3 phase power factor correction -> Capacitance

[DK PE]

here is pic

 

[snerts50]

I thought that for correction for motor power factor the units are usually connected in delta (but they're always spec'd in kVAR anyway, not uF). For transmission, maybe they are not connected in delta as it would increase the withstand voltage required for the capacitor?

For example, look up the spec for an Eaton Cutler Hammer 1543PCMRA, it will say 3 X 58uF Δ at 480V 60 Hz and it is spec for 15kVAR correction.

I have seen motor PF correction connected delta.

I worked for a utility for a year and swore I saw it there but cannot remember...

 

[snerts50]

I believe the concensus here was that the formula you show in step 1 doesn't make any sense because we can't see how it is derived or where it came from. Let us know if you figure out how to derive it somehow.

Hopefully we don't need this for the test but thought I'd contribute with an example I made up.

Say you need a total of -1000 VAR on a 480V/3phase 60Hz system.

For delta:

1) 1000=Q = sqrt(3) * Vline^2 / XC, total

2) Xc= sqrt(3) * Vline^2 / 1000 = 399 across all 3 phases, or = 133 per phase.

3) Use Xc = 1/(w* C) to find the value of C.

4) C = 1.99 * 10^-5 Farads.

To find the capacitance for a wye system you'd just use a different voltage

5) 1000 = 3 * Vphase^2 / Xc,phase

6) Xc,phase = 130

7) Cphase= 1/(w * Xc) = 2.04 * 10^-5 Farads.

Does this look right to everyone? The capacitance will depend on the arrangement of the capacitors.

I think the formula in step 1 is valid.

What about, Q=Vc * Ic (basic Power equation, usable for every type of power)

and Ic=Vc/Xc

So, Q=Vc^2/Xc

Using the root(3) would depend on if you are using per phase values or line values.

 

[EEVA PE]

Not wishing to trash Camara (although I do have to say he LOVES his Wheatstone Bridges), but I believe there may be errors that have not been reported. For example, on exam 1 problem 38 the solution says that option B is true, which says "per unit impedances on T1 (a transformer) differ from primary to secondary side", but then in the solution it states (correctly) that p.u.impedance is the same on both sides. In this case he is conceptually right but the correct answer would be "B and D" since both are false."

I am by no means the ultimate power guru, but I did spend at least an hour yesterday drawing capacitor connections, converting from line-to-line to line-to-neutral, playing around with the commonly known power formulas, and I absolutely could not find a way to get that formula in problem 10. But either way, the NCEES sample exam shows that they tend to ask for VARs and not capacitance, so we should be OK.

What is wrong with getting wheatstone bridge problems on the exam, they are easy. I know my blood pressure will drop 5 pts on Friday if the 1st question on the exam is a wheatstone bridge problem. My blood pressure will probably go up 10 pts if it is an op amp problem.