An active harmonic filter is something like a boost regulator. The concept used in an active filter is the introduction of current components using power electronics to remove the harmonic distortions produced by the non-linear load. Active harmonic filters are mostly used for low-voltage networks.
There are three types of active harmonic filters based on the way they are connected to the AC distribution network.
i) The series filter is connected in series with the AC distribution network. It serves to offset harmonic distortions caused by the load as well as that present in the AC system.
ii) The parallel filter is connected in parallel with the AC distribution network. Parallel filters are also known as shunt filters and offset the harmonic distortions caused by the non-linear load.
iii) The hybrid filter is a combination of an active and a passive filter and could be of a series or a parallel configuration.
Apart from the above classification, active harmonic filters can be either 3-wire or 4-wire filters.
3-wire filters: Are power units that are large and typically used in variable speed drives and other such applications, which have a large number of non-linear loads.
4-wire filters: Are those that can filter the neutral conductor of the triple-n harmonics. These filters are used to eliminate harmonics that are generated by switch-mode power supplies and Information Technology equipment, typically commercial applications.