Harmonics test

[Eduardo Maun]

Greetings!

Two different Company conducted harmonics test. But they got different result what are the factors affecting the harmonics of an Electrical System aside from Speed,Current,Voltage/Load?

Thank you.

God Bless...

 

[StephenSDH]

I am not sure what "Speed" is. If you do have an issue it will be because of your loads. If your issue loads are engergized / not energized you will get a different results.

You should post the results from you surveys.

 

[Besoeker]

Greetings!

Two different Company conducted harmonics test. But they got different result what are the factors affecting the harmonics of an Electrical System aside from Speed,Current,Voltage/Load?

Thank you.

God Bless...

Greetings reciprocated!
Lots of things can affect the magnitude and frequency of the supply harmonic content. Mostly is is non-linear loads like variable speed drives, rectifiers, almost almost all electronic equipment (computers, chargers etc), possible resonances...
If two different companies did the tests at different times and/or over different periods, it would be surprising if their results did match.

In my experience, it is usual to measure both the current and voltage harmonic content. I have known cases where the voltage distortion on the supply into a plant was greater than the utility company would permit without any of the plant running. In such a situation, if your brief is to ensure compliance you are on a bit of a sticky wicket.

 

[robbietan]

Greetings!

Two different Company conducted harmonics test. But they got different result what are the factors affecting the harmonics of an Electrical System aside from Speed,Current,Voltage/Load?

Thank you.

God Bless...

harmonics are tricky things, the difference in results could be explained by the loading of the facility tested if the testing was done on different days. if the tests were done at the same time, the difference could be in the meters that they used

harmonics in a system can be affected by the amount of harmonic loads and the amount of non-harmonic loads in the system. which is why we do harmonic monitoring for at least 7 days, just to have a good idea of how much harmonics are present in the system

 

[brian john]

IMO it is best to trend over time, watching for current and voltage comparisons along with notes made by production folks regarding equipment running. Additionally as noted is is a good idea to have an idea who your neighbors are, as they can contribute to the harmonic issue.

 

[PowerQualityDoctor]

Duration of measurements and values

Duration of measurements and values

Many people will say that one week measurement is a minimum to have accurate results, but if you can control which loads are running, you can have better analysis in shorter time.

Harmonic values are combination of different parameters, both from the utility side and the loads. Normally, the major changes are in the factory. I recommend to try different load conditions in short time to find the highest pollution level. This allows detecting potential problems. If the two have different values - peak the highest.

Another issue is the differences. Do they have different THD or different TDD? Comparing THD is useless - TDD is the only parameter that counts. You can have very high THD with very low pollution.

 

[zwodubber]

It is difficult to set a specific amount of time for monitoring harmonics when doing an overall power quality test. The tests I perform run anywhere from 24 hours to 2 weeks depending on the customers facility and what we are trying to accomplish.

Below is a shot of a facilities harmonics measured over a 48 hour time period. The customer requested that the test be done specifically over these 2 days as the heaviest loads are present.

This graph is from a dranetz power visa.

 

[robbietan]

Many people will say that one week measurement is a minimum to have accurate results, but if you can control which loads are running, you can have better analysis in shorter time.

Harmonic values are combination of different parameters, both from the utility side and the loads. Normally, the major changes are in the factory. I recommend to try different load conditions in short time to find the highest pollution level. This allows detecting potential problems. If the two have different values - peak the highest.

Another issue is the differences. Do they have different THD or different TDD? Comparing THD is useless - TDD is the only parameter that counts. You can have very high THD with very low pollution.

THD is the standard to which harmonic pollution in voltages that utilities adhere to. having a high voltage THD could affect the operation of certain electrical and electronics equipment - an electronic timer for one.

TDD is true for current harmonics. a high current harmonic THD is not a concern if the current TDD is low. which is why utilities use TDD in their current harmonics standards.

 

[G._S._Ohm]

Below is a shot of a facilities harmonics measured over a 48 hour time period.

Showing odd harmonics, which is a triangle wave in the time domain. . .

 

[Electric-Light]

I had to look up TDD. THD is only relative to the load you're measuring, while TDD is cumulative.

An ordinary CFL has a THD in excess of 100%, which doesn't seem to make sense but thats how it works out in the math.

Of course, the TDD of having a few CFLs in the office in an industrial plant is miniscule.


If each lighting ballast in a store had a THD of 75% though, it could have a significant TDD. (Total Demand Distortion)

http://ecmweb.com/design_engineering/electric_understanding_harmonic_indices/

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