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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
    Chris Pearson:
    John Peckham:

    It can't be that big a block of flats at 100A a phase?


    MS01:

    My thought was this is a big block of flats 300+ which are used by students and now with lectures online mostly will be using laptops....what are peoples thoughts? 


    I did wonder: that's 1 amp per flat.


    In fact if they are centrally heated bed-sits with a shower and WC, and no kettles, etc. allowed, they probably would get by with 230 W (after diversity).




    1 amp per flat is in my view possible at times of low load, That does not mean that a 100 amp 3 phase supply is enough for 300 flats, the load could easily average 5 amps or more per flat at peak times. But an average of 1 amp at times sounds reasonable.


  • 0
    Indeed - if the space heating is done by hot water or centrally pumped hot air, then the electrical load may well be mostly small stuff - which means electronic power supplies these days and quite a bit of it may be chargers of various wattages and small lights and generally kit that is below the power threshold for requiring any waveform correction at all.

    I'd like to think it could handle more like a kw per flat of ADMD, but maybe modern students  do not make coffee and tea at the same rate that some of us did in years past. Usually some one has a hairdryer and so on.


    regards Mike

  • 0
    MS01:
    The highest phase it at just over 100A with the other phases there or there abouts but the neutral current is at 50A




    How do you achieve balancing a three phase load in a residential building with single phase loads that will cancel the neutral current?


    You can balance the load to get an even load across the phases, but you cannot cancel the neutral can you?


    It’s over twenty years since I sat drawing diagrams during evening classes at Kidderminster College, but I don’t remember one where it showed how a single phase neutral current is cancelled.


     Andy Betteridge 


  • 0
    broadgage:
    Chris Pearson:
    John Peckham:

    It can't be that big a block of flats at 100A a phase?


    MS01:

    My thought was this is a big block of flats 300+ which are used by students and now with lectures online mostly will be using laptops....what are peoples thoughts? 


    I did wonder: that's 1 amp per flat.


    In fact if they are centrally heated bed-sits with a shower and WC, and no kettles, etc. allowed, they probably would get by with 230 W (after diversity).




    1 amp per flat is in my view possible at times of low load, That does not mean that a 100 amp 3 phase supply is enough for 300 flats, the load could easily average 5 amps or more per flat at peak times. But an average of 1 amp at times sounds reasonable.




    We haven't been told the capacity of the supply, just that the currents in the phases are about 100 A each.


  • 0
    We have had discussions about balancing single phase loads in installations with three phase supplies and meters regarding what the meter is actually recording as usage, the  highest phase, the average or usage by phase.


    But without a load connected across the phases within the installation nothing can cancel the neutral current can it?


    I know this is going right back to basics, but maybe I need a refresher.


     Andy Betteridge 


  • 0
    MS01:

    My thought was this is a big block of flats 300+ which are used by students and now with lectures online mostly will be using laptops....what are peoples thoughts? 


    We are doing some load montitoring and we noticed this issues. 




    So does each flat have an independent installation with a consumer unit in each supplied from a three phase distribution board that does not have any three phase loads connected into it at all?


    Andy B.


  • 0
    Hi Andy,

    I don't see how a load "connected across the phases" can help here. What balances the neutral is three separate phase-neutral loads.

    And in the case of a domestic single phase load the problem is just transferred to the street where a large third harmonic current could overheat the neutral.
  • 0
    With a three phase delta connection only one phase is live at a time and the return path is via the other phase conductors, so no neutral connection is required at all, because the load is connected across the phases.


    Edit- with a star connection neutral current will pass to the other phases at the star?


    With single phase the return path is always via the neutral and cannot be returned via the other phases.


    So how can you cancel single phase return currents down to zero in the neutral conductor?


    Can someone sketch out the layout of a building installation such as this with a X to mark the point where the neutral currents cancel each other out?


    Andy Betteridge
  • 0
    It’s over twenty years since I did three phase theory, so feel free to add a few sketches and notes to jog my memory, as I’m not getting a picture in my head of where the neutral current is supposed to cancel in this installation.


     Andy Betteridge
  • 0
    I have to go to a job now so I will read what I found later.

    Schneider

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