That WAS all true in the 'good old days' when the majority of the loads were resistive. In these days of non-linear loads (ie. anything based on a switch mode type front end) the presence of third order harmonics in the current waveform (which ADD in the neutral & don't cancel) mean its very possible that the neutral current can greatly exceed the highest phase current. Have a google for 'triplen harmonics' and the now realised need for INCREASED cross sectional neutrals of three phase circuits instead of the 'older' wisdom that the neutral could be reduced cross-section.
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That WAS all true in the 'good old days' when the majority of the loads were resistive. In these days of non-linear loads (ie. anything based on a switch mode type front end) the presence of third order harmonics in the current waveform (which ADD in the neutral & don't cancel) mean its very possible that the neutral current can greatly exceed the highest phase current. Have a google for 'triplen harmonics' and the now realised need for INCREASED cross sectional neutrals of three phase circuits instead of the 'older' wisdom that the neutral could be reduced cross-section.
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Well, yes, that's why I asked/said:
I wouldn't actually have thought that SMPSUs were a major issue in this respect, are they? Although they are certainly ubiquitous, they most commonly exist as part of pretty low power loads, so I wouldn't not have expected them to have much impact on the total neutral current in the sort of installation that warrants a 3-phase supply ... or have I got it wrong?
Kind Regards, John
There are two ways that the neutral current can exceed the phase currents.
One is if the loads on different phases have different voltage/current phase relationships. This can happen with capactive and inductive loads. It can also happen if there are loads on some phases and sources of power on others.
Another is harmonics. Third harmonic currents will sum instead of canelling in the neutral (assuming all the devices producting those currents do so with the same phase behaviour). In prinicple 6th, 9th and so-on harmonic currents will do the same but these tend to be much weaker than the third harmonic currents.
Lighting dimmers are apparently horrible for harmonics.
DC power supplies (switched mode or otherwise) have historically been pretty bad but in the last decade or so there has been regulatory pressure to clean up. However small power supplies below 75W are not covered so large numbers of small power supplies could theoretically still be a problem. There can also be problems if a large number of old PSUs or (more likely) imported PSUs that don't follow the regulations are used (AIUI the regulations aren't very well enforced).
Saying the neutral current will never exceed the highest phase current is not strictly true. In many cases it's a reasonable assumption to make but one should be aware that there are circumstances that can break it.
One is if the loads on different phases have different voltage/current phase relationships. This can happen with capactive and inductive loads. It can also happen if there are loads on some phases and sources of power on others.
Another is harmonics. Third harmonic currents will sum instead of canelling in the neutral (assuming all the devices producting those currents do so with the same phase behaviour). In prinicple 6th, 9th and so-on harmonic currents will do the same but these tend to be much weaker than the third harmonic currents.
Lighting dimmers are apparently horrible for harmonics.
DC power supplies (switched mode or otherwise) have historically been pretty bad but in the last decade or so there has been regulatory pressure to clean up. However small power supplies below 75W are not covered so large numbers of small power supplies could theoretically still be a problem. There can also be problems if a large number of old PSUs or (more likely) imported PSUs that don't follow the regulations are used (AIUI the regulations aren't very well enforced).
Saying the neutral current will never exceed the highest phase current is not strictly true. In many cases it's a reasonable assumption to make but one should be aware that there are circumstances that can break it.
Yes, I accept that.
Indeed - that's what I was acknowledging in my initial comment.
Yes, I understand that, although I had overlooked it when I made my initial comment. However, as I said in my last post, I would think that the great majority of sources of third harmonics (or 6th, 9th etc.), such as you mention, would relate to relatively low current devices (hence, presumably, relative low 3rd harmonic currents) - which is why I was suggesting that they were unlikely to have a major impact on total neutral current in a 3-phase circuit. What sort of high-current loads (if any) might produce very high 3rd harmonic currents (i.e. 3rd harmonic currents which were a significant proportion of the corresponding fundamental phase current)?
Kind Regards, John
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The two main scenarios I can see
1: large lighting dimmer setups (theatres etc).
2: situations were a large number of non-pfc DC power supplies are used either because the power supplies are very old, the power supplies didn't meet regulations but ended up getting used anyway or the power supplies are too small to be covered by the regulation. Each individual device may not be that bad but the cumulative harmonic current can get nasty.
1: large lighting dimmer setups (theatres etc).
2: situations were a large number of non-pfc DC power supplies are used either because the power supplies are very old, the power supplies didn't meet regulations but ended up getting used anyway or the power supplies are too small to be covered by the regulation. Each individual device may not be that bad but the cumulative harmonic current can get nasty.
Fair enough - so, as I expected, only pretty rare situations, then?
Kind Regards, John
Not as rare as you might think..... sitting here at home (4 bed house, 2 adults, 2 teenage kids) the load current at the moment is 17.9A of which 1.4A (8%) is 3rd Harmonic, pf = 0.991 cap (I know.... its sad that I can measure this in real time!). I suspect when the immersion heater cuts out, the ratio of 3rd to 1st will rise steeply!
edit: The immersion heater has know cut off-..... 1st harmonic is 6.7A, 3rd order is 1.2A = 16.5%, THD-I = 24%
This is coming from the 4 PCs we have on, each with its LCD monitor, the LCD TV in the lounge... all of which have SMPS at the front end, together with heaven knows how many wall-wart type phone/ipad/tablet chargers.
Another culprit is inverter drives running motors - these are becoming more common in washing machines, fridges and a whole host of domestic appliances.
As we get more & more devices using switch-modes I think harmonics in the distribution system are going to become more of an issue. We deliberately don't include OCPD in the neutral of a three phase system so nothing will detect overcurrent in that conductor - it will sit there quietly cooking!
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Fair enough, but I think you almost make my point. Yes, if one assumes that ~13A of that 17.9A is your immersion, when it goes off the 3rd harmonic will presumably become about 29% of the total, but it will still only be 1.4A. That sort of current is not go to be much of an issue - and (probably because I haven't thought enough about it!) I'm not sure what the problem is with harmonics within a single-phase system.
Right - so the problem, if harmonic currents continue to become more prevalent, is going to be in the (3-phase) distribution networks?
Kind Regards, John
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Yeah, DNO level distribution and distribution within large buildings or sites is where the concern is/was.
Looking at these numbers I think it's unlikely that they would cause a neutral overload unless combined with a very unblanaced loading situation where the whole load had similar harmonic characteristics. I think this is evidence that the current level of regulation is sufficient.
In that situation does CT no longer stand for Current Transformer?
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