Increasing or Decreasing Harmonics??

[robbietan]

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

and the utilities here do not permit certain ballasts as they have current THD of plus 20%, their current THD limit for small loads

 

[brian john]

see this topics it is 1.35 times

http://books.google.com/books?id=yFM...rmonic&f=false

see this topics it is 1.73 in one part and 1.5 and 2 times in other part.

http://docs.google.com/viewer?a=v&q=...BO_WcgN9wCdBDg

I have read NEC code and I have not seen such an issue but in these topics a little bit difference.

I have read a slew of stuff regarding possible upper limits of neutral current due to harmonics, all I can tell you is what I have seen.

 

[mivey]

Please see the below link part nuetral wire conductor size and please explain me your idea regarding this.

I really need to know this.


http://docs.google.com/viewer?a=v&q...hs6_ac&sig=AHIEtbT0AKg6k_IuFIWmBO_WcgN9wCdBDg

I did not read through every detail but he did recommend oversizing the neutral for the 1.73X theoretical extreme. In the cases of Bldg I & Bldg IV, he did not specify the existing conductor size and the % capacity utilized.

While those cases may be the extreme, our conductors are usually not loaded to 100% capacity so we might take that into consideration when oversizing.

 

[mivey]

I was reading one day regarding this topics I found the below links and studied please give me your comment regarding this.


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

Again, a theoretical extreme. The chart shows that at 85% 3rd harmonics the neutral current is near the 1.73X value.

 

[mivey]

see this topics it is 1.35 times

http://books.google.com/books?id=yFM...rmonic&f=false

see this topics it is 1.73 in one part and 1.5 and 2 times in other part.

I have read NEC code and I have not seen such an issue but in these topics a little bit difference.

see this topics it is 1.35 times

http://books.google.com/books?id=yFM...rmonic&f=false

see this topics it is 1.73 in one part and 1.5 and 2 times in other part.

http://docs.google.com/viewer?a=v&q=...BO_WcgN9wCdBDg

I have read NEC code and I have not seen such an issue but in these topics a little bit difference.

These links did not seem to work.

 

[mivey]

...I wonder what the special case is that would create the 2X condition.

I really don't recall the specifics. I'll see if I can find the reference, but Winnie may have caught it in #10.

 

[mivey]

I have read a slew of stuff regarding possible upper limits of neutral current due to harmonics, all I can tell you is what I have seen.

What % capacity use would you say you find on average? What about % capacity use for those on the "really loaded" end of the spectrum and what % of cables inspected would that represent??

 

[Besoeker]

I did not read through every detail but he did recommend oversizing the neutral for the 1.73X theoretical extreme.

I don't know how or where the "theoretical" 1.73 is derived. It isn't too difficult to come up with examples of how the neutral current can exceed the line current by more than that.
Take leading edge dimmers for example and have each of the three phases feeding some. If the loading is balanced and each is operating at say, 50 degrees conduction angle, neutral current is then 2.67 times the line current. It's one specific case I know, but it does show that there are circumstances where that 1.73 can be exceeded.
That doesn't mean that the distribution system current ratings will be exceeded. Each case needs to be looked at on its merits.

 

[mivey]

I don't know how or where the "theoretical" 1.73 is derived.

You can read some of the common examples in the links H-E provided.

It isn't too difficult to come up with examples of how the neutral current can exceed the line current by more than that.
Take leading edge dimmers for example and have each of the three phases feeding some. If the loading is balanced and each is operating at say, 50 degrees conduction angle, neutral current is then 2.67 times the line current. It's one specific case I know, but it does show that there are circumstances where that 1.73 can be exceeded.

Do you see much of that?

 

[Hameedulla-Ekhlas]

These links did not seem to work.

now try it please the below links


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


http://docs.google.com/viewer?a=v&q...LwwvEd&sig=AHIEtbSZZuD3rLZVm2lETXq859AEZOZSQA



http://docs.google.com/viewer?a=v&q...hs6_ac&sig=AHIEtbT0AKg6k_IuFIWmBO_WcgN9wCdBDg

 

[Besoeker]

You can read some of the common examples in the links H-E provided.Do you see much of that?

Some, yes.
But, given that a division of the company I work for makes lighting control systems, often for large new build, I may be more likely to than others in the electrical field to run into this.
Particularly as I'm usually called in after the the problems have come to light - no pun intended - and I'm left trying to defend the indefensible.
I have suggested to them a number of times that they would get more accolades if they looked at this at the design stage.
Intransigence is what I get.
Their view is that they are responsible for the product, not the installation.
Whilst that correctly states the contractual position, it doesn't sit well with me as an engineer. I like to see our customers the best solution for the job. They should too.

 

[mivey]

now try it please the below links...

Mo better .

 

[gar]

100309-1057 EST

Consider this theoretical circuit. Single phase center tapped secondary. Load on one phase to neutral is a pure capacitor with a reactance of X. On the other phase to neutral is a pure inductor of reactance X. What is the neutral current?

Note: no harmonics.

.