Neutral currents 1

[lyledunn]

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? Regards,

Lyledunn

 

[mpparent]

Lyle,

Sure have. Typically, I've either used the 200% neutral sizing rule of thumb or used a passive filter.

Mike

 

[infomaniac]

lyledunn,

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.

 

[dpc]

I agree that third harmonic currents could be the source of the excessive neutral current you are seeing. In addition to switching power supplies, fluorescent lighting ballasts and HID ballasts are sources of third harmonic 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.

 

[busbar]

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 Power Quality.htm

 

[lyledunn]

Many thanks guys. Busbar your references have been very useful and very enlightning.The wonderful world of electrical engineering, you never stop learning! Problem is the capacity of this old cranium is a bit like this old computer; filled and with little space left! Regards,

Lyledunn