# Harmonics produced by LCC converters

Considering an LCC HVDC system and that the flow of power is ALWAYS in the direction shown in the figure. I know that converters in general produce harmonics. My questions are:

1. Does this HVDC system produce harmonics only in the DC side in the middle and in the AC side circled in green? Doesn't it produce harmonics in the AC side circled in red too? I think not, but if the converters produce harmonics also in the AC side circled in red, why and how it do so, since that waves enter the converters and not the contrary?

2. Is the aim of the DC filters on the DC side to create a straight behavior of the voltage and current?

3. I know that in LCC systems, the AC harmonics currents are reduced. Since the voltage depends on the current, is there a decrease of the harmonics voltage too?

• 1) Harmonics will be produced everywhere. Mar 21, 2021 at 11:14
• @Andyaka why andy? Are you sure that the entering signal in the first AC/DC converter already contains the harmonics caused by the entering converter itself? It is too strange Mar 21, 2021 at 11:22
• Harmonics are always produced by any semiconductor AC to DC conversion. How big they might be is circuit dependent. Mar 21, 2021 at 11:25
• "But why they are produced also on the AC side marked in red?" The current drawn by the rectifiers / switchers will not be sinusoidal therefore there will be harmonics. Mar 21, 2021 at 11:44
• See this for the part with why on the red AC side. For 2) it's mostly yes (though different words are used), and for 3) yes, since the voltage will sag and swell depending on the nature of the load -- so if the load draws smoother currents, the voltage drops from the generating side are less. Mar 21, 2021 at 12:26

3. Point 1) shows what a nonlinear load that has no power factor correction (of any kind) does to the current. Since V=R*I, having those current peaks means greater voltage drops across the source which, in turn, can cause sags (sometimes potentially dangerous, e.g. in a weak grid). Improving the harmonic content of the current means less peaks which, in turn, means less sags, thus the voltage drops less. So, yes, power factor correction does improve voltage, too, though nothing is perfect: the switching regime and the parasitic inductances will cause additional filters, which, in turn, means more voltage drops. But, overall, the results can be much better than without, which is why there is a whole industry dealing with these matters.