Boost PFC input current has high Second Harmonic and Third Harmonic

[Longcircuit]

**Subject: High THD on 2nd and 3rd Harmonics of a 5kW Single-Phase UPS**

Hi everyone,

I'm measuring the input current harmonics on a single-phase 5kW UPS with a 120V AC, 60Hz input. I noticed that the Total Harmonic Distortion (THD) is high specifically on the 2nd and 3rd harmonics.the ups has a boost pfc and isolation transformer on input.

**My questions are:**

1. In general, aren't even harmonics (like the 2nd harmonic) usually a sign of a DC offset or asymmetry in the waveform?

2. Can such low-frequency harmonics be effectively filtered using an LC filter, or would other methods be more appropriate?

Thanks in advance for your insights!

 

[cupoftea]

I think "the "Line current dead zone" thing may well be your problem, because that deffo causes 2nd harmonic.
Its where the PFC cant control the current near the mains voltage zero crossings...and you get a wild current spike at every zero cross...that would be at 100Hz....so there is your 2nd harmonic. i think the LT1248 automatically solves this...but UC3854 does not.