harmonics

[havoone]

this question came up in a 2382 paper and i think the answer is A which ive gathered from App 11. however the answer paper states B as the answer.thanks
A TP&N system supplies an office with IT equipment and has high harmonic currents. How would the effects of harmonic distortion be controlled.
a)increse the size of neutral
b)add surge protection
c)fit a time delay RCD
[FONT=&quot]d)cannot be controlled[/FONT]

 

[netblindpaul]

Of the 4 options given, then B fits best.
Hopefully the surge protection would control the harmonic distortion by filtering out the foltage fluctuations.
However, none of those are really a cure. The root cause solution would be filters at the computers to prevent the distortions "leaving" each machine. That is filter them at source inside the switch mode power supply.
But this adds cost & complexity to what is basically a commodity product bought on price alone.
HTH
Paul

 

[Guest123]

Basically harmonics are caused by lots of electronic control circuits and equipment.

This equipment causes load currents to be out of frequency with our 50Hz system if you were to look at them through an oscilloscope instead of just the usual sine wave you would see another set on top.

These 3rd harmonics as they're known are what causes the humming sound you hear at mains positions and DB's etc on large installations.

Normal balanced 50 Hz 3 phase loads cancel each other out in the neutral but, 3rd line harmonics i.e not 50Hz but multiples of, dont they add up and become load current in the neutral conductor.

This obviously affects things like volt drop, cable sizing and OCPD's.


In answer to your question it is B, purely because of the way a surge protector works as some have a "wave generating chip" inside them which basically cleans up the voltage before it gets to the PC thereby making it more efficient. Also it reduces the harmonic produced by the electronics in the PC by reegulating the voltage to a constant.

 

[netblindpaul]

One of the problems with 3rd or triplen harmonics is that unlike the fundamental mains frequency they do not cancel out in the system, thus the sum of the harmonics can be found as neutral current even in a perfectly balanced 3ph system.
The 3rd harmonic is at 150Hz, this can also cause other disturbances as they can be reflected back onto the mains system.
Finally, some surge protectors are active as Lenny suggests, many are however simply passive, consisting of L-C-R filter networks and a few other bits perhaps a zener or two.
The inductors are tuned to restrict sudden increases in voltage at above the fundamental, hence why they resist the flow of 3rd harmonics.
The capacitors assist in the minimisation of brown outs by momentarily propping up the mains voltage.
Not perfect technical language I admit!
Paul

 

[havoone]

ok, you all lean towards surge protection, but can you advise where in BS7671 that it stipulates surge protection for such a situation or similar situation. reading through App 11 seems to suggest increased neutral to me, but i am not familiar with this, thank you

 

[Des 56]

I think the words "the effects of harmonic distortion" are whats to be concentrated on

Surge protection would aleviate the effects

Increasing the size of the neutral for ccc and safety

 

[netblindpaul]

I would NOT lean toward surge protection.
I would lean towards competent design and elimination of the possiblity of harmonics at source.
I went along with the surge protection bandwagon as it was the best of a bad job.
As des suggests neutral csa's may need to be looked at and even increased above that of phase conductors!
The issue is that this is a value engineered in problem it can be eliminated at source, but consumers willl not pay for this as they do not see the added value.
Harmonics can be generated from all sorts of devices.
They can also be eliminated in a plethora of ways.
As I see it the answers to the OP were given in the light of the options given there in.
Paul

 

[Guest123]

The question asks..."how can harmonics be controlled"...not what do we need to do as a result of.

 

[joe_berks]

i think a lot of people are misunderstanding this. i agree with the person who started this thread its answer A.
harmonics are not caused by lots of computers, it is where the three phases enter an enclosure in seperate holes you get a harmonic current, thats why your supposed to cut slots inbetween the holes.
computers have high earth leakage currents maybe thats what your getting mixed up with.
surge protection devices are designed to deal with transient voltages.
you do up the size of the neutral if its an unbalanced three phase system.
correct me if i'm wrong i'm not saying im mohammed "im hard" bruce lee.
just my interpretation of it.

 

[pevvers]

harmonics are not caused by lots of computers, it is where the three phases enter an enclosure in seperate holes you get a harmonic current, thats why your supposed to cut slots inbetween the holes.

I think that you'll find that those are eddy currents, and a completely different animal altogether...

 

[Guest123]

You might want to try googling harmonics.....


What you refer to there is eddy currents.

 

[netblindpaul]

The question asks..."how can harmonics be controlled"...not what do we need to do as a result of.

Lenny,
The answer options do not give the correct engineerign solution to harmonic currents, which is design them out at source.
Thus one has to select the best of a bad job.
I have been quite involved in harmonic issues from a mechanical and electrical standpoint in my last post to the extent of having formal training in the analysis and rectifications.
I was also lucky in that I was able to guide my undergraduate and post graduate studies to cover these areas.
So I have a moderate idea on their origin and the mitigation measures required.
Paul

 

[Guest123]

Given the question and where it's from (C&G) and the answers given B seems the only logical answer

I'm not saying it's right or wrong in it's correctness but thats what the C&G examiner will be looking for.

I simplified to the question to make it a bit easier to understand in it's meanings and solutions as it is off a 17th ed regs exam paper, not a degree in engineering one.

My answer was not aimed at you directly just a generalisation.

 

[netblindpaul]

i think a lot of people are misunderstanding this. i agree with the person who started this thread its answer A.
harmonics are not caused by lots of computers, it is where the three phases enter an enclosure in seperate holes you get a harmonic current, thats why your supposed to cut slots inbetween the holes.
computers have high earth leakage currents maybe thats what your getting mixed up with.
surge protection devices are designed to deal with transient voltages.
you do up the size of the neutral if its an unbalanced three phase system.
correct me if i'm wrong i'm not saying im mohammed "im hard" bruce lee.
just my interpretation of it.

joe,
You are talking about the "transformer effect" caused by eddy currents.
This is completely unrelated to harmonic affects and harmonic distortions of the incoming 50Hz sine wave supplies.
Harmonic currents are effectively transient voltages on the supply network due to reactive and other affects on the mains.
The issue with three phase conductors carrying different currents entering a ferrous enclosure via two or more different appertures relates to domain effects within the ferrous material caused by the differing current flows, resulting in the re-orientation of magnetic domains resulting in current flow being generated in the ferrous substrate. This can result in heating and EM issues, but NOT harmonics.
If you need to grasp harmonics draw out a 50Hz sine wave, then another 2 on the same graph, phase shifted by 120 degrees, then superimpose the 3rd harmonic on top of the same graph at 150Hz for each phase, then directly underneath graph the summation current of the 6 wave forms.
Easily done in software, a pain on paper but this will then give you an idea of the triplen harmonics and why they are so destructive.
Paul

 

[pmac10]

lenny. 3rd order harmonics do not cause what youre referring to as mains hum, mains hum you are referring to is the amplification of 50Hz signals by electronic equipment picked up normally from unshielded cabling in the same runs as mains supply, cant remember what this has got to do with anything, oh, computers and earth leakage