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High 3rd harmonic on the neutral

MS01
Afternoon,

I was wondering if anyone has nay experiences of having issues with high 3rd harmonic currents on the neutral on high-rised residential schemes? I appreciate  non-linear single phase loads will impact the 3rd harmonic and even on a balanced system harmonics are an issue but we are measuring it at 300% but I can’t think why this may be the case on a residential building.

Does anyone have any ideas?



M
  • 0
    If you have a laptop with a microphone input, you can feed it a suitably isolated and attenuated version of the waveform from a current transformer (the clip on kind made for energy monitors and variants on the theme), and there are a number of audio oscilloscope and spectrum analyzer programs that can show waveform and spectral views.

    Not perhaps for the faint of heart or those in a hurry, but not too hard to do on a bench top type set up rather than hand held.


    example clamp    you need to add your own load resistor and voltage divider before feeding it into the line input !

    example software. There are others.

  • 0
    So, put simply we aren’t going to spot the 150 Hz frequency using a electricians tester.


    I did an internet search for circuits with multiple frequency currents last night there wasn’t a quick and easy way to find different frequencies.
  • 0
    To a physicist, the definition of frequency is subtly different, and relates to the concept of cyclostationarity - not a made up word I promise. - namely that a full 'cycle'  has only passed when you get back to the same (or at least an indistinguishable ) place on the waveform again (so it looks stationary, if you move back and forth one cycle you cannot tell you have moved) - so the waveform may have all sorts of fast ringing, and maybe multiple zero-crossings, but the repetition period, and therefore the fundamental frequency only relates to the repetition of the full pattern.

    Anyone who has tried to line up flowered wall paper while decorating, where the pattern cuts more than one flower that looks similar, will be familiar with this problem, and the related one of variable phase modulation as the paper stretches a bit once pasted and you hang it under its own weight before brushing it - though not all of us will use the same language to describe it. ( even me ! )


     For a more electrical example,  if we had a circuit design that suppressed every 10th cycle from the mains, substituting a flat line for 20ms between bursts of 9 oscillations, then by the physics definition you would have a 5Hz fundamental, though the 50Hz component would be stronger (being present for 90% of the time after all). A spectrum analysis would show this correctly (thanks Mr Fourier), but any frequency counter based on zero crossings would read 45Hz, although that is one frequency clearly not really present in the generation of the waveform  (!)

    regards Mike.
  • 0
    Each flat has a single phase consumer unit fed via a 3-phase infrastructure


    Looks like from the amount of responses this is interesting topic to most, could be worth the IET techincal document about he effects on harmonics on electrical installations. 


    We are logging the installation as we speak so in a couple of weeks we will have a better set of results, as the figures quoted previously were just a snap shot in time. This was taken on the main incomer to the development whihc then is split into sub- switch panels for the different block then all the way down to single phase consuler units in each apartment. The building is no where near full occupancy as of last week, but this should now of changed over the weekend with alot of students moving back in. 


  • 0
    I particularly like Andy Jewsbury`s DC explantion.

    I do something similar myself when explaing apparent power and real power (even some electricians struggle to grasp it)

    I usualy say something like imagine A DC supply and remember Pythagoras and his right angled triangle. If you send 5V from a supply (battery or whatever) to whatever is using power then in in turn it sends 1V back because it does not need it, then it has only used 4V needs to create 5V just to send it. Of course you could imagine the same in Amps or Watts.


    Off course I then tell them that it is actually all a lie because power factors only work with AC not DC but I think it helps in simple terms to explain that you are generating large amounts to receive far smaller amounts and use them..

  • 0
    Sparkingchip:

    . . .

    What is frequency?



    Alternating current (ac) frequency is the number of cycles per second in an ac sine wave. Frequency is the rate at which current changes direction per second.

    . . .

    So what happens if there’s more than frequency in the same conductor, does the tester register the “dominant” frequency or a mish mash of them.


    Andy B.




    I don't like this as a definition of frequency. For a start, a frequency of 50 Hz involves 100 changes of direction per second. Also, if you take an open circuit, you can still talk about a supply frequency even though no current is flowing.


    Realistically, frequency is the number of times per second at which the electro motive force goes through a complete cycle of positive and negative direction.


    And as you say, Andy, there can be more than one frequency in the conductor, as this fascinating discussion has revealed. It is quite possible, with heavy harmonics, for a doubling of frequency to occur. Hence 200 changes of direction of current per second with an applied e.m.f. of 50 Hz.


  • 0
    Andy -  the data sheet says that it measures true RMS, so that is the reading you would get, the RMS value, i.e. the heating effect value of the waveform present.


    If you use the low pass filter, you will get the RMS value of the 50Hz component.


    David
  • 0
    AJJewsbury:
    A Megger MFT1741 will measure between 150 and 400 hertz, so if the harmonic current is high will it display 150 hertz rather than 50 hertz?

    Isn't it displaying the frequency of the voltage rather than the frequeny of the current though?

       - Andy.




    I  thought it was current, but I did an internet search before answering.



    What is frequency?



    Alternating current (ac) frequency is the number of cycles per second in an ac sine wave. Frequency is the rate at which current changes direction per second. It is measured in hertz (Hz), an international unit of measure where 1 hertz is equal to 1 cycle per second.

    https://www.fluke.com/en-gb/learn/best-practices/measurement-basics/electricity/what-is-frequency


    So what happens if there’s more than frequency in the same conductor, does the tester register the “dominant” frequency or a mish mash of them.


    Andy B.


  • 0
    "it made me wonder how much of the oversizing is precaution, and how much is still a necessity."


    A number of years ago I was helping out with the design of a series of data centres. The main designer was a bit of a salesman and he had put in a generator and main switchboard with a double sized neutral. After a while I twigged that all the outgoing supplies to the floors were three wire three phase supplies to delta star transformers at the DBs. and the only outgoing neutral fed the lighting supply. I asked him why the double sized neutral. Strange he didn't want me to work there any longer.
  • 0
    Sparkingchip:

    I have found a Diagram that shows the waves in relation to each other showing why you can end up 200% above where you expect to be.


    Andy B.


    Very useful diagram - explains the addition of third harmonics very well. Thanks for finding it, Andy.


    It does not, however, explain mathematically to my understanding why you can reach 200% in the neutral. However L & T has obviously done some research into this so I'll take its word for it.


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