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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.
 
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E

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
 
G

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.
 
G

Guest55

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

mechelec

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Arms
Esteemed
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)
 
G

Guest55

need that book biff lol
Its tough getting your head round it lol.
 

topquark

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

mechelec

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Arms
Esteemed
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

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Arms
Esteemed
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
 
B

Bobster

View attachment 13491 Here's something I found quite informative
 
E

Engineer54

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
The problem being, there isn't much these days that that won't cause harmonic distortion. Virtually every home appliance and audio/visual appliances will produce harmonics and or leakage currents to some degree. It's when you start to add them up over say a residential area or estate, that they start becoming a real and increasing problem. The other downside, is that nullifying these currents is an expensive business!!...
 

plugsandsparks

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Arms
Esteemed
The problem being, there isn't much these days that that won't cause harmonic distortion. Virtually every home appliance and audio/visual appliances will produce harmonics and or leakage currents to some degree. It's when you start to add them up over say a residential area or estate, that they start becoming a real and increasing problem. The other downside, is that nullifying these currents is an expensive business!!...
Product of the modern age - when i was a lad, PSUs always had a transformer, diodes, a big cap, and was the cheapest way. Nowadays you can make a switching PSU for a couple of pence, also large thyristors were megabucks and only affordable by the largest of industrial users, again they are ten a penny and i keep seeing smaller and cheaper VSDs for bog standard motors. Its not going to get any better anytime soon. Oh well
 
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