NCEES #111

[cruzy]

The load is a balanced delta, so when they do the single phase analysis to find the line-line source voltage, they divide the line-line load voltage of 12.5 kV by square root of 3. How do they know to do this, because you don't know if the source is delta or wye (or do you? it's not shown on the shcematic), and so I assume that the load line-line voltage equals the line-neutral voltage (to perform the single phase equivalent analysis)

 

[Flyer_PE]

The configuration of the load doesn't matter for this problem. They are dividing the line voltage by sqrt3 to generate a single-phase equivalent circuit. In the old NCEES sample questions, this one used to be problem 511. It's discussed here.

 

[cruzy]

I still don't understand why in a delta load you divide the V(L-L) by sqrt 3 when in delta V(L-L) = V(L-N).

Thanks,

Cruz

 

[Flyer_PE]

^Since it is a balanced load and you are given the line current, the configuration of the load doesn't make any difference for this problem. They are creating a single-phase equivalent circuit to simplify the math. If they aren't asking for current values for elements inside the delta, it's just a black box with three terminals on it. The analysis works out the same regardless of whether it is configured delta or wye.