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Why harmonics cause such voltage drops?

  1. Feb 26, 2005 #1
    I have recently come across the subject of harmonics in my new job and I am a little puzzled about them. I understand that they are produced by devices which draw current in a non-linear fashion and that they can travel throughout a network causing items to overheat and such like.

    BUT....

    I am a little unsure as to how they affect the voltage characteristics of the system they are present in. As I understand it they distort the current profile and thus distort the voltage profile. I also understand that harmonic current cannot be used as useful power and is termed 'dirty power'. I was told that they can cause voltage drops in other parts of the network. So my question is why do they cause such voltage drops? I am guessing it is because the power supply is having to power these useless harmonics :blushing: Any help appreciated :wink:
     
  2. jcsd
  3. Feb 26, 2005 #2
    Harmonic power is NOT unuseable. I assume you are working with the power company? In many applications harmonic power is unuseable because it is unwanted. But harmonic energy IS utilized in many cases.
     
  4. Feb 27, 2005 #3
    Thanks for replying that is something new that I have learned :smile: But I am still unsure as to how they cause voltage drops in the network they are attached to. Surely some components cannot utilise harmonics :shy:
     
  5. Feb 27, 2005 #4
    Guess on Harmonics

    Harmonics are formed when an otherwise sinusoidal signal or source encounters a nonlinear component a possible example being a cored transformer . If the original source is of fixed power then production of harmonics (which means energy at higher frequencies ) may reduce the Fundamental power.
    Whether the harmonic power is useful depends on the end device , a stove may not care but I think that a motor would. A transformer with a nolinear core may produce 3rd harmonics i.e for a 60 Hz source thats 180 Hz and it may have a shifted phase wrt the fundamental.
    Just some thoughts Ray -- I know little about Power distribution.
     
  6. Feb 27, 2005 #5
    Update

    Thanks for replying ray :wink: I have ordered a book on the subject as I would like to understand the concept thoroughly, hopefully the book will be a readable one and not like some of the over-advanced ones I have bought in the past :zzz:

    I always find that I cannot get on with books aimed at postgrad level even though I am a post grad :blushing: My boss said the harmonics cause voltage drops because of the impedance of the cables and such like in the networks. He didn't really explain it very well so hopefully the book will make more sense.
     
  7. Feb 27, 2005 #6

    Astronuc

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    Staff: Mentor

    Certainly the performance of a resistive element (e.g. electric heating element in stove or furnace) is not affected by harmonics.

    Some characteristics (reactive impedance, e.g. capacitance and inductance) and performance of inductive machines are frequency dependent. If something is operating at 60 Hz, and the voltage/current (and induced magnetic field) oscillate at 180 Hz, then during the 1 cycle of 60 Hz, the harmonic works against the device (although it is on a 1/3 cycle basis).

    Also, I would think that in general, the amplitude of the harmonic is not as great as the fundamental mode, but that may not be true in all cases, e.g. response of a power distribution system to a lightning strike. But that's why protective relays were developed - and these are now enhanced by digital protection systems.

    With respect to useful/dirty power, might this refer to real/reactive power. In power distribution systems, the objective to maximize real power and minimize reactive power.
     
  8. Jun 24, 2008 #7
    Re: Harmonics

    hi
    i have a problem related to harmonics too...i need to learn more about harmonic modelling.
    thanx
    madi nagar
     
  9. Jun 25, 2008 #8
    Re: Harmonics

    Harmonic content of voltage or current wave is periodical inherently but response of power system to lightning strike is not.
    Also existing of harmonics is useful sometimes, for example the third harmonic of electrical machine and power transformers magnetizing currents is necessary for suitable out put voltage wave.
    As you know, because the B/H curve of the magnetic material forming the transformer
    Core is not linear, if a sinusoidal voltage is being applied for a sinusoidal flux (and hence a sinusoidal secondary voltage), the magnetizing current is not sinusoidal. Thus the magnetizing current of a transformer having an applied sinusoidal voltage will comprise a fundamental component and various harmonics. The magnitude and composition of these harmonics will depend on the magnetizing characteristic of the core material and the value of the peak flux density. It is usual for third harmonics to predominate along with other higher third-order harmonics. If the harmonics cannot flow in any of these paths then the output voltage will contain the harmonic distortion.
    In power transformers with grounded star or delta connection, there are suitable low- impedance path for third harmonic and other zero sequence currents.


    REGARDS


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    Creative thinking is enjoyable,Then think about your surrounding things and other thought products. http://electrical-riddles.com
     
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