Current in Neutral Wire

Suggests that the load is not too well balanced across the phases. This may be a problem, maybe not, it all depends on the application and the network its connected to.

In = I phA + a.I ph B + a²I ph C = 0

if there is ballance load, I ph A = I ph B = I ph C, so In = 0.

ex : 100 + 100<120⁰ + 100<240⁰ = 0

100 + 200<120⁰ + 100<240⁰ = 100<120⁰

so, if we design Ib to have load twice than another, that will cause I in neutral 100 A.

100 + 150<120⁰ + 100<240⁰ = 50<120⁰

so, if we design Ib to have load 1.5 times than another, that will cause I in neutral 50 A.

Is 37A much or not much depends on the size of the transformer.

Is this a single phase transformer or a three-phase?

Is your plant equipment working correctly (ie- have you got a phase imbalance due to an equipment fault)?

When did this start (was everything fine before you installed that new piece of machinery)?

First guess (and it is a guess based in the little information provided) is it is due to harmonic currents from variable speed drives or lights (etc) on your site.

http://en.wikipedia.org/wiki/Power_factor

Either that or single phase load current due to single phase loads connected between phase and neutral (hence the neutral current).

Can you please supply more information.

Information is not enough to guide you. What is transformer rating? Primary/ secondary voltages. When 37 Amps on neutral what the Phase currents. In majority what types loads? What is the PF and lastly what is the value of capacitor you are using to get this corrected PF?

Using many single phase Switching power supplies also cause high neutral currents.

The neutral current is the arithmetical sum of the single phase loads, but taking into account the angle of the phases.

So if you are measuring a heavy current on the neutral, then the phases are unbalanced for load. Redistribute the loading till the phases are balanced out better.

Though its usually difficult to get and keep such loading equal all the time.....time of day etc. will make a perfect balance usually impossible to achieve. Some thoght needs to be incorporated into the load balancing.......

May I suggest a clamp on ammeter and make a few measurements during a typical day in the week and a day at the weekend say measuring once per hour the load on each phase and noting it down carefully. You may see a way to balance things out far better....

If this is a main line to an average house ( single phase ) and your main 110 volt lines are rated for 100 amps, it would be considered excessive. Since the neutral shows the difference in current between each 110 volt line, balancing the load on each line should get rid of most of this. It will change as different loads are applied to each line, so this current will keep changing as things are turned on and off. Try to get this value as close to zero as you can while you are running the house normally. Any heavy load devices such as 115 volt air conditioners , washers and dryers, pumps, etc should be divided up between both lines if they are usually on at the same time. If you're not talking about the average house, let me know and I will try to give you advise as everything I just said probably will not apply.

Assuming that the transformer is a step-down distribution transformer with secondary star-point solidly earthed with copper/aluminium conductors.In India,the normal LV supply should be 3 phase 415V and 240V 1 phase.As no KVA of transformer is given,I am taking the example of 1000KVA which can supply 1390A per phase.Proportionate accordingly.

37A in neutral is not a problem at all, if the neutral conductor is large enough.By regulation for very old installation it should be approximately half the cross section area of the phase cables.For already many years,IEEE requires full size neutral ,the same as phase conductors.For me,I shall check the temperature of this neutral cable by touching it with my hand if it is insulated.If it is bare conductor,I touch it with 2 layers of dry paper wearing dry shoes.If I can touch it for a few seconds,it is less than 60 degree centigrate, there is no danger of failure.

The tendency is for electriciams to tap single phase power supply from the Red or phase A when new devices are added.Accumulated over the years,this cause single phase power comsumption to be unbalance over the 3 phases.This cause the neutral current to flow which is obeying Kirchoff"s current law.Measure or note currents on each phase at different time of the day and shift the load at sub-DB or DB from the phase that is carrying more to the phase that is carrying less.Neutral current due to harmonic is more difficult to eliminate.

Regards,

Khor

Current in the Neutral due to Harmonics can either be accomodated or suppressed. To accomodate the effect of third Harmonics, neutral size can be increased, transformer capacity can be increased, zig-zag transformers can be use etc. etc. There is also a Patented Technlogy available in USA which can suppress (or elimate) the effects of third Harmonics in a three phase four wire distribution system with non-linear single phase loads connect between line and neutral of a delta-wye distribtuion transformer.

Please contact me at mmfasf@yahoo.com to get more details of this commercailly available Patented Technology in USA suitable for 415/230V 50Hz Transformers as well.

Need not worry as long as the neutral wire is sized to carry this current continuously.

In a existing 3 phase 4 wire electrical distribution system of a building the neutral size will not be sufficient to handle the additional current when non-linear loads are added to the existing system. Because the neutral wire of the exisitng system may have been sized ONLY to handle the imbalance currents.

Third Harmonic zero sequence current due to non-linear loads add up in the neutral. Additional non-linear loads cannot be plugged into a existing distribution system with normal size neutral