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PF angle = Cos^-1(.92)=23 Cos^-1(.67)=47.93 Cos^-1(.8)=36.86 Cos^-1(1)=0 P1 angle= Cos(PF angle - 30) and P1 angle= Cos(PF angle + 30) Plug in all the angles and see which one gives you the answer that you're looking for.

You can't parallel a delta and a wye transformer. The 30 degree phase shift will cause the magic smoke to come out. It's best if both are exactly the same. You can parallel a delta wye to another delta wye though as long as they are the same. They can have different MVA and impedance but...

Under most circumstances Z1 operates instantaneously. If it is delayed you would need to know why it's being delayed. If it's coordinating with another relay or element then that information would need to be provided. 100%-15%=85% of line impedance = (0.217+j0.637)(.85) = 0.184+j0.539

akyip you used P-N voltages for the transformers that are connected P-P.

It's given in the problem "transformer with 4% impedence" For transformers the PU impendence quantity is always given for the rated voltage and MVA. If you're operating the transformer at a different voltage then you have to change the impendence but if you're operating it at the rated voltage...

Your KV pu is not correct. 12.47 System - Vbase=12.47 V=1 pu, MVAbase=1MVA MVA= 40 pu Z= (1)^2/40=.025 pu 12.47 side Transformer = Vbase= 12.47V V=1pu, MVAbase=1MVA MVA=1pu, Z= .04pu 480 side Transformer = Vbase=480V V=1pu, MVAbase=1MVA MVA = 1pu, Z= .04pu I=V/Z Z=Ztransformer + Z system =...

From what I've seen if it's over 1 the equipment is put back in service assuming there hasn't been a decrease over time. (if historical records are properly maintained and checked) There a lot of different policies on this though depending on where you work. I've never heard anyone debate over...

This looks like a max power transfer theorem question to me with no calculation required.