Unbalance loads on a 208Y lighting panel 3

[Thedroid]

We were installing a 225A MCCB in a lighting panel today and needed to make sure that the Breaker feeding the transformer was the proper one, so we hooked up an analyzer and shut down some of the lighting to make sure that we were shutting theright breaker off.

As we were doing this we noticed that the neutral was carrying 14A and A,B,C were all carrying about 33A.

What causes this? The panel feeds lighting and GFCI outlets in a control room. The service is 480/208Y 225A.

Can this be harmful, and how could we go about balancing everything?

Each phase measured within 1/2A of each other.

None of the plc's or control room computers are fed from this service.

Cement Plant Electrician and
Instrumentation Technician

 

[dpc]

1. Third harmonic current. Any third harmonic currents in the phase conductors ADD together in the neutral.

2. The neutral current is the vector (phasor) sum of the three phase currents. Just adding the phase currents together doesn't work if the power factors of the various loads are different.

Most of the neutral current is probably harmonic current if you have a lot of lighting on the circuits.

 

[roydm]

I think what you see would be normal for a 3 phase 4 wire system. Even though your phase currents looked about the same. I think Power Factor differences would cause the neutral curent. No doubt someone will add to that.
Regards
Roy

 

[Thedroid]

Is there any danger or problems associated with this current in the neutral. Just for laughs what what would I reference for a voltage measurement?

Most of the loads are T-8 flourecents, with a few pc's, some small breakroom appliances and several GFCI outlets.

I don't really have a grasp of harmonic currents yet. So many things to study in this field, that I havn't got there yet. Are harmonic currents the same electrically as the current used by a load? Are they dangerous?

Cement Plant Electrician and
Instrumentation Technician

 

[BJC]

Two seven amp loads on "C phase, an equal number of lighting fixtures on each of A, B and C would do it.
Are the lighting fixtrues 120 volt or 208? It won't make much differenc.

 

[Thedroid]

All of the loads from this panel are 120VAC.

I forgot to mention that this is a 3 phase 4 wire system.
I'm not sure if it makes a difference.

My understanding is that this is regular 3 phase power coming into the box and just being split up into a bunch of single phase lines, with each individual phase sharing the same neutral.

Going down the panel cb's 1 and 2 are fed from A. 3 and 4 from B, and 5 and 6 are from C and then just repeats the pattern down to the bottom of the panel.

Cement Plant Electrician and
Instrumentation Technician

 

[waross]

Your flourecent ballasts and the PC power supplies will draw harmonic currents. Third order harmonics add on the neutral instead of canceling. When you know you will have a lot of harmonics use a neutral the same size as the phase conductors and consider a panel with a 200% rated neutral.
What are harmonics etc may be a topic for a new thread.

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

 

[Thedroid]

Forgot to mention that the reason for changing the mccb was that a prior electrician had installed a 125A breaker when the panel is designed for 225A. Can't say why, but this was how it was done.

It worked fine for several years, but has beem tripping

Cement Plant Electrician and
Instrumentation Technician

 

[Thedroid]

Sorry hit enter too fast.

It worked fine for several years, and lately seemed to be nuisance tripping. None of the 120V breakers were ever tripped, and all of the loads are operating properly.

I'm wondering if current through the neutral could have be causing this problem?

Cement Plant Electrician and
Instrumentation Technician

 

[waross]

No

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

 

[sibeen]

Thedroid, a third harmonic is a current or a voltage that is three times the frequency of the fundamental. So, for a 60 Hz system the third harmonic will be a current at 180 Hz.
To get some concept of what is going on a diagram can help. If you draw a standard sine wave, and then on the same graph and starting at the same point, draw another sine wave that is three times the frequency you will see that the ‘harmonic’ frequency will start a new sine wave at 0 degrees, 120 degrees and 240 degrees.

In a three phase system the phases A, B + C are offset from each other by 120 degrees. If you draw the three phase sine waves and then draw the harmonic (three times) sine wave for each of the phases you will find that the harmonic sine waves are the same for all three phases.

The third harmonic for phase A will start at the same point as the third harmonic for phase B and the same for phase C.

It is for this reason that the third harmonic is known as a ‘0 sequence’ harmonic, ie: it doesn’t have a positive or negative phase sequence as does the fundamental voltage. It is also why the third harmonic currents add up in the neutral in a three phase system.

I hope that isn’t too confusing. If you have any questions I’m sure someone will be happy to provide advice.

 

[Thedroid]

Thanks. Third harmonics wasn't something I've put alot of thought into. I'll definately do my studying now that it seems like I'm surrounded by them.

 

[roydm]

This is interesting, I need to know more!
What causes the third harmonic, is it something peculiar to an electronic ballast?
Is it only 3rd order that add up on the neutral?
A new buzz phrase I can use in the office. "it's the 3rd harmonic problem"
Seriously though, tell us more.
Roy

 

[waross]

Because of the hysteresis of the iron in the ballast, the current is not proportional to the voltage. The voltage referred to is not the RMS voltage but the instantaneous voltage. This distorts the current wave form. A distorted wave form contains both the fundamental frequency and multiple or harmonic frequencies. The characteristics of a ballast are such as to produce a high percentage of third harmonics, but other frequencies may also be present.
A wave form may look like a sine wave but actually the wave front may be a little steeper or a little narrower.
And, the distorted current wave form flowing in the resistance of the conductors develops a distorted voltage wave form.
I'll let someone else take it from ere.

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

 

[sibeen]

Roy, this is one of those concepts that can get highly technical fairly quickly and reasonably hard to explain without the aid of a whiteboard, but I’ll give it some form of attempt.

As waross stated, “A distorted wave form contains both the fundamental frequency and multiple or harmonic frequencies”. Now, this is true for any form of distorted wave (non-sinusoidal). Via Fourier analysis, any distorted waveform can be deconstructed into a fundamental frequency and harmonics of the fundamental frequency.
As an example, a square wave can be shown to consist of the fundamental frequency + 3rd harmonic/3 + 5th harmonic/5 + 7th harmonic/7 + 9th harmonic/9….etc, etc.

So, to address your questions:

What causes the third harmonic, is it something peculiar to an electronic ballast?

No, many different loads can draw substantial third harmonic currents. The power supplies on a PC are a switched mode power supply (SMPS) and these can draw a current waveform that is severely distorted and contain a high percentage of third harmonic current.

Is it only 3rd order that add up on the neutral?

No, although it is generally only the third harmonic that causes problems. Any harmonic that is a triplen of the fundamental, but not an even harmonic, will add in the neutral, So the third, ninth, fifteenth, twenty first harmonics etc will all add. As the triplens above the third harmonic are normally of a much lower magnitude they are of less of a concern.

 

[sibeen]

I've decided to do it in pictures. See the link and if you have any questions, fire away.

 

[roydm]

Sibeen,
You obviously spent a lot of time on those graphs, unfortunately pages 1 thru 3 don't show up correctly. (no coloured traces, on my Mac anyway), I will check it again on the PC in the office.
Fortunately page 4 where you show the 3rd adding and the rest are OK so I was able to get the idea.
Since you have put so much work into explaining the subject it would be a pity not to correct pages 1,2,& 3, I'm sure it will be appreciated by all who read it.
Over 40 years in the trade & I was never aware of this particular 3rd harmonic problem.
Thanks again for the simple explanation.
Roy

 

[sibeen]

Hmm, what a pain in the arse.

If you download the PDF you then need to zoom to about 75% and it should show up OK.

Thanks for the heads up, Roy.

Actually, it didn't take too long to put together.