Harmonics 1

[Boof87]

Hey, was wondering if anyone would be able to give me a bit of guidance on the analysis of THD%, having trouble discerning between which is causing problems and what isnt

boof

 

[rbulsara]

What is the problem? Harmonics are present every where, not all harmonics create problems.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Measuring the voltage THD will tell you what is on your system.
Measuring the current THD will indicate what is causing the distortion.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[magoo2]

I'm afraid you've worded the problem much like someone going into a doctor's office and saying "I'm sick. Tell me what's wrong?"

What problems are being experienced, or imagined?

What types of equipment are you concerned with?

Have you made measurements or are you just trying to learn about harmonics?

 

[cherry2000]

If you have the individual device contributions, you can calculate the total THD at PCC, using free softwares.

 

[LionelHutz]

There really is no guideline that says "this level of harmonic will cause trouble with this equipment". You have to figure out for yourself if you have a problem with harmonics unless, of course, the utility comes along and tells you that you do because you are messing their system up.

 

[VTer]

Typically, if you have harmonic problems on a system, your low order harmonics will be the problem. What those exactly are depends on the equipment that is causing them. For example, in a 6-pulse rectifier the harmonic of interest would be the 5th. Harmonics get very detailed very quick and your questions is just the opposite, so the best thing I can do is tell you to look at some power quality books and come back with a more detailed question. I recommend, “Power Quality” by C. Sankaran.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[cherry2000]

IEEE Std 519 has tables which give the allowable THD values at point of common coupling (typically the MCC/Switchboard Incomer). Now comes the calculation part. Use the excellent free software from Danfoss drives netsite:

http://www.danfoss.com/BusinessAreas/DrivesSolutions/Software+MCT31/MCT31+Downloads.htm

The values of harmonic contribution will be available from each equipment vendor. Some safe assumptions can also be made in case of missing data.

 

[Boof87]

Sorry wasnt the way was meant to come across that way. I no what my THD% voltage as within limits of the power authority, its more in regards to THD% of current, i've been told just because they may have a high individual % doesn't mean they cause problems. At the moment ive got a swithcboard humming and vibrating, seems as though it is coming from the 3 phase bus (neutral is run in a seperate section) running through the switchboard to a mechanical services distribuition section which has a fair number of vsd's. is it possible for high end harmonics been induced from the vsd's back down the phases to the bus causing the issue. Every time ive been to the site it isnt happening and mechanical services isolated which is why i think its the vsd's, i haven't yet had a chance to get a 434 or data logger on there to get real world value's, am i anywhere near the right track?

 

[marks1080]

vsd'd are notoritous for producing harmonics. I think you are on the right track. What would you do to fix this? Is it really a problem? Will the issues affect your motors?

 

[waross]

It just takes a loose bolt on an insulating mout to set a switchboard humming and vibrating. This is usually a current issue and the 60 Hz current will have by far the most effect.
Is the hum the normal 120Hz (Often called 60 Hz hum) that we are all familiar with? If harmonics are causing the hum and vibration it will probably be at twice the harmonic frequency.
Check your neutral for any loose mounts or loose ferrous metal near the neutral. Third harmonic currents flow in the neutral when they can. Unbalanced currents flow in the neutral. Investigate the possibility of single phase or unbalanced loads, and neutral currents in general when the plant is operating.
If you always have humming, that increases in volume when the plant is running, look for loose hardware. If you only have humming when certain equipment is running, check the neutral.
The first rule of trouble shooting is to properly define your trouble.
I believe that your trouble is humming and vibration.
To assume that your trouble is harmonics may cause you no end of grief if the problem is not caused by harmonics.
If you are still unable to monitor working currents, you may physically check the neutral supports and the surrounding metal for looseness, and inquire about line to neutral loads.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Boof87]

cheers for the info guys, just never seen anything like this before. its a 50Hz 240/415 system, the only reason was heading in the direction of harmonics was the vibration only seems to be coming from the portion of bus supplying the mechanical services and none of the sub boards have any filters for the VSD's. Will definatly check neutral mounts and loading. thanks again much apreciated.