Fluorescent Lighting?

[Mr. Bill]

What kind of system are we talking about? A WYE system may have a larger neutral current due to harmonic loading caused by the ballasts. A single phase system wouldn't have harmonic currents.

I'm just curious why you think a single phase system wouldn't have harmonic currents.

He didn't specify if the ballast was magnetic or electronic. I'll assume electronic since magnetic are being phased out. An electronic fluorescent ballast creates harmonics. It's the nature of an electronic load. You can choose either <20% THD or <10% THD ballast. But there will always be some harmonics.

A single phase system might be able to cancel the harmonics on the neutral better when phase 'A' and 'B' are together. But that would assume the phases are loaded equally and the electronic ballats have no manufacturing tolerances. You can still expect to see harmonics on a single phase system.

I also think there sould be a section added for Lighting so these topics don't keep showing up under Lightning.

 

[winnie]

I'm just curious why you think a single phase system wouldn't have harmonic currents.

[...] But there will always be some harmonics.

A single phase system might be able to cancel the harmonics on the neutral better when phase 'A' and 'B' are together.

A very good point.

If the load produces harmonics, then harmonics will be present. This is true for three phase systems, single phase systems, two phase systems, delta systems without a neutral, etc. These harmonics will be present on both the phase conductors and the neutral if loads are connected to the neutral.

The reason that we focus on the three phase neutral is that 'triplen' harmonics tend to be additive in this situation. In _most_ circumstances where harmonics may be present, they are simply additional load on the conductor. For example, in a single branch circuit (one hot, one neutral), harmonics may be present, and will be present in equal amounts on the two conductors. If there is lots of harmonic current, then this will show up as additional heating of the conductors, but no more than a similar RMS fundamental current (approximating to ignore skin effect, dielectric heating, etc).

In a single phase system, odd order harmonics will tend to cancel on the neutral, and so again are not a problem. The neutral acts the way that we've been trained, always having less current flow than the 'hots'. Harmonics may be present, but they don't generally cause unexpected loading. It is possible to come up with situations where harmonic currents will add on a single phase neutral, but to do so would require a contrived system, one that produces even order harmonics or which produces seriously strange and non-symmetric phase offsets for the harmonics on the two legs.

But in a three phase wye system, triplen harmonics tend to add. All the other current components cancel as expected, but in this system a subset of the harmonics _add_ on the neutral. This means that neutral loading will be greater than expected. If the harmonic content of the various load currents is high, then the neutral current (which is the _sum_ of currents from _three_ phase conductors) will be greater than the current on any of the individual phases. This is no stranger than the neutral current being less than the current on any of the individual phases, in the normal case where currents from the different phases cancel.

Summary: harmonics can be present on single phase systems and on the phase conductors of three phase systems...but they are particularly annoying when present on the neutral of a three phase wye system.

And this goes back to answering the original poster's question:

Question is it true that the nutral carrys more on a fluorescent lighting load as opposed to in incandescent lighting load? and if so why?

Fluorescent ballasts _may_ produce harmonic current flows. If you have ballasts which produce harmonic current flows, and have a system where harmonic currents will add on the neutral, then the neutral current will be greater than expected. This is _not_ the case for a single phase system.

-Jon

 

[Michael15956]

Hi All,

I have to agree with the boss.

Electronic ballasts cause increased harmonics. The ballast changes a 60Hz input sine wave somewhere up to 20,000Hz, output. This action can result in the heating of circuit conductors and neutrals. Source: Studies completed by IEEE and others; Source: Data Sheet written by CDA. Although this data sheet only referrers to a 3-phase circuit, but on a separate "Power Quality Issues and Recommendations" sheet it recommends not using one shared neutral among electronic equipment on "single-phase branches." It advises to use (on single-phase branches) "separate full-size neutral for each phase, back to panel."

Also, it is common trade practice not to install rows of electronic ballast fluorescents on a 3 wire (shared neutral) circuit. Reason is that the shared neutral may become overheated due to the harmonics.

So, guess the hard headed boss is right again this time. I feel sorry for his employees.

Michael

 

[infinity]

I'm not convinced that there would be any additive harmonic currents in a single phase 120/240 volt system. Anyone have any proof?

 

[winnie]

I'm not convinced that there would be any additive harmonic currents in a single phase 120/240 volt system. Anyone have any proof?

Quoting myself:

In a single phase system, odd order harmonics will tend to cancel on the neutral, and so again are not a problem. [...] It is possible to come up with situations where harmonic currents will add on a single phase neutral, [...]

The only requirement for harmonics to be additive on the neutral is that the _harmonic currents_ be in phase.

For single phase systems this situation requires either _even_ order harmonics, or it requires different phase displacement of the odd order harmonics.

_Most_ of the common non-linear loads which cause harmonic current flow will produce far more _odd_ order harmonics then even order harmonics. This means that current flowing at 3x, 5x, 7x, ... the fundamental frequency will be much larger than current flowing at 2x,4x,6x,... the fundamental frequency. There will be _some_ even order harmonic; but it can essentially be neglected.

Because the common harmonic loading is mostly odd order harmonics, harmonic currents simply are not found to be a problem on single phase systems. The even order harmonic loading will be present, and will be additive, but it is also likely to be insignificant.

-Jon