harmonics

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blackknight6945

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I just completed a power quality test on building with power coming to 480 volt service panel showing 99.8% THD has the max and 35% THD min. Has anyone ever come across THD results that high?
 
If it is a very light load, the harmonics you read could be actual poor power quality or inaccuracy of your meter at low load. Either way, the result is not bad to the distribution.
When I perform a PQ study, I normalize the THD values by weighting them by multiplying them by the % of load for the full feeder.
So a circuit with a THD of 50% at 5% load is not as bad as a 20% THD at 80% load.
 
blackknight6945 said:
I just completed a power quality test on building with power coming to 480 volt service panel showing 99.8% THD has the max and 35% THD min. Has anyone ever come across THD results that high?
Click to expand...

High WHAT PHASE Voltage THD, Current THD, Neutral VOLTAGE THD, CURRENT THD.

Yes and no depending what you are measuring.
 
Percent versus Amps/Volts

Percent versus Amps/Volts

I agree, talking about THD as a percent is misleading. Get a number for harmonic current or harmonic voltage. A 20A circuit with 1A of load, all high harmonics, is not a problem. A 20A circuit with 16A of load and 1A of harmonics, is probably not a problem. A 20A circuit with 16A of harmonic current is a problem.

Also, harmonic current is less of a problem than harmonics seen in the voltage. A 20% harmonic current that produces 1% harmonic voltage may not be a problem, except if there is an overloaded transformer. It is when high harmonic currents cause high harmonic voltages that then cause grief for other equipment, that you have a problem.
 
beanland said:
I agree, talking about THD as a percent is misleading. Get a number for harmonic current or harmonic voltage. A 20A circuit with 1A of load, all high harmonics, is not a problem. A 20A circuit with 16A of load and 1A of harmonics, is probably not a problem. A 20A circuit with 16A of harmonic current is a problem.
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Or an unloaded transformer.
 
blackknight6945 said:
I just completed a power quality test on building with power coming to 480 volt service panel showing 99.8% THD has the max and 35% THD min. Has anyone ever come across THD results that high?
Click to expand...
Assuming you mean TDH for current, then a figure of 35% wouldn't surprise me. Today I completed an analysis of a piece of kit that had about 48% of just 5th harmonic. Even visually, it is a very distorted waveform.

I'd be interested to see what kind of waveform produces close to 100%
 
blackknight6945 said:
I just completed a power quality test on building with power coming to 480 volt service panel showing 99.8% THD has the max and 35% THD min. Has anyone ever come across THD results that high?
Click to expand...


Not trying to bust on you BUT, can I ask how you can be performing a PQ study and not have an idea about what you are doing OR are you working under an engineer and are just curious?
 
No, I really don't know what I'm doing. I just decided to throw a PQ analyzer on a service panel for kicks. I asked a simple yes or no question.
 
Understand, when one has the test equipment and is unsure NOTHING beats doing what you did. I have learned a lot doing the old hands on and then trying to determine what is this?

There are some excellent books available and quite a bit on the WWW that will offer detailed explanations, just remember that much of the data is from manufactures pushing equipment and is slanted towards the fact that global warning is due to harmonics
 
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