PPI Exam 2 #108

The solution to this question notes that "Since the source is ungrounded, the phase-to-phase voltage is the line voltage".



Does this mean that if there was a neutral you can't assume the phase-to-phase voltage is the line voltage?  I have been treating phase-to-phase voltage as line voltage for my entire time studying and haven't seen an issue yet.

ME->EE said:

The solution to this question notes that "Since the source is ungrounded, the phase-to-phase voltage is the line voltage".



Does this mean that if there was a neutral you can't assume the phase-to-phase voltage is the line voltage?  I have been treating phase-to-phase voltage as line voltage for my entire time studying and haven't seen an issue yet.

Click to expand...

For a Wye 3 phase-3 Wire system, Phase-to-Phase Voltage is equal to the Line voltage.  But Phase-to-Phase Voltage is never equal to sqrt of 3 time Phase Voltage.

I wouldn't read too much into the answer key on that one. Just by inspection, the voltage across the delta load connected to phases A & C is the phase-to-phase voltage of 12.5kV.

ME->EE said:

Does this mean that if there was a neutral you can't assume the phase-to-phase voltage is the line voltage?

Click to expand...

Imagine if there was a neutral connected to ground, or point C on the load. Phase C on the source would be grounded on both sides, and that wouldn't work.

@BebeshKing so in general should I not assume that V phase = V Line /sqrt(3)? I should try to prove it each time?

Thank you both for the comments!

If I see a problem with Wye-3phase-3w system, I usually covert it to delta so that VL will be equal to Vphase. But for this problem, the configuration at the source does not matter weather it is a Wye or a Delta.