Neutral Current Standard

[MCWONG]

Dear all,
Is there any one clear about the standard of neutral current ? I search for many data ,but there is unbalance standards for voltage. I cannot find any standards for neutral current.
Please, help!
MC

 

[Shortstub]

The question you pose is analagous to "what color should a car be?" I'm not trying to be facetious, but, there are too many variables to consider for a standard. Following is an overview of the discipline referred to as "Neutral Grounding:"

In low voltage systems the neutral is frequently directly or "solidly" grounded. Thus, the resultant current flow is significant enough to operate protective devices.

In medium and high voltage systems, the neutral current is intentionally restricted so that ground-faults currents are not injurious to motors and generators, yet high enough to counter the deleterious effects of system capacitance to ground. The former condition insures that, althought detectable, they won't melt stator iron. conversely, the higher magnitudes are used to offset resonance conditions.

If I have misunderstood your query, or you have a particular application in mind, provide details!

 

[busbar]

To add to Shortstub's worthwhile comments — In ANSI regions, neutral current seems to be viewed as a side effect of harmonic flows—indirectly associated in IEEE Std 519. 519-1992 is based on problems at the “point of common coupling;” id est, where the utility and their customer meets. For that reason, hard numbers in consensus standards are harder to find. The typical fix at lower levels is to install a retrifit or new [possibly “K-rated”] delta-wye transformer that effectively ‘traps’ neutral current, preventing it from being reflected into heavier power circuits.

 

[busbar]

I should qualify the previous posting as only my impression of the situation. [I could be way off.]

 

[peebee]

The standard for neutral current is the same as for phase current: any current less than the ampacity of the conductors is acceptable.

 

[MCWONG]

Thank you for your relpy!
I checked the IEEE519-1992, but there is only "Ground Current : 0.5A rms" , and "the ground-fault current is limited to be less than 60% (ususlly less than 25%) from IEEE C62.92.4-1991". However, in normal operation, it seems that "Any current less than the capacity of the conductors is acceptable". In our site, the rule is half value of the rated phase current during construction is the capacity of the neutral conductor. But, it may effect the output of the transformer's voltage.
For example, the THD of current for low capacity is 20% that is the limited value for good quality of power from IEEE519-1992.
I try to find that number for neutral current with good power quality. I am not sure whether the "Ground Current : 0.5A rms" is the answer. Due to the limited resource, I don't know whether there is any other standards.
I make a system to compensate the THD of current under 20%, but I have to find a number to compare the neutral current under a limit. Please, hlep. Thank you very much!

 

[jbartos]

Suggestion: There are various neutrals in power distribution systems. E.g. there is a transformer neutral, generator neutral, motor neutral, system voltage neutral.
The system voltage neutral is the one that can have the current as high as the conductor ampacity rating permits. In some voltage systems, it is actually doubled.
The transformer/generator neutral current strongly depends on the number of wire of the power distribution system. E.g. if you have three-phase four-wire system, i.e. three-phase plus neutral, then the neutral current can increase up to neutral conductor ampacity rating.
However, if you have a three-phase three-wire system, then the neutral current is usually limited by some system grounding method protective scheme at transformer/generator, e.g. high resistance grounding, etc. In this case, the transformer/generator neutral current can be limited to 5A or 10A depending on the system grounding design.
So, all what is needed is to make certain that the neutral is interpreted and treated correctly, and relevant current values are being considered.