose drawing out the circuits would help, ey?

 

[Flyer_PE]

There is some discussion of problem #508 here.

For question 509: The only part that matters about the utility being a wye configuration is that the neutral is ungrounded. That is what allows grounding of phase A at the load without a causing a fault condition. For either a delta or ungrounded wye, if one phase is grounded, the magnitude of voltages for the other two phases will be the line voltage.

For question 510: It will probably help to draw the circuit. Having a single-phase load connected between phases B and C will result in current flowing in phases B and C only. This one breaks down to a single phase problem where all you need to find is the current drawn by a 500 kVA load at 13.2 kV. The currents in phases B and C will both be equal to that value. Remember that since this load is connected phase-to-phase and not phase-to-ground, the line voltage is used directly.

I = S/V = 500kVA/13.2kV = 37.88 A (Answer C)

The only real reference I had for these types of power problems was a couple of college text books. They are:

Elements of Power System Analysis - Stevenson

Power System Analysis and Design - Glover/Sarma