Hi,

To my knowledge, Active Filter is more reliable

The active filter is more reliable in the AC side HVDC

HVDC links are used for bulk power transfer, so these are connected to atleast 220kV or 400kV AC transmission system. At that voltage level, the active power filter (which has to be essentially a PWM Voltage source inverter) may not be feasible, even with isolating transformer. Moreover, the grid tied PWM inverters injects voltage harmonics in the system, that is also undesirable at that voltage level. Active power filters are mostly used at distribution level, as PWM can be used at distribution voltage level. 

Thats why the FACTS devices which are commissioned at transmission voltage levels are all fundamental switching frequency Inverters not PWM. They use multipulse and multilevel Inverters.

active filters are not practical in HVDC

why active filter dont practical in case of hvdc??? #Sabah Vs

Active filters are frequently considered for HVDC systems because they provide superior harmonic performance and avoid problems which are often encountered in passive filter applications,but also more costly!