The presence of harmonic currents in electrical networks generated by non linear loads such as AC/DC drives, inverters, UPS systems, saturated reactors and transformers, etc., often cause problems to other equipment sensitive to high frequency currents.

Harmonic filters are often conceived as passive single stage units operating in only two states, on or off. Their sizing is based on the maximum harmonic current to be filtered and all power components, reactors, fuses, contactors, must be rated accordingly. Additionally on variable load applications for example DC motors operating between no load and full load, a power factor greater than unity may be produced which could cause considerable damage to sensitive electronic equipment. In general electricity supply authorities require an inductive power factor between 0.9 and 1 .0. The design characteristics of single stage filters make then difficult to modify in the event greater filtration is required since all power components must be changed.