Reduction of the harmonic currents in LCC HVDC systems
That is incorrect to assume the current is sinusoidal.
Current is defined when rectifier conducts when VHDC is less than VHAC ie. when the rectifiers are forward biased.
Knowing Ic=Cdv/dt and Resistive damping results in ripple . I do not know the ripple specs but for 10% ripple induced the current crest factor Peak / average is 10x. More rectified phases raises the average so the crest factor ratio and harmonics reduce.
The line impedance affects conduction pulses so reactor  filtering is required and harmonics are now &gt; 6x line frequency must be controlled.  This may include notch filters near the 11th or 12th harmonic.
Anecdotal
I am not an expert in HVDC yet this observation comes from fundamentals. (I am an expert in PD however on HV transformers and stuff alike) When I was a student in Winnipeg, I used to vacuum the ceiling of the 7 story scaffolding inside  Dorsey HVDC station to remove construction dust (very low levels already) with portable battery backpack vacuum machines to do the ceiling. This was to reduce the risk of Partial Discharge(PD). I also operated the wet-vac floor cleaners and applied concrete sealer daily. At that time English Electric was the primary contractor and they used Mercury valves 2m tall to operate as inverters.