Harmonics produced by LCC converters

Question

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:

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

@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 — Samuele Benito Di Gioia, Mar 21 '21 at 11:22
Harmonics are always produced by any semiconductor AC to DC conversion. How big they might be is circuit dependent. — Andy aka, Mar 21 '21 at 11:25
@Andyaka yes I already know that they produce harmonics. But why they are produced also on the AC side marked in red? — Samuele Benito Di Gioia, Mar 21 '21 at 11:34
May I suggest you simulate to find out. I'm not going to answer this because you can answer it fully yourself with the right tools and a detailed design. — Andy aka, Mar 21 '21 at 11:44
"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. — Transistor, Mar 21 '21 at 11:44
@Andyaka Yes, with the simulation I can verify the presence, but I want to understand the nature! Since harmonics born when we have not a ''clear'' wave, I understand the nature of DC harmonics since we have not a perfect straight behavior caused by diodes, but for AC side? why diodes distorce the signal in input and not only in output? — Samuele Benito Di Gioia, Mar 21 '21 at 12:15
Embed your simulated circuit into your question if you want an answer. I'm not giving a generalized answer. — Andy aka, Mar 21 '21 at 12:20
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. — a concerned citizen, Mar 21 '21 at 12:26
@aconcernedcitizen thank you very much! I have understood with your answer! Can you change your comment into an answer so I can close this question? — Samuele Benito Di Gioia, Mar 21 '21 at 12:32
It's best if the mods mark this as a duplicate, then. No need to repeat the answer. — a concerned citizen, Mar 21 '21 at 12:35
Does this answer your question? Why Does DC Load Draw Twice As Much AC Current Through Transformer? — a concerned citizen, Mar 21 '21 at 12:35
@aconcernedcitizen that question answers the first point, that is mainly the non-linearity of the diodes that causes the harmonics in the AC side marked in red. About the second and third question I think that that question doesn't answer them, so if you want you can answer this ;) Thank you for your help!!! — Samuele Benito Di Gioia, Mar 21 '21 at 12:41

score 1 · Accepted Answer · answered Mar 21 '21 at 15:07

The linked answer should answer it (you say you agree, I won't repeat everything in there). I'll add that there is no such thing as harmonics on the DC side. Harmonics are associated with harmonic signals, and DC is not harmonic.
Those are called DC link capacitors and their purpose is to store the energy for the next stage, such that the higher their value, the lesser the voltage fluctuations. There is a limit to the size of them, both in terms of physical realization, and as far as the control loops go (whose purpose is to maintain the voltage at a certain level).
The fluctuations need to be kept low because, usually, the AC-DC-AC chain implies the usage of a switching inverter. These use PWM to deliver the AC, and they rely on the power supply to be delivered by the DC link capacitors. Due to the nature of the PWM, a varying supply means a varying amplitude, other than sinusoidal, which acts as an amplitude modulation, introducing not only harmonics, but risking causing sags that can damage the loads (and not only).
Note that the DC links are not only capacitors, they can be inductors, too, since they, too, are reactive elements. In this case, the topology of the inverter changes, but the principle remains: having a large storage for the energy to maintain a steady delivery of voltage/current.
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.

Thank you very much again. I think that these are basic things very useful to understand a lot of phenomena, so you gave me some tools to improve my knowledge!!! I appreciate a lot this! — Samuele Benito Di Gioia, Mar 21 '21 at 16:10

Harmonics produced by LCC converters

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