Triplen Harmonics

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The only thing which I am not able to understand is that how a 6 pulse VFD automatically removes the 3rd harmonic ?
Also as there is no triplen harmonic in VFD circuits, so in which circuits does the triplen harmonic exist?
Getting back to the basics, first of all there are no even harmonics in the VFD input currents because it's a full-wave rectifier.
There can be no triplen harmonic currents because as Hv&Lv mentioned these are zero sequence currents and would therefore require a neutral connection for them to be conducted. A 3-phase 6-diode bridge in a VFD does not have a neutral connection.
So if we eliminate the triplens (all odd multiples of the third harmonic: 3rd, 9th, 15th, 21st, ...) then we are left with the 5th, 7th, 11th, 13th, 17th, 19th, 23rd, etc. So as Besoeker3 mentioned these are at harmonic numbers n = 6k+-1, where 6k-1 are negative sequence and 6k+1 are positive sequence.
 
By the way, my comments above assume that the rectifier is getting balanced voltages 120 degrees apart on its line inputs. If the voltages are not balanced and phased properly, then triplen harmonics can start to appear because the diodes for each phase do not have the same current waveforms displaced from eachother by 1/3 of a cycle.
 
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The opposite is true; triplen harmonics are more of an issue on three-phase systems.


No, they actually add in phase because of the frequency. Look again at the pic I posted, at the dotted-line column.
I know...
it was a rhetorical question.
thats why I said zero sequence currents.
 
It is important to differentiate between 3rd harmonic voltages and 3rd harmonic currents.

The output of a normal 3 phase VFD is often _intentionally_ rich in 3rd harmonic _voltage_. The reason this is done is to shave the peaks off of the desired fundamental frequency output, thus permitting a higher output voltage for a given DC rail voltage.

However the 3 phase third harmonic voltages are all _in phase_ at each of the terminals, thus 'zero sequence', meaning that there is no third harmonic voltage _difference_ between the phases. This third harmonic voltage does not cause third harmonic current to flow in the _balanced_ three phase motor. Doesn't matter if it is wye or delta, what matters is that it is balanced.

Non-linear single phase loads on a three phase system can cause 3rd harmonic current flow. Again the 3rd harmonics are _in phase_. But in this case this means that the 3rd harmonic _currents_ sum on the neutral rather than canceling.

-Jon
 
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