PCC - VFD harmonics

[mayanees]

I'm measuring harmonic distortion caused by VFDs at the Point of Common Coupling at an industrial facility - in order to verify that the system is in compliance with IEEE-1592.
My question is - at what frequency would a given drive exhibit its worst harmonic distortion?
Logic says it will occur at the lowest frequencies - but it would be nice to hear from some of our resident experts.
I expect to vary the frequency from startup to 60 hertz, since this will be the real world application.
Thanks for any input.
John M

 

[winnie]

The harmonic distortion that you are looking at is caused by the VFD _input_ rectifiers, not the output.

I would expect this distortion to change with the over-all load on the drive, which is of course dependant on output frequency and load characteristics.

I am guessing here: as the load goes down, the rectifier conduction periods will get shorter, and the % harmonic current will go up. Also, as the load goes down, there is less total current drawn, so I would expect the total harmonic current to go down, even as the % harmonic current goes up.

-Jon

 

[Besoeker]

I'm measuring harmonic distortion caused by VFDs at the Point of Common Coupling at an industrial facility - in order to verify that the system is in compliance with IEEE-1592.
My question is - at what frequency would a given drive exhibit its worst harmonic distortion?
Logic says it will occur at the lowest frequencies - but it would be nice to hear from some of our resident experts.
I expect to vary the frequency from startup to 60 hertz, since this will be the real world application.
Thanks for any input.
John M

The input harmonic current spectrum will vary with speed and loading. At low speed and low load and discontinuous current the proportion of harmonic current in relation to fundamental but maximum voltage distortion will usually occur at maximum loading.

 

[Jraef]

This is why there is (was?) a movement to change to looking at what is called Total Demand Distortion, because as was said, the current distortion itself increases inversely to speed, but the total current is decreasing at the same time, so the negative effects are in fact decreasing. But IEEE had not yet made that change AFAIK.

 

[mayanees]

Thanks guys. That's just what I was looking for.
jraef, I found this article when searching out the TDD vs THD difference, and their contention is that TDD is what is intended to be used in the comparison calculation, although the basis for the statement isn't referenced.

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

I had an IEEE 519 copy but do not now. I'll get one to make sure the calc I do is appropriate. But my approach for determining if the facility meets the standard will be to measure at the PCC when the facility is running in a peak-demand level time of day, getting a 15-minute demand number along with the associated current and voltage distortion numbers. If the measured V and I distortion are within limits, the test is over. If not, I'll drill down to individual drives to determine the best possible areas for improvement based on size and distortion measurements. This of course means that I will have determined the strength of the electrical system at the PCC prior to the test.
This is all brought on by a specification that holds the manufacturer of the drives to meeting the requirements of IEEE 519-1992.

Thanks again jraef and besoeker.

John M