11th Harmonic Only

[ZeroSeq]

Hello,

I have an interesting current waveform at an industrial facility that appears to be quite sporadic (potentially occurring only once or twice a week), which very clearly has high harmonic content. This oscillography is only captured as there are fairly sensitive alarm elements programmed in the relay (i.e. no faults are present). The interesting part is that there are *only* 11th harmonics present, at about 8% - 12% of fundamental for each event record. The next highest harmonic is the 3rd, at approximately 0.25%.

Does anyone have any thoughts on what type of load (or other phenomenon) can produce only 11th harmonics? Not 5th, 7th, 9th, or 13th - just the 11th.

 

[davidbeach]

What's the sampling rate for the measurements? A 1.5kHz sampling rate will give 11th Harmonics while ignoring the 13th.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[ZeroSeq]

Good point, although 4kHz in this case.

 

[jghrist]

Could the current be flowing into a single tuned 11th harmonic filter? Maybe a switched capacitor on the secondary of a transformer that is resonant with the transformer inductance at the 11th harmonic?