Harmonic currents

[La Poste]

Greetings

I have a quick question about neutral harmonic currents.

I have done a little reading on the subject and I understand how harmonic currents from the three phases are in phase with each other and hence sum in the neutral.

The third harmonic has three times the frequency of a standard 50 HZ signal and there is also an eleventh harmonic that I presume is eleven times the frequency of a standard 50 Hz signal, all these harmonics will sum in the neutral and create neutral currents that may well exceed a single phase current and so overload the neutral.

I have read that things like switch mode power supplies produce harmonics but I don't know why.

What is the mechanism, how does clipping a waveform produce a signal that has three times or eleven times the frequency of the original 50 HZ signal?

Does anyone know what other things produce harmonic currents?

Any comments appreciated.

Thanks.

 

[Farmelectrics]

am i right if all 3 phases are balanced theres no current in the neutral

 

[Engineer54]

am i right if all 3 phases are balanced theres no current in the neutral

YES!!

But that doesn't count where harmonic currents are concerned, they will still add up and circulate at the suppliers distribution Transformer. The 3rd harmonic, is just the most common found, there are many more and all will have detrimental effects on the supply.
This is now becoming one of the biggest concerns to all the DNO suppliers regarding their supply cables and Distribution TX's

 

[Farmelectrics]

ok e54 understando

 

[Guest55]

am i right if all 3 phases are balanced theres no current in the neutral

This is true , but a bit more complex with harmonics.
Third harmonics in the phases combine to create a larger harmonic in the neutral , which may have negative effects for a circuit
such as derating cable and volt drop.
As for waveform clipping - pass lol.

 

[Guest55]

Eng54 beat me to it lol , must stop dithering ;-)

 

[Farmelectrics]

need that book biff lol

 

[mechelec]

YES!!

But that doesn't count where harmonic currents are concerned, they will still add up and circulate at the suppliers distribution Transformer. The 3rd harmonic, is just the most common found, there are many more and all will have detrimental effects on the supply.
This is now becoming one of the biggest concerns to all the DNO suppliers regarding their supply cables and Distribution TX's

You are right about it being a concern. All Lv distribution pillars are now speced with the neutral having the same current carrying capacity as each phase. They used to be half size. (in my area anyway)

 

[Guest55]

need that book biff lol

Its tough getting your head round it lol.

 

[Farmelectrics]

what the book or harmonics

 

[Darkwood]

Harmonics Explained

Harmonics is a complex subject but the link above has a simplified explanation.

 

[topquark]

Have a read of this little article: http://www.powerstudies.com/articles/Harm_Intro.pdf Should help to explain the basics.

 

[mechelec]

As I understand it switched mode power supplies and other non linear loads draw a non sinusoidal current from the sinusoidal voltage supply. The triplen harmonics are zero phase sequence in nature and need a neutral conductor or delta connected winding to flow in. This causes problems in neutral conductors that are undersized.

 

[plugsandsparks]

I have read that things like switch mode power supplies produce harmonics but I don't know why.

What is the mechanism, how does clipping a waveform produce a signal that has three times or eleven times the frequency of the original 50 HZ signal?

.

Whenever a square wave is introduced, there are the multiples of the fundamental frequency introduced also as a by product of the fact that the frequency components of a pure square wave are in theory infinite. Square waves are typically produced by semi-conductors used in their switching mode - hence SMPS. E.g. a transistor can be designed as a current amplifier, like an audio amp or it can be used as a switch. The very fast times these devices can switch can create a near perfect square wave but with all the associated harmonics.
VSDs also use semi-conductors in switching mode and can create similar problems.
So basically whenever we use semiconductors in switching mode we will get lots of harmonics which have to be dealt with.
If you want to read up more on it - plenty of electronics books deal with the principles of switching

 

[Bobster]

View attachment 13491 Here's something I found quite informative