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About dc line harmonics

  1. May 16, 2009 #1
    Hi all, :smile:

    My understanding of harmonics in general is quite sketchy and as a result my query might appear a bit lulu! :rolleyes: I tried the net but could not get suitable info on the same.

    This is what i'm looking at;
    Assume a battery hooked up to an inverter (1 phase or 3 phase doesn't matter). The output of the inverter is connected to a typical AC load. I do not know the relation between nature of AC load (resistive, inductive or capacitive) and hence the resultant harmonics injected into the DC line due to the same, hence I would be unable to elaborate on the same.

    What I want to understand is; whether rapid switching of the inverter inject harmonics into the pure DC line? If so, what is their nature and waveform like? I would be grateful if someone could provide me some articles or an on-line link on the same.

    Thanks & Kind regards,
  2. jcsd
  3. May 16, 2009 #2


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    You are asking a somewhat broad question. An IDEAL inverter will fully isolate the output from the input. No matter what kind of load is hooked on the output of an ideal inverter, current will be drawn from the DC source in a smooth manner. There will be no ripple current on the input. It all goes downhill from there.
  4. May 16, 2009 #3


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    whether rapid switching of the inverter inject harmonics into the pure DC line?
    Yes it does.

    There is always a risk that any device where rapid switching is happening will let some of the energy from this switching escape back down the DC power line.

    To avoid this, filtering is used. Bypass capacitors from the DC line to ground and series inductors in the line attempt to minimise this effect.

    Often such devices are also enclosed in metal boxes to make the chance of radiated energy reentering the power line less likely as well.

    There is another post like this on this page:
    See the 5th entry which has some circuit diagrams in it and you will see the extent of the bypassing that has to be used.
    Notice the right hand diagram where 3 capacitors are used in parallel and two inductors are also used (in the left diagram ) to avoid this.

    The waveform is unpredictable but would be related to the frequencies being used in the inverter. It would always be a variation imposed on the supply line. So, it is still DC but varying slightly (or a lot) from being perfectly smooth.
  5. May 16, 2009 #4
    :smile: Thanx guys, but some waveforms will be of much help

    Best regards,
  6. May 16, 2009 #5


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    The waveforms will vary.

    One reason this happens is that the inverter will probably not be drawing constant current from the power supply. On peaks of current, the power supply voltage will drop.

    The waveform in this case will possibly be pulses of square wave tending to reduce the power supply voltage below its steady value.
    These pulses can be any shape from an actual square wave or they might be very brief pulses.
    You might have a 14 volt supply with various shapes of square wave dropping the voltage down to 12 volts periodically.

    Bypassing capacitors try to fill in the drops in power supply voltage

    And if there is bypassing, this waveform will vary again depending on how effective the bypassing is at particular frequencies.

    One waveform you are unlikely to get is a sinewave as inverters use rapid switching and this is what is likely to get back into the power supply line.
    Last edited: May 16, 2009
  7. May 17, 2009 #6

    That's perfectly fine. I understand that the nature of waveform will depend on several factors, but a typical waveform would provide me an insight into the concept.
    Thanx :smile:
  8. May 17, 2009 #7
    There aren't any
  9. May 17, 2009 #8
    There aren't any and topology varies.
  10. May 17, 2009 #9
    Don't know if this is what you have in mind, but might be of interest. Shown is the DC source current, it's FFT and the circuit from which it came - not mine - PSIM Demo.

    Attached Files:

  11. May 17, 2009 #10

    The Electrician

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    Tell us more about this inverter. Is it's output a so-called "modified sine wave", which is really a square wave with some dead time? Or, is it outputting a fairly good sine wave, using PWM modulation? Or, is it the "stepped approximation" type (such as the Xantrex SW4024)?

    Are you only interested in the high frequency harmonics, due to rapid switching edges, with frequencies up in the radio frequency range? Or, are you also interested in the harmonics of the line frequency (perhaps up to the 51st harmonic) caused by the inductance of the battery cable?
  12. May 18, 2009 #11

    I think this what I've have in mind. However, the variables do not present detailed info.
    But thanks very much for the attachment, i'm grateful for that. :smile:

    Further inputs are welcome

    Best regards,
  13. May 20, 2009 #12
    Someone plz guide me! Thanx :smile:
  14. May 20, 2009 #13


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    If you want further input, please post exactly what you are looking for, and what your application is. Inverters can be very noisy, both on the AC Mains output and the input DC supply. If you invest more in the cost of the inverter and the power lost/dissipation, you can have a cleaner output and less hash on the DC input.

    Why are you asking this? What is your end goal? So far, your questions in this thread are a bit too open-ended for us to answer efficiently. I hope you understand that.
  15. May 20, 2009 #14


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    Really bad input waveform

    ------------------------------------ 0 volt level

    Bad input waveform Slightly better input waveform Good input waveform
    VVVVVVVVVVVVVVVV vvvvvvvvvvvvvvvvvvvvvvvvvvv ------------------------------

    ____________________________________________________________________0 volt level

    These waveforms do not really have a shape and, on an oscilloscope, they usually cannot be synched to give a steady pattern. This means they just look like a noisy blur.
  16. May 20, 2009 #15

    I agree. But this is not a project and my query is general in nature. I was just curious about typical waveform pattern for injected noise in DC line due to inverter switching. Thanx for reply. :smile:

    Thanx very much for reply. :smile: A very innovative way to explain a concept i must say....I'm impressed! :wink:

    Best regards,
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