Harmonics in neutral conductor

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fsavoie

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There is some place here that their ballast (T8 120V electronic) keep failing (several on each circuit per month).

Here what I have :

Phase A: 82A
Phase B: 94A
Phase C: 81A
Neutral: 19A

THD%f

Phase A: 9
Phase B: 6.9
Phase C: 7
Neutral: 485

All the neutral harmonics are coming from the 3rd one in the neutral cable (473.4).

I know that electronic ballast have harmonics between 18 to 32%. So the harmonics in the phase A,B,C seems quite normal, but the neutral one is alot? can that be the cause of the ballast failling?
 
090218-1010 EST

Your % harmonic content is measured in relationship to the magnitude of the fundamental.

Your neutral fundamental is relatively small because the 3 loads are nearly equal and the fundamentals cancel in the neutral. The third harmonics do not cancel and therefore relatively speaking become large compared to the fundamental.

My guess on failures would be overvoltage, marginal design, or marginal components, and the last two are a manufacturer quality issue.

.
 
Remember that the harmonics are being expressed as a percentage of the fundamental current flow. Further the 3rd harmonic currents will add on the neutral.

If you somehow measure the number of 3rd harmonic amps, you will find that they agree across all of the conductors.

As to the third harmonic somehow damaging the lamps, I don't know if that is a possibility. You have provided the 3rd harmonic _current_ numbers, but would also need to see if that harmonic current flow is causing significant _voltage_ distortion.

-Jon
 
The harmonic on voltage is

Phase A 3.1%
Phase B 3%
Phase C 3.1%
Neutral 105.4%

3rd harmonic on neutral is 81%

The neutral to ground voltage is 0.03 so I guess, the harmonics can't be the cause of the ballast damage.

Thanks for your helps!
 
I don't much about harmonics but I'd call the ballast manufacturer. My guess would be that either they had a bad run of ballasts or that they in general produce low quality units. If it's that they had a bad run of them they might offer to replace them if they are out of warranty.
 
76nemo said:
FOUREIGHTYFIVE, did I read that right???
Click to expand...

Right, neutral harmonic distortion is 485% compared with fundamental 100%.

So 4.85 times more harmonic current measured than fundamental current. This is certainly possible and is explained by gar and winnie above with relatively balanced loads, the fundamental currents will cancel and subsequently will be small, third harmonic currents will add.
 
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