Minato Sama
Member
- Location
- Phnom Penh, Cambodia
I have problem! The amp of each phase is 25A, 20A, and 22 A. But the current on the N cable is 53.7A. How can I solve this problem? and what is the reason of this problem?
Unexplained neutral current
- Thread starter Minato Sama
- Start date
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kwired
Electron manager
- Location
- NE Nebraska
The neutral imbalance from your loads should be about 4.5 amps. The rest is probably coming from outside someplace. There are several possibilities depending on what you have there but is likely either from bad neutral at a neighboring facility or a bad POCO primary neutral. How much current is measured on your GEC? If you have other conductive paths between your facility and other facilities the current could be coming via those paths also.
- Location
- Massachusetts
Minato Sama, I moved your question into it's own thread.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
I know low power factor can increase neutral current, but 50 amps is a lot.
He already has his own thread...
http://forums.mikeholt.com/showthread.php?t=139525
...and you closed shockingdave's...
http://forums.mikeholt.com/showthread.php?t=136651
- Location
- Massachusetts
He originally posted his question in shocking Dave's thread, I moved his question from Dave's thread to here and closed Dave's thread from last May.
Any other questions?
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
I didn't have any questions to begin with...
... but I'm sure I can pull a few out of my... e-r-r-r hat, if I thinjk really hard
kbsparky
Senior Member
- Location
- Delmarva, USA
What happens to your neutral amps readings as you shut down loads on each phase one at a time? Can you do this, and report your findings back here?
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
What are the loads? You may be looking at a serious harmonics issue.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
Since this is apparently a three phase unbalanced system, then it requires analysis using symmetrical components. Where did this 4.5A come from?
Anywho........
Non-linear devices, which includes florescent lighting, can have a substantial third harmonic component. The third harmonic, or triplen, as well as other harmonics divisible by three (i.e. 6th, 9th, 12th) are in-phase on the supply phases. These currents will sum up on the neutral to three times the individual phase currents. So, in this case you could have a neutral current as high as +60 amps.
Switch mode power supplies can result in neutral currents that exceed the current of an individual phase. When operated on a three-phase system, they can also produce neutral currents up to 1.4 times the phase current.
My gut instinct says the underlined statements are contradictory. :slaphead:
Perhaps just a slight error in the thought-to-word transference occurred. Happens to me, too [occasionally]. Just saying, "These triplen currents will arithmetically sum on the neutral" would suffice. In order for triplen currents to effect a neutral current of three times the individual phase currents, the triplen currents would have to have the same magnitude as its fundamental. Though possible in theory, and perhaps producible in a staged experiment, the occurrence in practice is very improbable... as your second statement correlates.
:thumbsup:
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
I see your point, maybe too much info was provided.
The two statements relate to two different pieces/types of equipment. One is fluorescent lighting/non-linear devices and the other is specific to switch mode power supplies. Perhaps it was not that clear.:huh:
The two statements relate to two different pieces/types of equipment. One is fluorescent lighting/non-linear devices and the other is specific to switch mode power supplies. Perhaps it was not that clear.:huh:
lauraj
Senior Member
- Location
- Portland, Oregon
It was my understanding that neutral current caused by harmonics should not be more than 1.73 x phase current.
Well the device will make some difference, so let's talk about the one you say will have up to 3 times the neutral current. That being flourescent lighting and other similar non-linear devices. I will still say you cannot have 3 times the neutral current as each of three phases. For the discussion, the fundamental frequency is 60Hz and the power factor of the three phases is identical. I'm sure we will agree that on a balanced system the fundamental frequency currents cancel at the common neutral node.
Now, say for example you only had third harmonics (180Hz) on each phase conductor. You will have 3 times the third harmonics on the neutral as you do on each phase. Right? (<redundant) But what about the fundamental currents that are canceling out at the neutral node. Are they not also measured as phase current? (<also redundant). So in order to have 3 times the neutral current as on each of three phase conductors, there cannot be any fundamental current...???
Perhaps... but I see that as more of a practical limit rather than a theoretical one. For a theoretical example, let's say we connected, on each of the three phases, "perfect" line-to-neutral 180Hz sine-wave current sources with identical loads. The neutral current would be 3 times that of each phase. The challenge is then creating the "perfect" scenario, which is not practical. Yet we can see that a 1.73 max factor would be one of practicality.
slalom skier
Member
- Location
- Royal Palm Beach FL. USA
Say you had this problem on a single phase panel would there be any additional effect from it being fed from a 3 phase panel?
The problem of differing power factors and or harmonics are, shall we say, less discussed for single phase... ahhh, correction... split phase panel feeder neutrals. That is not to say the potential does not exist, but with the ungrounded conductors being 180? out-of-phase with respect to the neutral, the the conditions which cause additive neutral current are different and not as likely to occur. This would include single phase panels fed from a typical delta-configured 3? 4W secondary.
However, a single phase 120/208 panel is entirely different in that it is [typically] fed from a wye-configured 3? 4W secondary. In this case, there is no third "phase" current to cancel fundamental current from the other two "phase" conductors. Even the fundamental currents of line-to-neutral loads are partially additive towards neutral current. For example, two resistive-only 10A L-N loads on different phases will contribute 10A [5A each] to neutral current. Triplen harmonics will add more yet if the loads were non-linear.
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