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Harmonic Currents
2

Harmonic Currents

Harmonic Currents

(OP)
I am weak on Harmonic currents, so I would like some opinions.  I have a project for a small office area.  The building is an existing shell.  The service is 3 phase, 240 V, Closed, center tapped delta from the utility.  I will have a couple of single phase panels (fed from phases A & C) that will feed lighting and receptacles.  Since this will be all fluorescent lights, PCs, copiers, scanners, printers, etc, I am concerned about harmonic loading of the neutral.  Is this a concern in this application?  I know it is a not a concern in a single phase system, but I was not sure in this application?  Thanks for any advice you may provide!

RE: Harmonic Currents

If center tapped delta means 3 lines with 240 V and 120 V from two corners to neutral, then the neutral will not see the sum of triplen harmonics.

Triplen harmonics only add in systems with star connected secondary and neutral connected to star middle.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonic Currents

I believe that you are correct about the system, Gunnar.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Harmonic Currents

There is a demo where you can see the harmonic components live and added to the fundamental voltage vectors as the system rotates. Explore different phase configurations, add harmonics, click *show sum' to see neutral current and more. Available as 'trefas.exe' on http://www.gke.org/pub/
 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonic Currents


I got a phone call from a guy that was very frustrated about the rotating vectors in my harmonics demo (in the 6 Aug 10 2:26 posting).

He said that, since fifth harmonic is negative sequence, it should rotate backwards. "Because it does".

I know for sure that the vectors shown are correct and that they rotate the correct way - but couldn't find a good answer. I have to think about it. Or has someone else a good answer?

Zero sequence vectors all point in the same direction. But they rotate. So, when I asked him if they shouldn't stand still, he first said "Yes", but when I asked him if the ninth shouldnt rotate faster, although it also is zero sequence, he wasn't quite so sure any more.

I think that a good explanation is needed. Anyone?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonic Currents

That is exactly where we disagree. I still mean that all vectors have to rotate in same (positive) direction and he says that minus sequence should rotate in the opposite direction (which also should mean that zero sequence doesn't rotate at all - and that we know is wrong).
I think there is a simple semantic mistake somewhere, but can't put my finger on it.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonic Currents

If you represent the three negative sequence phasors in different order (A-C-B)then they "rotate" in the same direction as the positive sequence.  What matters is the sequence, not the rotation, I think.  

One of my college textbooks was very insistent that phasors do not rotate at all.  
 

David Castor
www.cvoes.com

RE: Harmonic Currents

I think that is a 'Bingo moment', David! Semantics, it is.

Whether phasors rotate or not is really a question about what the phasors represent. In AC systems, they have been chosen to illustrate behaviour of sine shaped variables and to be able to do so, and for the notion of 'phase angle' to have a meaning, they have to rotate.

Yes. It is about sequence, not how the system or its vectors rotate. I shall try and get used to that insight. PLS for you, sorry that I cannot give you more than one sad

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonic Currents

Gunnar,
I believe that you are correct.  The third harmonic vectors all have the same angle, as would zero-sequence vectors.  If you stop the vectors with one phase horizontal to the right (0°), fundamental ØB is at -120° (correct for positive-sequence), 3rd harmonic ØB is at 0° (correct for zero-seq), 5th harmonic BØ is at +120° (correct for negative-sequence).

The vectors all rotate counter-clockwise, but ØC of the 5th harmonic follows ØA by 120°, not ØB.

You might want to modify the program so that you can change the relative magnitude and angle of the harmonics.
 

RE: Harmonic Currents

Yes, I see that now.

Stopping the rotation and putting a zero in the 'degrees' box and then pressing '+' or '-' to rotate the system shows this clearly. Funny how you think you understand things and suddenly realize that there's more to it than you knew. Now, I wonder what else I have missed?

Re different amplitudes: we didn't want to make the use too complex. This is just to show that harmonics vectors rotate with different speed depending on numbers. And also to show why higher harmonics create steep edges. Check all harmonics and see the whip-lash when passing zero degrees (and 180 degrees when also even harmonics selected).  

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

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