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High 3rd harmonic on the neutral
MS01
28 Posts
Question
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?


59 Replies
I assume the overall neutral current is nearly zero. The harmonic currents do not necessarily cancel on the neutral so the %age figure is rather misleading, and not of much use. Can you measure how many amps this is, and compare this to the phase currents, you will be much happier, or perhaps something strange is going on but I doubt it.

Regards
David CEng.
mapj1
2467 Posts
300% of what ? The average neutral current ? The most heavily loaded phase? -
As noted above, if the 50Hz neutral current is low, then even a modest 150Hz contribution from each phase may look enormous as a fraction of that. No need to panic unless it is more than perhaps 20% of the most heavily loaded phase.
MS01
28 Posts
The highest phase it at just over 100A with the other phases there or there abouts but the neutral current is at 50A
Those figures are not possible. If all 3 phases are taking 100A there will be zero neutral current. The vector difference of 50A says that they are not all similar. What are you testing with?
AJJewsbury
1881 Posts
Those figures are not possible. If all 3 phases are taking 100A there will be zero neutral current.
Isn't that just the OP's point though - with 3rd harmonics the currents DON'T cancel neatly in the N (indeed they can add).
    - Andy.
MS01
28 Posts
Yes we know in a normal installation with the phases balanced the neutral current would be zero but odd harmonics do not cancel out and infact add current to the neutral which is why I have a high neutral current. Anyone any ideas why, surely this just can't be non linear loads.
I remember someone who was better at maths than I explaining that the worst case is a neutral 3rd harmonic twice the phase current.
And this happens when computer power supplies are being run at about 60% of full load - which, of course, is what most of them do run at.
However I would be a bit suspicious of any meter which is designed for use on 50Hz giving an accurate reading at 150Hz. I'm sure some do, but maybe not all and I would not expect this in a block of flats, a data centre yes.
AJJewsbury
1881 Posts
I would not expect this in a block of flats, a data centre yes.
I wonder. If the flat's don't have a large resistive load (e.g. space/water heating is by gas or communal heating system) - what's left? Much of the stuff that's left on standby these days will have electronic PSUs (most likely switchmode) the washing machine's motor is likely got the domestic equivalent of a VSD in front of it, probably the vacuum cleaner too (or is battery powered with an electronic PSU to recharge it). Even the simple oven I installed recently demanded an A-type RCD as presumably it temperature controls via power electronics rather than the old clunk-on-clunk-off thermostats. Lighting is now CFL or LED with electronic ballasts/drivers. So maybe it's only the kettle and toaster left as nice clean resistive loads, maybe a tumble dryer - but they're not on very often. Maybe we are drifting towards domestic being as troublesome as datacentres.
   - Andy.
Sparkingchip
3167 Posts
If there are lots of people working from home are blocks of flats data centres?
Throw in all the entertainment kit and as Andy J says there’s not much else left in the way of a load.
Denis McMahon
292 Posts
What type of substation transformer supplies this?  Delta-star?  Star-star?  Something else?
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.
mapj1
2467 Posts
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.
MS01
28 Posts
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. 
mapj1
2467 Posts
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
John Peckham
494 Posts
It can't be that big a block of flats at 100A a phase?
 
Chris Pearson
1763 Posts
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).

OMS
719 Posts
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
Sparkingchip
3167 Posts
What’s the earth leakage like?
MS01
28 Posts
 it's not anywhere near full occupancy yet. The majority students are due back in the next couple of weeks. 
Chris 223
13 Posts
Are there any other sources in parallel? Sometimes you can get circulating currents if installation is not looked carefully...
broadgage
382 Posts
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.

mapj1
2467 Posts
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
 
Sparkingchip
3167 Posts
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 

Chris Pearson
1763 Posts
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.

Sparkingchip
3167 Posts
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 

 

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