CI Exam #2, Problem 17

[HopefulPE]

Going through these exams one last time, and was reintroduced to the one problem that didn't make much sense to me. 

The answer to the problem states " 3 phase cap banks, whether they are connected in series or parallel to each phase, their total kvar contributions are added". Is this true? I never could find any references on the banks themselves. This seems contrary to the fact that the kVAR contributions of a cap vary with the square of the voltage across the cap. 

Thanks!

 

[Troll]

it's simply conceptual. The total amount of energy is a sum.

if you work out the numbers using V and I you'll see that S is ALWAYS the same whether they are parallel or series.

 

[HopefulPE]

Guess I'm missing something. Say I have a single cap with -j1 impedance, and 1V line voltage. By itself I get |1| VAR, and let's say that's it's rating. In parallel with the same unit, I get |2| VARs. If I put it in series with itself I then only can supply 1/2 Var for pfc (voltage constant for same circuit).

I found this thread though:



... It looks like they couldn't figure it out either however. 

 

[rg1]

Going through these exams one last time, and was reintroduced to the one problem that didn't make much sense to me. 

The answer to the problem states " 3 phase cap banks, whether they are connected in series or parallel to each phase, their total kvar contributions are added". Is this true? I never could find any references on the banks themselves. This seems contrary to the fact that the kVAR contributions of a cap vary with the square of the voltage across the cap. 

Thanks!

The author seems to be talking about this particular problem meaning thereby you should not worry about the series parallel connection of the units and simply do the maths for each phase to get 3360KVAR in total for 3 phases @140 Kvar a unit. 

 

[TNPE]

All banks are generally connected in parallel to the grid (usually Y-GND).  I may be misunderstanding you, but in AC circuits, all values of impedance (reactance or vector sum of reactance and resistance) are combined exactly like resistors in DC circuits.  Recall that impedance and capacitance/inductance are not the same (i.e. XL=jwL and XC=1/(jwC)).  As for analyzing PFC problems, implementing the impedance triangle is the safest and most fail-proof approach, at least IMHO.  Not to sound discouraging, but these types of problems are the proverbial "softballs" and should be gimme points.  Don't overthink it.  The bank is connected in a 3-phase configuration, but the bank itself is in parallel with the grid.  Hope this helps!