Shunt Capacitors in Electrical Systems

Discussion in 'General Electronics Chat' started by daylonh, May 17, 2016.

  1. daylonh

    Thread Starter New Member

    Jan 22, 2016
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    I know that in electronics, a shunt capacitor decouples the AC signal by effectively shorting it to ground, but in a power system, when a shunt capacitor is used for power factor correction, this doesn't occur. I know that if you calculate the current as if the capacitor were ideal, you only get reactive current at a negative angle, and it would make sense for no real current to be shorted to ground. But the capacitor isn't ideal, there is resistance in the copper wires going to the capacitor.

    Why does a shunt capacitor in a power delivery system behave differently than a decoupling capacitor (such as in a 4R biasing circuit)?
     
  2. SLK001

    Well-Known Member

    Nov 29, 2011
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    What is a 4R biasing circuit? Could it be AC vs DC?
     
  3. daylonh

    Thread Starter New Member

    Jan 22, 2016
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    [​IMG]
    It's an amplifier circuit which uses a resistor at the emitter to provide feedback so that the output voltage remains within a tolerable range of its "bias point." It's useful since BJT manufacturing can be pretty inconsistent.

    The capacitor I was talking about in the circuit is CE1. It decouples the AC signal from the DC signal before RE2.

    In this circuit, the AC signal is shorted by CE1, but in a power system, a shunt capacitor doesn't behave in the same way.
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    1,790
    Have you considered the possibility that the frequency of a power system (50/60 Hz) is different than the frequency of an audio amplifier. CE1 is more of a short at audio frequencies than at power frequencies.

    At 50 Hz. CE1 has an impedance of 3183 Ω
    At C above middle C (512 Hz.) CE 1 has an impedance of 311 Ω
    At 10 times that frequency CE1 has an impedance of 31.1 Ω

    You can see the pattern here that CE1 tends to block low frequencies and pass high frequencies to GND. Since the power line consists of a single frequency, this effect is not relevant.

    Can you compute the effective impedance of 4.5K || CE1 at various frequencies?
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Yes, there is.
    But the wire gauge is large enough that the low resistance dissipates an insignificant amount of power at the reactive currents in the capacitor.
     
  6. daylonh

    Thread Starter New Member

    Jan 22, 2016
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    Ah, I can't believe I didn't see that myself. I knew it was something simple that was slipping my mind.
     
  7. daylonh

    Thread Starter New Member

    Jan 22, 2016
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    Sorry, I wasn't being super clear when I said that.

    I meant to say that the resistance of the wire added a real impedance to the otherwise completely reactive ideal capacitor. In my mind that would have caused large amounts of real current to flow to ground.

    I should probably do some digging and find a value for the capacitance of a cap bank used by a power company and work the circuit through...
     
  8. crutschow

    Expert

    Mar 14, 2008
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    You need to recalibrate your mind. ;)
    The resistance is in series with the capacitance, so it doesn't add any additional current, it just adds a small real component to the large capacitor reactive current.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    The real wire and leads of the capacitor change the ideal resistance and impedance from zero to some small value that is, from a practical view, insignificantly more than zero. Both the resistance and inductance are in series with the cap, so tend to reduce any real current, not increase it.
     
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