High 3rd harmonic on the neutral

Perhaps someone who can would like to illustrate a SMPS which gives a harmonic content of 50%? Unfortunately they don't work like that. This, as someone said above, must be odd harmonics, and every supply must be the same. The way forward is to show us the neutral current waveform, this is not rocket science. The waveform would have to be almost completely square to have a harmonic content of 50% (from Fourier). I suggest this is simply a measurment problem.

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.

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.

Well LEDs below 25watts per fitting can have a power factor as low as 50%, (40% below 5 watts)  (above 25 watts it is a more sensible 90%), and laptop power supplies have a similar stepped rating, but the step is at 75 watts input power, above which the current waveform has to be properly corrected.

I'd be surprised if it was that bad just from a few laptop supplies, but if you have LED lights as well. 

Very roughly you can relate PFC to harmonic content as

1/(1+(THD)^2)


Where THD is the total harmonic distortion,  THD= sqrt(  I 2F^2 + I3F^2 ..)  /I fundamental.


So if you had basically a bridge rectifier feeding a capacitor gives the 'bursty' current waveforms

some images to help, I hope.  

Supply voltage black, current green, heavy black the 'DC' on the smoothing capacitor.



On the AC side this becomes more like the following - Voltage is red, current is blue ; some waveforms are sine, this one really isn't.




  regards Mike

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

I think the OP is suggesting that the total harmonic distortion as a % is quite high - as only triplen harmonics will be additive in the neutral, it would be useful to see the harmonic distortion of just the triplens 


% Harmonic distortion is a useful metric, but it needs other data to draw a conclusion as pointed out above


300% THD doesn't seem a credible value to me


Regards


OMS

What’s the earth leakage like?

it's not anywhere near full occupancy yet. The majority students are due back in the next couple of weeks.

Are there any other sources in parallel? Sometimes you can get circulating currents if installation is not looked carefully...