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In what stage should you design the distribution circuits containing third harmonic current?

eizo

In design stage of the larger installation, I think, it is quite difficult to decide which (three-phase) distribution circuit has third harmonic current.

 

But BS7671 523.6.3 says that if the total harmonic distortion due to third harmonic current or multiple of third harmonic is greater than 15% of the fundamental line current, the neutral conductor shall not be smaller than the line conductor.

 

I imagine that the sub-main to a distribution board which supply many articles of elecronic equipment has the third harmonic current.

But how do you estimate that the circuit has third harmonic being greater than, for example, 15% of the line conductor.

 

I think there must be an established method or a traditonal practice or a rule of thumb that a few like me don’t know.

 

I am very grateful if someone tell me what phase is appropriate to design the distribution circuits containing 3rd harmononic current and how to estimate 3rd harmonic content of line current in a design stage, if any.

  • 0

    I would be inclined to provide a full size neutral conductor for distribution circuits, unless satisfied that third harmonic currents will be very low.

    If the load is predominately resistance heating, then a reduced neutral may be used. If the load is predominately three phase motors, then without any connection to the neutral no neutral current can result.

    In the rather unusual case of most loads being connected between phases then no neutral current can result.

    In almost all other cases where the load details are unknown or are liable to change during the life of the installation, then a full size neutral is good practice.

  • 0 in reply to broadgage

    o broadgage

    Thank you for your response.

    I understand what you say.

    Thank you again.

  • 0

    A large fraction of 3 phase distributions suppliers single phase loads spread over the 3 phases. In such a case you need to allow for the unbalanced cases, when perhaps all the loads on one phase are switched off but the others are not - buildings with one  phase per floor or something can be like that.

    In such a case again the full size neutral is needed.

    Actually it is better to assume the regulation is written backwards, and the neutral is always full sized, except in the rare event when the load is fixed and its power factor/ waveform quality is well known.
    Basically fully balanced loads of  good power factor are the only ones that you can be sure will not stress the neutral conductor.

    A poor power factor can be a measure of both harmonic content (non-sinusoidal wave-forms) or phase shift,. (good waveform but current and voltage not in phase) so a poor power factor does not mean there will always be a lot of harmonics, it could just be a lot of inductance, but the reverse is always true, a good power factor means there cannot be harmonics.

    You may find the more detailed explanation here is more help.

    Mike

  • 0 in reply to mapj1

    To Mike

    Your words ‘A poor power factor can be a measure of both harmonic content (non-sinusoidal wave-forms) or phase shift’ open my eyas.

    Thank you.

  • 0 in reply to mapj1

    I might be wrong here, but I don't think you can get TPN cable with a reduced neutral any more. I haven't seen any for years.

  • 0
    eizo said:
    I think there must be an established method or a traditonal practice or a rule of thumb that a few like me don’t know.

    Have a look at Chapter 11 of the IET's Electrical Installation Design Guide, and Chapter 17 of the Commentary on IET Wiring Regulations.

    It would require a calculation knowing the harmonic contents of the line currents of the loads, OR for relevant loads you could assume worst-case conditions using BS EN IEC 61000-3-2:2019, Electromagnetic compatibility (EMC). Limits. Limits for harmonic current emissions (equipment input current ≤16 A per phase)  and multiply by an estimate of the quantity of devices.

    For larger installations, a study would be required for G5 compliance in any case?

  • 0 in reply to gkenyon

    I have already read Chapter 11 of the IET's Electrical Installation Design Guide,but don't know Chapter 17 of the Commentary on IET Wiring Regulations and  BS EN IEC 61000-3-2:2019. So I will look at them.

    Thank  you.

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