Neutral currents
Neutral currents
(OP)
In the 3phase and neutral 400v supply cables in an office block my simple (but recently calibrated) clamp meter is indicating considerably more current in the neutral than would have been expected with vectoral addition.
Now there are no problems as the neutral is of appropriate cross section, however, I was wondering if this is a result of harmonics due to the Switch mode power supply units that I guess are in much of the IT equipment.
I understand that in such a system it is possible for the rms neutral current to be 1.73 times the rms current in the phase conductors. This is really the first time I have come across this in practice. Has anyone else met with a similar problem? Is it likely that csa of neutrals is likely to be carefully considered in future instead of just selecting same csa as associated phase conductors for load?
Now there are no problems as the neutral is of appropriate cross section, however, I was wondering if this is a result of harmonics due to the Switch mode power supply units that I guess are in much of the IT equipment.
I understand that in such a system it is possible for the rms neutral current to be 1.73 times the rms current in the phase conductors. This is really the first time I have come across this in practice. Has anyone else met with a similar problem? Is it likely that csa of neutrals is likely to be carefully considered in future instead of just selecting same csa as associated phase conductors for load?
Regards,
Lyledunn






RE: Neutral currents
Sure have. Typically, I've either used the 200% neutral sizing rule of thumb or used a passive filter.
Mike
RE: Neutral currents
Your assumption is coorect I believe. If you have a lot of switch mode power supplies, there could be a significant amount of neutral current. This occurs because switch mode power supplies produce a large amount of 3rd harmonic currents. Unfortunately, 3rd harmonic currents are zero sequence components and therefore will add together in the neutral. Therefore, it will not take long before a sufficient level of harmonic content is present. I understand that the 200% rule is often used.
RE: Neutral currents
In the old days, we use to specify reduced size neutrals for panelboards, switchboards, etc. No one does that anymore - everything is at least 100% of phase rating.
RE: Neutral currents
An alternative to neutral heavyup in feeders is a local zig-zag autotransformer connected to the 4-wire wye system.
Thread238-28245
http://www.iee.org/Policy/Areas/Electro/annb2.doc
http://www.liebert.com/support/whitepapers/documents/sl_24230.asp
http://www.pdicorp.com/engineering/SS-TT.pdf
http://www.ladwp.com/energyadvisor/EA_47.html
http://www.acmepowerdist.com/ael12.html
http://www.lovornengineering.com/Protecting%20Power%20Quality.htm
RE: Neutral currents
Regards,
Lyledunn