Harmonics?

[joeyww12000]

Why does sharing neutrals cause potential problems with electronic ballasts?

 

[Smart $]

Why does sharing neutrals cause potential problems with electronic ballasts?

I think you are being misinformed.

 

[dbuckley]

Because on a polyphase system with non-linear loads the triplen harmonics add, which can cause the neutral current to be greater than any phase current, and often twice the phase current.

Thus your 100% rated neutral may overheat through overloading.

 

[brian john]

and often twice the phase current.

Though I have never seen this on lighting multiwire circuits.

 

[Smart $]

Though I have never seen this on lighting multiwire circuits.

Theory vs. Reality... the source of a lot of disinformation.

Some people get stuck in theory and forget we actually get to 'practice' it :grin:

 

[brian john]

ZACTLY


But then again Not twice the phase current, but HIGH

 

[Smart $]

Because on a polyphase system with non-linear loads the triplen harmonics add, which can cause the neutral current to be greater than any phase current, and often twice the phase current.

Thus your 100% rated neutral may overheat through overloading.

Possible... but highly improbable, IMO.

 

[Smart $]

ZACTLY
But then again Not twice the phase current, but HIGH

But then again, that's not exactly a graph of an electronic ballast lighting system... :grin:

 

[cripple]

Harmonics?

You might want to look this web page; harmonics are real and here to stay.
http://www.dranetz-bmi.com/pdf/harmonicsall.pdf

 

[don_resqcapt19]

You might want to look this web page; harmonics are real and here to stay.
http://www.dranetz-bmi.com/pdf/harmonicsall.pdf

Like all most all information about the problems caused by harmonics, this one is written by a company that has a vested economic interest in solving or detecting the problem. I am not saying that they don't exist, but they are not near as big of a problem as you are led to believe they are.

 

[76nemo]

If one is to pull one neutral per phase from the get go, how can we go wrong???? Isn't that better than jeopardizing overheating the tranny? Harmonics are real, and will become more of an issue as the electronic world evolves.

If anything installation wise is worth doing, it worth doing it right from the get go.

 

[winnie]

I'd suggest not turning this into a 'shared neutral' versus 'separate neutral' debate, and just focus on the issue of harmonics.

To the extent that harmonics are an issue, running separate neutrals to the various branch circuits will not change the harmonic neutral loading of the transformer in a significant way. The triplen harmonics will add up, on the common neutral of the MWBC, or if you don't use MWBCs, then they will add up at the neutral bus of the panel, and if you have separate panels for each phase, they would add up at the transformer X0 terminal.

If you wanted to eliminate the issue of triplen harmonics, then you could use single phase systems, with or without MWBCs, as you prefer. (Note, in this case by 'single phase' I am specifically excluding 'network' services derived using two legs of a three phase wye service.)

I tend to agree that they are more of an issue in theory than in practise, there are infrequent reports of neutral overloading caused by harmonics. While we are seeing more and more switching power supply loads, more and more of these loads are being designed with power factor correction, specifically to avoid the harmonics issues.

-Jon

 

[brian john]

76:

The separate neutral will do nothing to protect the transformer, the 3rd harmonics will still be there.

cripple:

If I sold harmonic analyzers I'd be pushing the product hard (as Don pointed out). I have been doing harmonics surveys since the word became prevalent in the late 70's. In all those years, I have seen 4 or 5 major issues* on utility and maybe 20 or more while connected to a generator. And none to date have been from lighting branch circuits.

In addition my firm perform IR surveys, scanning 1000's of lighting panels a year most utilizing multi wire branch circuits, all thermal issues noted are from improper phasing of the conductors.

*most of these issues are from early UPS's, large printing presses with multiple frequency drives on under sized distribution systems, casinos (electronic slots) and call centers.

 

[76nemo]

I'd suggest not turning this into a 'shared neutral' versus 'separate neutral' debate, and just focus on the issue of harmonics.

To the extent that harmonics are an issue, running separate neutrals to the various branch circuits will not change the harmonic neutral loading of the transformer in a significant way. The triplen harmonics will add up, on the common neutral of the MWBC, or if you don't use MWBCs, then they will add up at the neutral bus of the panel, and if you have separate panels for each phase, they would add up at the transformer X0 terminal.

If you wanted to eliminate the issue of triplen harmonics, then you could use single phase systems, with or without MWBCs, as you prefer. (Note, in this case by 'single phase' I am specifically excluding 'network' services derived using two legs of a three phase wye service.)

I tend to agree that they are more of an issue in theory than in practise, there are infrequent reports of neutral overloading caused by harmonics. While we are seeing more and more switching power supply loads, more and more of these loads are being designed with power factor correction, specifically to avoid the harmonics issues.

-Jon

Regardless of what we are now seeing as far as loads being determined with PF in mind, I see no harm in running multiple or oversized neutrals. How can you determine what is plugged in at the recep.? Whether or not you are seeing neutrals overcome with extra loading, what may be added later may make a big difference. I am not talking of residences, I am talking of commercial/industrial facilities. One may speak of initial cost, I am talking about rework. Run it right from the get go.

 

[76nemo]

76:

The separate neutral will do nothing to protect the transformer, the 3rd harmonics will still be there.

cripple:

If I sold harmonic analyzers I'd be pushing the product hard (as Don pointed out). I have been doing harmonics surveys since the word became prevalent in the late 70's. In all those years, I have seen 4 or 5 major issues* on utility and maybe 20 or more while connected to a generator. And none to date have been from lighting branch circuits.

In addition my firm perform IR surveys, scanning 1000's of lighting panels a year most utilizing multi wire branch circuits, all thermal issues noted are from improper phasing of the conductors.

*most of these issues are from early UPS's, large printing presses with multiple frequency drives on under sized distribution systems, casinos (electronic slots) and call centers.

Brian,

Undersized neutrals ARE a problem. I am not sure what exactly you are trying to point out:-?

 

[brian john]

Undersized neutral have not been proven IN THE FIELD to be a problem, even in the example I posted the neutral current was close to the phase currents.

Isn't that better than jeopardizing overheating the tranny

How will individual neutrals resolve this?

 

[don_resqcapt19]

Brian,

Undersized neutrals ARE a problem. I am not sure what exactly you are trying to point out:-?

Can you cite a documented study of a real world installation, outside of a data center, that has had a problem because the grounded conductor was not larger than the ungrounded conductors?

 

[brian john]

What my point is

If I sold harmonic analyzers I'd be pushing the product hard (as Don pointed out).

In my expierence MOST harmonic issues are over blown and the issue is STRESSED by manufactures of equipment that can locate/analyze harmonics, or mitigate/mask them.

I have been doing harmonics surveys since the word became prevalent in the late 70's. In all those years, I have seen 4 or 5 major issues* on utility and maybe 20 or more while connected to a generator. And none to date have been from lighting branch circuits.

Most issues I have seen are related to 3 phase devices resulting in 5,7-11,13-17,19 ......harmonics. The neutral issues are all as mentioned in data centers, casinos and call centers NEVER WITH LIGHTING CIRCUIT's even with the issues I have seen with MWBC's (non-lighting) the neutral current never exceeded the phase currents (to the best of my recollection we are talking a period of almost 30 years)

In addition my firm perform IR surveys, scanning 1000's of lighting panels a year most utilizing multi wire branch circuits, all thermal issues noted are from improper phasing of the conductors.

1000's a year times almost 27 years of IR scanning is a fairly large data base with no MWBC neutral thermal issues other than the phasing issues noted on lighting circuits.

I think this is a non issue, in regards to lighting. BUT I am more than willing to review any valid field data that proves me wrong.

 

[pbeasley]

Can you cite a documented study of a real world installation, outside of a data center, that has had a problem because the grounded conductor was not larger than the ungrounded conductors?

Brian John, Try reducing current on any one leg to zero amps and see how your neutral is loaded. Harmonics are not the only reason to carefully evaluate neutral sizing.

There have been some instances of neutral overloading on both temporary and permanent installations of SCR/SSR based Dimming control systems. The idea being that in Live Entertainment it is extremely difficult to maintain a balanced load through out a performance and even if it is relatively balanced, the loads are almost never running at full voltage, the waveform is usually being chopped ALOT and the harmonics can be amazing. Most of the temporary systems now include connections for double neutrals... If the dimming system is loaded to capacity, then the neutral feeders should be doubled to accommodate. If your system is not fully loaded (which most systems end up at about 40-60% of rated capacity) then a single neutral is usually sufficient..

Let me know if you want more details and I'll see what I can pull together.

Back to the OP, as electronics get smarter and smarter they can be susceptible to irregularities in the wave form. I can see where, theoretically and depending on the technology employed, 1 ballast on one phase could confuse the zero crossing reference point for another ballast on a different phase when sharing the same neutral. I have also seen problems with some electronic low voltage transformers being very unhappy with irregular wave forms (running off of the aforementioned dimmers).

 

[dbuckley]

If the dimming system is loaded to capacity, then the neutral feeders should be doubled to accommodate.

I think you'll find its a NEC requirement to have uprated neutrals in theatrical and similar installations, in addition to being a good idea.