A passive harmonic filter is built using an array of capacitors, inductors, and resistors. It can take the form of a simple line reactor or may use a series of parallel resonant filters to eliminate harmonics. Passive harmonic filters are also divided based on the way they are connected with the load.
i) A series filter: Here the filter is placed in series with the load and uses parallel components, i.e., inductors and capacitors are in parallel. This filter is a current rejecter.
ii) A parallel filter: The filter is placed in parallel with the load and its components are built in series. This filter is a current acceptor.
Based on the components used to build the passive filter, there are the following types:
A Band-pass filter is a common passive filter that is built using a capacitor connected in series with a resistor.
A High-pass filter has a resistor connected in parallel with a reactor. This helps in reducing the q value of the filter, which will in turn help reduce the higher frequencies.
A High-pass filter when used in combination with a band-pass filter will provide a solution for medium voltage and sub-transmission voltage networks, which have moderate harmonic distortions.
A C-type filter is used for complex loads, cycle converters and electric arc furnaces and is a special variation of the high pass filter. This filter will provide the load with reactive power and avoid forming parallel resonance circuits with the load.