Current Harmonics based on Source Impedance

[VTer]

Hi All,

I posted this question on the Motors & Generators Forum and it got no traction, so I will try it here.

Does anyone have any reference on calculation of harmonic currents and THD-i for 6-pulse diode rectifier based on the impact of source impedance?


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

 

[DM61850]

Each VFD has a profile based on how many pulses it has. You probably can get a pretty good idea of what harmonics the recifier produces based on how many pulses it has.

I don't know if you can back up what the harmonic currents will be based on the source impedance. You can have series and parallel resonance and you are not going to be able to determine that from the source impedance.

In my limited experience, your step down transformer usually does a good job at keeping harmonics inside of the plant. The transformers inductive impedance will scale (2*pi*L*f) with the frequency. As you move up in frequencies, it starts becoming really hard for the harmonics to escape the plant. If you have a capacitor bank installation near by, you can still have resonance. (1/(2*pi*(L*C)).

I believe you want someone to do an evaluation of everything on the grid that is electrically near you.

 

[VTer]

Hi DM61850,

Thank you for your reply. I have seen VFD manufacturers for example publish tables that show what the individual harmonics will be as a percentage of fundamental based on variation of effective source impedance. For example, for the same 6-pulse drive, they publish data from somewhere 0.25% effective impedance to 12% for prevalent harmonics and THD-i. I was wondering if anyone knew how those numbers were established.

I am trying to understand how to calculate the magnitude of the current harmonics for a six pulse diode bridge rectifier?

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

 

[7anoter4]

See for instance:
Control of Harmonics in 6-Pulse Rectifiers

https://digitalcommons.du.edu/cgi/viewcontent.cgi?article=1527&context=etd

 

[DM61850]

Vter,

I think you want to get a copy of the IEEE Brown Book and IEEE 519. The brown book has a section in it explaining harmonics, how to model equipment, and how to mitigate issues. IEEE 519 has all the THD requirements for defining healthy voltages and currents.

When I did a study of a plant that was having issues, I modeled the entire plant and included a current source for the VFD, which generated all harmonic voltages and currents. The lower the impedance path for your harmonic, the less harmonic voltage is created inside of your plant. One quirk is that the odd triplen harmonic source through your grounding so your grounding will impact the path for those currents and their impact on your system voltages. That study was compared to what was found with a power quality meter and a harmonic filter was installed in the plant to mitigate the problems. Thinking on it because the VFDs are modeled as harmonic current sources, you should be able to get a rough idea of the harmonic currents with maybe an exception for the odd triplen harmonics.

 

[rcw retired EE]

Check IEEE Standard 3000.2-8-2018 " Recommended Practice for Conducting Harmonic Studies and Analysis of Industrial and Commercial Power Systems"

Table 2 in the standard shows the characteristic line harmonics for 6, 12, 18 & 24 pulse inverters.

For 6 pulse harmonics are: 5th: 20.0%, 7th: 14.20 %, 11th: 9.09%, 13th: 7.69%, 19th: 5.26, etc. up to the 43rd at 2.04%. These are ideal values for a perfect square wave rectifier, actual values will be lower. These are in % of fundamental current.

A high source impedance may decrease the current harmonics (higher reactance) but will increase the Voltage harmonics on the bus due to the voltage drop of the harmonic current flowing through the source impedance.

 

[mparenteau]

I like to think of current/voltage harmonic distortion this way:

1) Current distortion is path dependent
2) Voltage distortion is not path dependent

Mike