High 3rd harmonic on the neutral

These numbers are high but certainly not impossible for a building with a lot of electronic control. I remember a post by Zs a few years ago, relating I think to the large art gallery in the house of someone very rich in London, where all the lamps were on dimmers for  computer controlled fading.

Basically this arises if the current drawn per phase is 100A RMS, but that takes the form of a waveform with a lot of conduction at one point in the cycle, so each phase is 100A RMS, but that is not at 50Hz, but has a 50A 150Hz component - as Dave notes that is quite square, (with a large dead-band so a 3 level waveform up zero and down) but certainly not impossible, and used to be quite common on early power supplies before regulations on waveform quality came into effect .

As an example imagine bursts of current flowing between L and N of twice the RMS current but only for the +/-45 degrees nearest the peak of the voltage, and very little current flowing on the side-slopes (this is not an incredible waveform and is typical of rectifiers driving capacitors with inadequate pre-rectifier filtering).

You will need 3 coloured pens and the ability to sketch ;-)

If you repeat this for all 3 phases, it is clear that there are 6 isolated bursts of conduction that do not overlap, so the current in the neutral is burst of alternating polarity at 150Hz - with no 50Hz component.

The fact the phases are so balanced is suspicious - maybe the bulk of the load is made up of 3 banks of identical single phase pumps with VSDs without reactors or something, or do you have hundreds of LED drivers each below the minimum wattage to require built in correction..

Is the neutral wiring running hot, and have you been asked to look at this because there is a problem ?

Are there unusual loads in the building?


regards Mike.

These numbers are high but certainly not impossible for a building with a lot of electronic control. I remember a post by Zs a few years ago, relating I think to the large art gallery in the house of someone very rich in London, where all the lamps were on dimmers for  computer controlled fading.

Basically this arises if the current drawn per phase is 100A RMS, but that takes the form of a waveform with a lot of conduction at one point in the cycle, so each phase is 100A RMS, but that is not at 50Hz, but has a 50A 150Hz component - as Dave notes that is quite square, (with a large dead-band so a 3 level waveform up zero and down) but certainly not impossible, and used to be quite common on early power supplies before regulations on waveform quality came into effect .

As an example imagine bursts of current flowing between L and N of twice the RMS current but only for the +/-45 degrees nearest the peak of the voltage, and very little current flowing on the side-slopes (this is not an incredible waveform and is typical of rectifiers driving capacitors with inadequate pre-rectifier filtering).

You will need 3 coloured pens and the ability to sketch ;-)

If you repeat this for all 3 phases, it is clear that there are 6 isolated bursts of conduction that do not overlap, so the current in the neutral is burst of alternating polarity at 150Hz - with no 50Hz component.

The fact the phases are so balanced is suspicious - maybe the bulk of the load is made up of 3 banks of identical single phase pumps with VSDs without reactors or something, or do you have hundreds of LED drivers each below the minimum wattage to require built in correction..

Is the neutral wiring running hot, and have you been asked to look at this because there is a problem ?

Are there unusual loads in the building?


regards Mike.