Passive harmonic filters are series filters (which means that current goes through the filter) that are used to reduce or mitigate, harmonics to tolerable levels as defined by IEEE-519.
Passive filters use the following items to ‘filter’ (or clean) the current wave:
Input reactor
Output reactor
Shunt reactor
Capacitor
 
A passive filter consists of a series circuit of reactors and capacitors. Harmonic currents generated by, for example, a frequency converter are shunted by this circuit designed to have low impedance at a given frequency compared with the rest of the network.
 
Among several harmonics solutions, a passive filter is a very common and effective mitigation method. Passive filters are designed to provide a low impedance shunt path for harmonic currents. In this way, the harmonic currents are deflected to the ground. Another function of passive filters is to suppress the flow of harmonic currents between parts of the system by tuning the passive elements to create resonance at a single frequency or a band of frequencies.
Passive harmonic filters generally consist of capacitors, inductors, and resistors. An array of these elements is arranged in one or more shunt arms to form different topologies. Among popular topologies of passive filters are single tuned, second-order, third-order, and C-type filters.