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#1
11-26-2009, 09:03 PM
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Thd%
THD%
Total Harmonic Distortion Has anyone really look into this? I have been reading up on it,however most examples are of a electronic background. I did find this link which give it the power side of THD. However what is the norm for the THD% in a system, I will have to do more looking but could be different for every system. I ask because I was in a factory and I happen to look at the THD for the main electrical service and found the THD to be 427%  I looked at it because the plant had 6 small 60kvar capacitor banks and all blew up. The Number one fighter of capacitors is harmonics.
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MIKE SHAKE THE TREE Last edited by miketrek : 11-26-2009 at 09:20 PM. 
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#2
11-26-2009, 09:24 PM
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Re: Thd%
From the 435 Manual......
Positive sequence harmonics try to make a motor run faster than the fundamental; negative sequence harmonics try to make the motor run slower than the fundamental. In both cases the motor looses torque and heats up. Harmonics can also cause transformers to overheat. Even harmonics disappear if waveforms are symmetrical, i.e. as equally positive and negative. Zero sequence current harmonics add in Neutral conductors. This can cause overheating of these conductors. Distortion. Current distortion is to be expected in a system with non-linear loads like DC power supplies. When the current distortion starts to cause voltage distortion (THD) of more than 5 %, this signals a potential problem. K-factor: this is an indication of the amount of harmonic currents and can help in selecting transformers. Use the K-factor along with KVA to select a replacement transformer to handle non-linear, harmonics-rich loads. I have not recorded any of our sites where the THD is over 3%. The rectification equipment has a really high PF, so non-linear loads have not been an issue. Most of the AC powered equipment is served from an inverter bank anyway. 427%......that is unreal......Did you measure that with the 434 or 345?
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#3
11-27-2009, 04:20 AM
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Re: Thd%
Hey Tommy!
Thanks for the scoop. The readings came from a 3 phase power meter, connected to the main buss in the switch gear.....made by AB I was scrolling through, and looking into THD and yup.......427%
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MIKE SHAKE THE TREE
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#4
11-27-2009, 07:33 AM
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Re: Thd%
Quote:
I forget the term used but it's the THD's relationship to the load. I think that I posted here long time ago about a circuit in my home that has a RCA flat screen on it where THD is very high also but less than 100%. THat THD is with the TV OFF or when the current draw is very low. Turn it ON and the THD drops a large amount. Turn ON other loads on the same circuit and it continues to drop. Whatever term it is that I can't remember right now isn't anything that's found on any PQ device, it has to be calculated with a formula. I just found it and it's called "total demand distortion" or TDD.
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Bill Last edited by WPTSKI : 11-27-2009 at 07:38 AM.
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#5
11-27-2009, 08:59 AM
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Re: Thd%
Quote:
i have see high numbers like that but only on the current not the voltage. I think in florida anything more than 4 percent requires action from the utility company.
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#6
11-27-2009, 10:44 PM
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Re: Thd%
gee mike are we working on the same site
 ...i have a similar problem the installation was done 18 months ago and all 6 x 60 kvar caps are history...they all need to be replaced...i have instructed that i need to do further investigation before i replace them because something must have caused them to pop...capacitors should last longer than 18 months unless there is a problem in the system or design.
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#7
01-02-2010, 10:51 PM
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Re: Thd%
Quote:
TDD is calculated as: THD * Il / Ipeak Il is the measured RMS current and Ipeak is the peak demand current from the previous 12 months. TDD is useful in a different way compared to THD. Think of it this way -- if THD is high but the load is very low, that means a large fraction of a small amount of current is "distortion current". On the other hand, if the load of the facility is high and the THD is high, the TDD will consequently be high. This means the facility is either producing a lot of "distortion current", or is drawing it in from the grid.
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