# Power Factor Correction

Discussion in 'Electrical Engineering' started by George Tsakalos, Apr 11, 2006.

1. ### George TsakalosGuest

Hello,

i'm interested in learning a little bit about PF Correction, by means of
using compensation capacitors. More specifically, i want to search for
products in this category for OUTDOOR use in distribution networks (not in
industries, buildings etc. where capacitor banks are used).

Any suggestions?

And more importanly: i've heard that capacitors used for PF Correction cause
harmonics in the network. Is this true? Because it doesn't seem very logical
(to me at least). Why would a capacitor be responsible for harmonics?

George Tsakalos

2. ### Ben MillerGuest

Here is a simplified explanation of what happens. The capacitor resonates
with the system's inductance. If that resonance occurs at a harmonic
frequency, it greatly amplifies the effect of that harmonic throughout the
system. The whole thing can look like a short circuit to the source at the
harmonic frequency, causing protectors to trip, etc. The solution is to
"detune" it by adding inductance so that the resonant frequency is between
harmonics.

Ben Miller

3. ### JimGuest

Bill, you don't usually play the troll. Why this time? Bad day
maybe?

4. ### George TsakalosGuest

Ben, first of all thanks for the answer, it covers me completely. I thought
i was right to suspect that a capacitor can not produce any harmonics (a
passive element cannot produce anything right?)

My special interest is in capacitors that are pole-mounted in MV networks.
Is there any theory or "algorithm" that states where we should place
capacitors to correct the cosö in the span of a MV line?

And by the way, can we use filters instead to filter out anything else than
the 60 or 50 Hz signal and stop worrying about any high harmonic currents?
You mentioned the inductance in series solution with the capacitors. But,
wouldn't that work in the opposite way we're trying to go? Wouldn't that
reduce the amount of Var the capacitors are "feeding" the network?

George Tsakalos

5. ### Don KellyGuest

----------------------------
----
The capacitors, in the situation that you mention, do correct the power
factor However, their main purpose is to improve the voltage profile.
Suppose that, on a feeder, the voltage must be kept within bounds of +/-
5%. and if no capacitors are used. it can happen that the voltage will be at
the upper bound at the source end and below the lower bound at the other.
Then a capacitor bank at an intermediate point can help by improving the PF
at that point, resulting in a lower voltage drop to that point (beyond that
point it has no effect).
Instead of having a source at 1.05 pu and a mid point at 0.99 pu and end of
line at 0.93pu, with capacitors at the mid point, the profile might be
The problem with a set algorithm is that the load distribution is not the
same in all cases. it may be necessary to actually determine a voltage
profile by measurement at different times of day and home in on correction
for the worst case by considering various amounts of compensation at
different points. This could be an interesting optimisation problem. There
probably are many "rule of thumb" procedures out there as this is a common
problem.

The inductance in series is already there- that is the problem. It is this
inductance that leads to higher voltage drops in the first place.