High 3rd harmonic on the neutral

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.

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.

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?

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.

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.

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.

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.

What type of substation transformer supplies this?  Delta-star?  Star-star?  Something else?