harmonic currents

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electricalperson

electricalperson

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massachusetts
today i opened a 3 phase 120v/208v panel and taken current measurments. phase A was around 7 amps B and C were 10 amps and about 8 amps. the neutral was 10.8 amps. the panel fed computers and other nonlinear loads. i believe the current i measured was mostly harmonic currents instead of 3 or 4 amps of unbalanced current. in a book i read it says the 3rd harmonic can double the neutral current. it shown a diagram of each phase with 20 amps and a 40 amp load on the neutral.

was that book showing the worst case scenario with harmonics and what i measured more what reality is like?
 
Well i dont do service work i mostly do new commercial /industrial factory plant new work some or most of the jobs we do the engineer calls for double neutrals on any panel /trans that feeds computers or communication data or such , most run thur a ups back up system. Ive put a amp probe once or twice on a panel just for fun to see if the loads were balance and ive seen 18 amps or so tops on the neutral this is 500 mcm 2 runs meaning 4 neutrals they spec double neutrals per run on the power one line risers . We also install harmonic or k type transformers as spec bigg thicker windings at same kva . I guess its for the harmonics and the heat generated that extra current flowing in the neutral , ive always wondered about it myself i bet there is a better way to stop harmonics like filters or maybe a AC to PWM dc supply to feed pwm to a transformer and feed pulse dc at 60 cycles to the loads saw tooth wave to your electronics instead ? ill bet old Thomas Edison would have liked to see what we can do with dc power today i think some day in the distant future we will go back to dc power ,comments Best to yas
 
dbuckley said:
There's lots of people who think this neutral harmonic stuff is something you never see in practice, so its nice to have a posted account.
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What size feeder? Based on the load unless the feeder is #16 AWG I do not see the issue.

Do harmonics occur? YES, do they CAUSE problems? Sometimes. IS the issue overblown IMO YES.

I was on a job Friday night with harmonic issues that caused problems, but for everyone 1 see that have real issues I see or hear of another 100 that are overblown.
 
brian john said:
What size feeder? Based on the load unless the feeder is #16 AWG I do not see the issue.

Do harmonics occur? YES, do they CAUSE problems? Sometimes. IS the issue overblown IMO YES.

I was on a job Friday night with harmonic issues that caused problems, but for everyone 1 see that have real issues I see or hear of another 100 that are overblown.
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it was a 100 amp sub panel fed with #3 copper conductors. there was no problem when i was there working. the place is a testing lab that does top secret things for the government. its a lockheed martin building. they stop working when were in there. i was just wondering why the neutral didnt have unbalace current but had more. i think it was harmonics that i read about but never seen in person yet unless i never noticed
 
electricalperson said:
was that book showing the worst case scenario with harmonics and what i measured more what reality is like?
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It can be.
I don't often get involved in single-phase harmonics problems. But I can recall one spectacularly bad case. It was a hotel in the middle east, supposedly the only seven star hotel in the world. From memory, there was around 3 MW of controlled lighting (dimmers), some VSDs on escalators and a number of other non-linear loads.

At some phase angles dimmers can produce up to 77% third harmonic plus other higher order triple-n harmonics. These add arithmetically in the neutral so it isn't too hard to see how the neutral could have to carry twice the line current or more.

Of course, there will be some diversity and not all loads will be at worst operation at the same time. It's becoming more common here (UK) to specify oversized neutral bars in switchgear for commercial applications.
 
Besoeker said:
It can be.
I don't often get involved in single-phase harmonics problems. But I can recall one spectacularly bad case. It was a hotel in the middle east, supposedly the only seven star hotel in the world. From memory, there was around 3 MW of controlled lighting (dimmers), some VSDs on escalators and a number of other non-linear loads. ...
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Did you mean "single phase"? It is my understanding that harmonics do not add in single phase systems. The neutral overload as a result of harmonics is only a three phase issue.
 
don_resqcapt19 said:
Did you mean "single phase"? It is my understanding that harmonics do not add in single phase systems. The neutral overload as a result of harmonics is only a three phase issue.
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It was a 3-phase 4-wire system. The non-linear loads were mostly single phase connected line to neutral.
Single-phase non-linear loads generate the triple-n harmonics which sum in the neutral.

I mostly deal with three-phase power electronics and triple-n isn't normally an issue. You get 6n+/-1 harmonics which can also be troublesome.
 
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Besoeker said:
It was a 3-phase 4-wire system. The non-linear loads were mostly single phase connected line to neutral.
Single-phase non-linear loads generate the triple-n harmonics which sum in the neutral.
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Yes, the nonlinear line to neutral loads on three phase systems can cause excessive neutral loads on the neutrals of multiwire branch circuits and feeders.
I thought you were saying that these types of loads cause excessive neutral current on single phase systems.
 
how do non linear loads produce harmonics? what ways can we take the harmonics away if there really is a problem? what did you do at the job you done brian?

im not really too worried, but that panel was fed from another sub panel that also feeds more nonlinear loads. i hope nothing happens to the feeders. theres a 500kva 277/480v - 120/208 transformer that feeds a 1200 amp 120/208 volt switchgear. that transformer is fed from a breaker inside the main 3000 amp switchgear. the 500kva transformer is pretty hot. that 1200 amp switchgear feeds tons of computers and ALL of the 120 - 208 volt test equipment. i would say more than half of the loads in the place are nonlinear

also there is tons of 277 volt 2x4 fixtures and motors all over the place for the heating system and whatever else. there power factor must be pretty bad too. the fixtures are T-8 we replaced all the T12 and we added T5's
 
76nemo said:
I just have thee old Fluke 39 which went to the 41 and replaced by the 43B. There's a couple of guys here with the 43B.

The 39 doesn't have software for interface with a PC :(((
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im going to buy a nice PQA one of these days. just cant afford it right now and only thing my company has is the DM-II from amprobe. i dont know if you can use that as a PQA or not we just use it for data logging
 
Save up for a Ferrari :D:


435.jpg




http://us.fluke.com/images/Products/Industrial/Power_Quality_Tools/435.jpg
 
electricalperson said:
how do non linear loads produce harmonics? what ways can we take the harmonics away if there really is a problem? what did you do at the job you done brian?

im not really too worried, but that panel was fed from another sub panel that also feeds more nonlinear loads. i hope nothing happens to the feeders. theres a 500kva 277/480v - 120/208 transformer that feeds a 1200 amp 120/208 volt switchgear. that transformer is fed from a breaker inside the main 3000 amp switchgear. the 500kva transformer is pretty hot. that 1200 amp switchgear feeds tons of computers and ALL of the 120 - 208 volt test equipment. i would say more than half of the loads in the place are nonlinear

also there is tons of 277 volt 2x4 fixtures and motors all over the place for the heating system and whatever else. there power factor must be pretty bad too. the fixtures are T-8 we replaced all the T12 and we added T5's
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The problem with harmonics and transformers is that if they are delta/wye transformers (most are) then the higher frequency of the harmonic currents never leaves the delta side and causes higher currents in the core causing it to heat up. if the problem is severe enough and you cannot eliminate the harmonics the use of a "K" rated transformer is recommended. In addition, as many a one neutral per phase may also be required due the the fact that the 3rd and all 3rd 3rd harmonics add in the neutral (These are called triplens).
 
76nemo said:
That's a three phase PQA:wink:
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i read the product notes for the 43b and it says it can calcuate 3 phase stuff from single phase measurements. i might just save up the 8000 dollars for hte 434. the guys at work will think im nuts for spending that much money on a meter :D maybe ill buy a cheaper one first :wink:
 
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