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Thread: Harmonics and PF

  1. #1
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    Harmonics and PF

    Compliments of Power Monitors, Inc, I read a short paper on how harmonics affects PF. library.powermonitors.com/understanding-how-harmonics-affect-power-factor
    I'm now fully informed and competent.

    We are still working with a dairy on new lighting and eventually fans. Currently they have a PF penalty that kicks in when the existing fans run. We managed last year to reduce those penalties charges to about 9 months of the 12. POCO brought me that report last week. Those penalties amount to about $1000 per month IIRC. (CRS)

    One brand of LED lights we are looking at has a PF of >.9. Great, but now I am concerned about the harmonics created by these power supplies and it's affect on the resulting PF. These or any other fixture will be connected 480. No L-N loads even though it is a Wye. I do not want to reduce the PF enough in the winter months when the lights will be on the longest nor do I want to add to the problem when the fans come on for cooling. Is there a way for me to be able to predict what will happen or do we just wait and add filters as needed?

    Now add that the new fans will most likely have VFDs.
    Tom
    TBLO

  2. #2
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    If the LED fixtures have a .9 PF, that is all you really need to know. The effect of harmonics is already included in that. You just cannot compensate for it with capacitors.
    As for the VFDs, with a decent design VFD the current harmonics at the input will be fairly low, and more than compensated by the VFDs' taking care of the motor PF for you.
    A win-win.

  3. #3
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    The key is to understand what 'power factor' is. As you know the voltage and current in an AC circuit is continuously changing. To make life easy, we _describe_ an AC circuit by its RMS voltage and RMS current, neither of which tell you the actual voltage and current at an instant in time, but rather give a sense of the average voltage and current over a period of time.

    When the current flow is perfectly in sync with the changing voltage (as in feeding a resistance) then power factor is 1.0

    If anything makes the current flow not match the changing voltage, then the power factor is less than 1.0

    The current flow could not match the changing voltage by having the same shape but different timing (displacement power factor seen in motors) or the the wave shape of the current flow could be different from the wave shape of the applied voltage (harmonic power factor) or a combination of both.

    Unfortunately there isn't an easy way to figure out how the power factor of different loads will combine. What you basically need to do is figure out how the waveshape of the current drawn by one load will combine with the waveshape of the current drawn by the other load to see what the _net_ current waveshape will be.

    If you switch to LED lighting and VFD fed motors, then you will pretty much change to pure harmonic power factor with very little displacement. You might be able to mitigate this by using distribution transformers with different phasing, so that harmonics from one subset of your load are out of phase and cancel with harmonics from other subsets of your load.

    -Jon

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    Quote Originally Posted by GoldDigger View Post
    As for the VFDs, with a decent design VFD the current harmonics at the input will be fairly low,
    Depends on what you call fairly low. With a good sized DC choke and nearly level DC current, the current harmonics are roughly the reciprocal of the harmonic number.
    For a six-pulse rectifier front end you get 6n+/-1. So 5th, 7th, 11th, 13th etc.
    So 5th will be at 20% of the fundamental. Or worse.

  5. #5
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    How much harmonic current are we talking about here? I’ve been using Active Harmonic Filters on some projects now for a few years, the results are great, both from a harmonica standpoint and from a PF correction standpoint. They automatically adapt to ongoing changes without manual intervention.
    __________________________________________________ ____________________________
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    Quote Originally Posted by Jraef View Post
    How much harmonic current are we talking about here? I’ve been using Active Harmonic Filters on some projects now for a few years, the results are great, both from a harmonica standpoint and from a PF correction standpoint. They automatically adapt to ongoing changes without manual intervention.
    Yes. AFE is also a good approach but, at nearly the price of a standard VFD, rather expensive.

  7. #7
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    Quote Originally Posted by Besoeker3 View Post
    Yes. AFE is also a good approach but, at nearly the price of a standard VFD, rather expensive.
    Not AFE drives, an Active Harmonic Filter that connects to the distribution panel or MCC bus and injects corrective current which corrects all harmonics up to about the 50th from any source on that bus.

    My favorite lately. I’m not associated with them in any way other than I like their customer service and support (when I have screwed up).
    https://transcoil.com/products/hga-5...rmonic-filter/
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  8. #8
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    Quote Originally Posted by Jraef View Post
    Not AFE drives, an Active Harmonic Filter that connects to the distribution panel or MCC bus and injects corrective current which corrects all harmonics up to about the 50th from any source on that bus.
    I know.
    That's why I said also.

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