Increasing or Decreasing Harmonics??

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jclint07

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Hope all are hanging in there! Need help with harmonic currents, which are not my specialty. I am bidding a lighting upgrade in a factory, which currently has 333- (400 Watt) Metal Halide High-Bay fixtures to 333- 6 lamp T8 Fluorescent High-Bay fixtures. I believe Fluorescents will draw around 225 watts each. Factory motors & lights are 277/480 3 ph. Not sure how many VFD are in service now. Lighting upgrade is all we have asked to bid. My question is would we be increasing Harmonic Currents or decreasing Harmonic Currents? Plant runs 24/7, 50 hours week, 100, 000 square feet. Any help would be appreciated! :-?
 
It will depend upon the specifics of the ballasts involved.

Do you have a THD figure for the high bays that you will be installing?

-Jon
 
jclint07 said:
Hope all are hanging in there! Need help with harmonic currents, which are not my specialty. I am bidding a lighting upgrade in a factory, which currently has 333- (400 Watt) Metal Halide High-Bay fixtures to 333- 6 lamp T8 Fluorescent High-Bay fixtures. I believe Fluorescents will draw around 225 watts each. Factory motors & lights are 277/480 3 ph. Not sure how many VFD are in service now. Lighting upgrade is all we have asked to bid. My question is would we be increasing Harmonic Currents or decreasing Harmonic Currents? Plant runs 24/7, 50 hours week, 100, 000 square feet. Any help would be appreciated! :-?
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Harmonics can be likened to water waves, adding waves could either cancel out or increase harmonics. In your case, if the motor loads are much greater than the lighting loads (which I suspect), there may be a small increase in current TDD. maybe not enough to affect the operation of the plant's "sensitive" equipment.

you really have to look at what kind of harmonics producing loads the plant has, the harmonics from the vfd, etc to get a good feel for the overall plant system
 
jclint07 said:
Lighting upgrade is all we have asked to bid. My question is would we be increasing Harmonic Currents or decreasing Harmonic Currents? Plant runs 24/7, 50 hours week, 100, 000 square feet. Any help would be appreciated! :-?
Click to expand...


some electronic ballasts produce harmonics which can boost neutral current beyond that expected in some situations.

Some people are installing electronic tube light ballasts to save power but ultimately loose much on the other side. Your neutral wire will draw high currents and you need to install neutral wire that is bigger than all the three phases put together


if you have a balanced load of all complex circuits like computers, copy machines, electronic ballasts and other non-linear loads, the current in the neutral can be up to 1.414 x the current in one of the line conductors. This would amount to up to 21.2 amps for the 15 amp balanced loads above.

.

ballasts will vary from one brand of electronic ballast to another, depending on internal design. It is possible that the big companies will be willing to supply this information (try Phillips, Advance, etc.).
 
brian john said:
In my experience looking at lighting panels with a variety of lighting types, neutral current did not exceed the phase currents and were well within the rating of the conductors utilized.
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That is common real world data. Even in extreme real world cases I don't think you would ever need a neutral conductor 3X a phase conductor. The best of my recollection, 1.73X was a theoretical extreme with 2X possible in a special case. Sounds like H-E was fed some H-Y-P-E.
 
mivey said:
That is common real world data. Even in extreme real world cases I don't think you would ever need a neutral conductor 3X a phase conductor. The best of my recollection, 1.73X was a theoretical extreme with 2X possible in a special case. Sounds like H-E was fed some H-Y-P-E.
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1.73 factor is a theoretical one but I have never heard of anyone actually measuring a load that high. I wonder what the special case is that would create the 2X condition.
 
Consider a line-neutral load which only conducts during the peak middle third of the AC half-cycle. Now consider a balanced wye array of such loads. When the loads on phase A are conducting, the loads on phases B and C are not conducting, thus all the A current must return on the neutral. Similarly for B and C. With this _extreme_ type of loading, the neutral current could equal the sum of the phase currents.

The real world experience reported here suggests that such an extreme case is very unlikely.

Going back to the original post, it seems that many (most??) T-8 electronic ballasts are now designed to give high power factor and low harmonic distortion. A quick looks shows that common values are <10% THD, though the numbers can be as high as 25%.

Lamps with magnetic ballasts have much higher THD; so if you are replacing old magnetic ballast HIDs with modern electronic ballast T-8 systems, I would expect harmonic currents to go down considerably.

-Jon
 
mivey said:
Not likely. Who told you that?
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friend mivey,

I have just considered the harmonic problem regarding to neutral size. I see the below links and topics which I had studied regarding nuetral size.


http://books.google.com/books?id=4i...q=neutral conductor size for harmonic&f=false


Harmonics and Neutrals
In an ideal power system, current flows to and
from loads in a standard sine wave form and is
equal on all three phases. In a 3-phase, 4-wire
power system, single phase line-to-neutral
load currents flow in each phase conductor
and return in the neutral conductor. If loads
are equally balanced, the sum of the currents
in neutral is zero.
Harmonics, on the other hand are high-frequency
waves that distort the current flow. A measure
of harmonics is total harmonic distortion
(THD). THD represents the deviation of a
distorted waveform from the ideal wave.
These distorted wave forms may not cancel in
the neutral. Triplen harmonics, the product of
devices with electronic power supplies, create
the greatest current in the neutral.


In four wire WYE systems such as 120/208 and 277/480-volt systems; harmonics
may cause a problem with overheating of the neutral wire. The phase wires should
also be designed for the increased harmonic current, but since the ?triplens? are
additive, the problem is especially critical on the neutral. The third harmonic and
other ?triplens? (9th, 15th, etc.) are additive. Total Harmonic Distortion is the
percentage of all of these additive values in relation to the total load. The sum of
triplen harmonics greater than 33 percent will result in neutral current greater than
individual line currents. The resultant current exceeds the neutral conductor?s rating
and causes overheating of the neutral and/or transformer.
It is a common misconception that electronic ballasts increase THD. Currently
available electronic ballasts actually decrease the THD on an electrical system
compared to a system applying magnetic ballasts. ANSI C82.11 requires that the
maximum THD of electronic ballasts not exceed 32 percent and the maximum
triplens not exceed 30 percent. Electronic ballasts today are rated at less than 20
percent, 15 percent, or less than 10 percent THD. The magnetic ballast is rated in the
20 to 28 percent range.
Electronic ballasts reduce THD in two ways.
1. The electronic ballast has a lower THD percentage than the magnetic ballast.
2. The biggest reduction comes from the fact that electronic ballasts reduce the
total load.
For lighting systems, the following table provides some comparisons (3):

http://docs.google.com/viewer?a=v&q...LwwvEd&sig=AHIEtbSZZuD3rLZVm2lETXq859AEZOZSQA
 
Thanks for the replies. Advance T8-Lamp Electronic Ballast w/10% THD, is what the fixture supplier bid out for this project. Lights are 277V. Motors are 480V. I did not go as far to see if each lighting circuit had its own grounded conductor(aka neutral). I assumed since we would be:) replacing older magnetic with new electronic ballast (and that we were not adding any more fixtures), that harmonics would decrease, but could not find anything to back up my theory. Hope we are awarded this project, but wanted to make sure I was not overlooking a possible problem.
 
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