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Power Factor Correction

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

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  1. 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?

    Thanks in advance,

    George Tsakalos
     
  2. Ben Miller

    Ben Miller Guest

    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. Jim

    Jim Guest

    Bill, you don't usually play the troll. Why this time? Bad day
    maybe?
     
  4. 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?

    Thanks in advance again,

    George Tsakalos
     
  5. Don Kelly

    Don Kelly Guest

    ----------------------------
    ----
    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
    1.05->1.03->0.97pu at heavy load and yet be OK at light load.
    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.
     
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