3rd Harmonic Current

Any non-sinusoidal waveform will have components of multiples of the fundamental frequency present, with the third harmonic, being three times the fundamental frequency, being the most prominent in the mix. Multiples of the third harmonic can also occur.

http://en.wikipedia.org/wiki/Voltage_optimisation#Harmonics

Non-sinusoidal waveforms and a greater-than-zero phase angle between the voltage and current waveforms will introduce third harmonics, which appear principally on the neutral conductor in three-phase systems. Possible solutions are suggested in the link ↑.

Dear Mr. PWSlack,

During my college days, our Professor has taught, that the 3rd Harmonics is annelled or wiped out on account of phase angle of 120 Degrees, between the 3 phases, and it is only 5th, 7th 11th, 13th Harmonics, and so on.

As the Harmonics Order increases, the INTENSITY COMES DOWN, but the pinch will be severe and cumulative resultant is to be considered for FILTERING etc.

Very recently we ordered to a multi-national company, 5 nos. of 600 KW VFD DRIVES, 1000 RPM Base Speed and the Manual for the VFD Drives indicates, 5th, 7th 11th, 13th Harmonics, cumulative effect has been derived at 11.2% impact. and we faced some problem, and some Chokes were replaced and the effect came down to 8.2%

Now I want to understand how 3rd and multiples of 3rd Harmonics is existing. May I request you to pl. clarify.

I also request CR4 MEMBERS of ELECTRICAL Engg. and electricalexpert65 and Ramconsult explain this.

DHAYANANDHAN.S

I suppose that in some cases, what you were taught is correct. But there are many more cases at hand because one condition can not usually be considered in isolation. Consequently it is difficult to make a general statement that works with every problem. That being said, the specific nature of harmonics has been well defined. Unfortunately, the application of that information is frequently beyond the average user (or engineer).

One particular characteristic that is very important is the magnitude of the distortion at each of the harmonics. Harmonics can be present only on one phase or all three. And the source of these harmonics are usually power supples that draw gulps of current. This happens because a DC power supply only conducts current when the line voltage (at the peak) is greater than the voltage already stored.

As a result, these gulps may be wide or narrow or constantly changing as the load changes. This pulse of current contains characteristics, specifically the rate of change, that can be represented by the sum of various odd harmonics at their respective magnitudes. Sometimes it is fairly constant and other times it is not.

In a three phase system, when these gulps occur (at the peak of the sinusoid) it becomes a very brief single phase condition and then shuts off. Consequently the short term current path will be between the leg (at peak voltage) and neutral. These peak current draws make it necessary to have a neutral as big as the phase conductor. It is not reasonable to try to balance this current flow because it constantly changes.

Interestingly enough, the power company does not care much about harmonic currents as long as it does not distort the sinusoidal wave shape of the supply voltage. In most cases, the effects of harmonic currents will be burnt off as heat in the iron core of your supply transformer. If it is enough to distort the voltage wave shape, they fix it by using a larger transformer.

Not too long ago, we had a large power factor correction capacitor fail, just outside our building. It was mounted on the power pole where we made our connection at 15KV. I looked up the voltage rating for the capacitor and calculated the actual voltage based upon our current draw at 60 Hz and the current at 180 Hz and found that the rated voltage was being exceeded. That explained why it caught on fire. That is actually a common problem, as it turns out. But harmonics can cause other problems including motor overheating and problematic circuit breaker tripping. And most of the time, that breaker will seam to have nothing in common with the source of the harmonic distortion.

Thank you Mr.NotUrOrdinaryJoe, for your reply.

A good presentation, I am sure this will give further intrest for other CR4 Members.

I have some view on your point and I shall collect the data and post it here.

DHAYANANDHAN.S

Hallo Mr. Dhayan...

Your professor is right with the classic 3 phase source feeding a 3 phase VFD. Everything is inherently symmetrical. The conclusions are correct.

On the other hand, OP happened to have 3 phases, on each of which individual dimmers (an undefined multiple) produce harmonics independently of each other, and definitely independent of what happens on the other phases.

The harmonic transformer attempts to distribute the harmonics evenly between the phases. These balancing act steeply reduces the current in the neutral. Also eases further filtering, if and when it is needed.

Please see my previous note too.

Thank you Mr.leveles, for your reply.

DHAYANANDHAN.D

What exactly is the dimmer system dimming?

2000A busbar, I am guessing this is not some simple light dimmer.

increase the neutral conductor size?

That is not a fix of the problem.

Can you provide more information.

The dimmer system is to control the lighting at meeting rooms and ballrooms.

There are some feeders tapped from 2000A busbar which are lighting bustrack, general C form socket outlet and dimmer system itself.

There are total 6 nos. of grandballrooms of dimmer system tapping from 2000A system.

I believe this 6 nos. of dimmer panel accumulated harmonic currents and cause the busbar tripped.

Are the dimmers provided with filters or are filters recommended in the supplier literature?

What dimmers are you using, do you have an internet link to part you can post?

There are no filters provided in the dimmer.

The dimmer system is using 'Lutron'. All the dimmers are interlink each other and go back to the main processor.

Can you narrow it down to a Lutron product range and model please.

At least, where I am living, neither code nor the power company permits such a "backfeed" such a load represents.

You need filtering. The simplest one is a harmonic transformer. It basically forces the harmonics from a phase onto the others. Balancing it between the phases, and tripling its frequency for further, easier filtering, if needed. Make a drawing with the 3 phases and their dimmers (when they fire) to see, what I am saying.

Measuring on the neutral indicates the success of the method.