Excessive Neutral Current

[DistCoop]

All,

Is there a standard or reference out there describing neutral current or voltage/current imbalance limits (three phase 4.16kV wye overhead, United States)? I'm working on a study and am finding imbalance as much as 30% which I know is high, but I'd like to be able to point to a standard.

Thank you

 

[stevenal]

ANSI C84.1

https://www.nema.org/Standards/ComplimentaryDocuments/Contents-and-Scope-ANSI-C84-1-2011.pdf

 

[stevenal]

Annex C in the above recommends a voltage unbalance of no more than 3%. The accompanying graph shows a motor derating factor of about 0.9 at the 3% mark.

 

[jghrist]

There is no standard for current imbalance. Neutral overcurrent relaying on a feeder cannot be very sensitive with a high imbalance, and you have to account for the possibility of a single phase branch fuse blowing as well. Losses and voltage drop on the distribution system increase considerably with imbalance. Correcting imbalance to some degree can usually be justified by loss reduction and/or voltage drop reduction.

We have used a criteria of 15% maximum current imbalance in studies. We have recommended shifting loads to achieve 10% if the imbalance is more than 15%. This is based on the practicality of shifting loads, however, not on an industry standard.

One problem with studies is that records of what phase transformers are connected to are often not accurate.

 

[prc]

With present day electronic loads at distribution level, neutral may carry substantial third harmonics currents, sometimes exceeding phase current.So in K -Transformers neutral leads are made with same or more area of cross section as line leads.

 

[bacon4life]

To qualify for BPA's distribution efficiency funding, BPA requires no more than 15% imbalance and no more than 40A of neutral current.

http://www.bpa.gov/energy/n/industrial/Distribution/pdf/Final Draft_VO_MV_Protocol 4-27-10.pdf

Although we are attempting to achieve this goal, there are some feeders where phases have variations in daily or seasonal loading (i.e. phase A is always high in the summer/day but phase C is high during the winter/night). We also have a number that are reasonably balanced on normal load days, but are dramatically more unbalanced during extreme temperature.