Harmonics in electric supply

Replies

• 

  freak16

  Answer it friends..
• 

  shreyasm89

  We can prevent harmonics by using a Low pass filter with the value of the cut-off frequency equal to the fundamental freq.(1st harmonic). This filter can be realised using an integrator ckt., also digital filtering using windowing is used. The theory is very exhaustive of digital filtering.
  harmonics are harmful as in they reduce the output power in the main lobe of the passband.(Frequency domain representation by Fourier transform) As the energy is redistributed over the multiples of cutoff frequency aliasing problem could be created. Also it creates Total Harmonic distortion in the signal.
• 

  freak16

  Thanks shreyasm89!
  Can you please explain what is THD??
  harmonics are harmful as in they reduce the output power in the main lobe of the passband.
  I doubt it will it not increase the output power
  As vcosa+vcos3a + vcos5a..like that so if we multiply v and i will it not increase the output???
  and also what is digital filtering using windowing??
  can you please refer me any book related it...
  I will be thankful
• 

  shreyasm89

  When I say main lobe I mean the passband in which we require the o/p(for e.g. 50/60 Hz in case of main lines) Your eqn.(vcosa+vcos3a + vcos5a..) itself indicates that there are higher frequencies present in the o/p(read 2a, 5a etc. ) Since total energy remains same the energy is redistributed in the higher frequencies & hence we get Harmonic distortion. Total harmonic Distortion is the mean RMS value of the distortion in the o/p power. It is just theory & I'm not aware of its practical measurement. Digital filtering is the phenomenon where you convert an analog filter to digital filter & then design the filter. Refer any good book on Electronic Devices & Ckts. for harmonic distortion.
  As for digital filtering using windowing it is a subject in final year & can be implemented in MATLAB.
• 

  freak16

  I got it.
  Repeating my words but cannot help to say it again
  " you are amazing"
  THanks.
• 

  Arp

  its not always harmonics are harmful only.
  sometimes we want them a in the case of transformer for obtaining a perfectly sinusoidal waveform.
• 

  freak16

  @ arp
  sorry ! but Can you please elaborate your previous post.
  I couldnot get your point..
• 

  Arp

  for voltage to be sinusoidal flux should be sinu for that magnetising current should be peaky which can be obtained by adding fundamental frequency with enhancing 3rd harmonic..
• 

  therealabdo

  guys in the factory i work in the Harmonics were 4% before installing a power factor correction panel. After installing this panel the Harmonics jumped to 14%..Does this harm my factory (the machines?)
• 

  narayana murthy

  hmmmmmm as per my knowledge nothing happens
  because harmonics means not getting pure sinusoidal wave from supply i.e., adding of additional frequency
  so i think it doest effect much unless your company has frequency dependent components
• 

  freak16

  @therealabdo
  Seriously!
  Is the power factor correction panel contains power electronics device like thyristors and all.
  Becoz power electronics device pollute the electric power by harmonics and all.\
  and harmonics are harmful mind this thing for machine specially for induction motors and all.
  Harmonics caused more noise and vibrations..reduce life of machines..
• 

  Arp

  this is from the best of my knowledge..
  what i suppose as u r using APFC pannel so due to the automatic switching operation it should have power electronic devices which introduces harmonic content.
  if supply contaminated with harmonic is given to motor then crawling may be done if 7th or so harmonic are present which may result in heavy power loss in the windings of the motor and if run for longer on that then temp may rise much more than normal resulting in burning of winding or damage of insulation.
• 

  therealabdo

  Actually I had a problem 3 months ago with one of the compressors. We had to change it. I don't know if it is from the harmonics increasing. Do u think that increase in harmonics (from 4% to 14%) would harm that much?
  The power factor correction panel contains capacitors, contactors thats all...
  thank you very much for ur reply guys
• 

  freak16

  as ARP said they may do this..
  Damage of insulation or winding burning..
  can you please tell the problem in the compressor?
• 

  Arp

  i've no great idea about compressors.
  but from the best of my knowledge compressors use single phase induction motor
  and as IEEE Standard 519-1995 provides a good guideline for the acceptable levelsof voltage distortion to loads (including motors). A broad recommendation is to establish the voltage distortion monitoring limits at 5% total harmonic distortion (THD) and at 3% for any particular harmonic frequency. The types of connected loads and system impedances will be a major factor in the level of exposure a motor has to voltage distortion.
  so i suppose 14%is too much and then the motor has the problems listed in my previous post
• 

  freak16

  @arp..
  you are amazing.
  highly informative post..
  you rock buddy..
  keep the spirit high..😀
• 

  shreyasm89

  @Arp- *bows* to you. But you have mentioned "crawling" in your post? How is it used to reduce harmonics in supply?
• 

  Arp

  no not al all it is only one of the problem due to harmonics.
• 

  sau

  for a basic idea, go to this link...-> #-Link-Snipped-#
  i would come up with more info that i know soon...
• 

  sau

  i think 3rd harmonics help in obtaining a sinusoidal waveform!