Passive filters consisting of a bank of tuned filters and/or a high-pass filter have been broadly used to suppress harmonics because of low initial costs and high efficiency. However, passive filters have the following drawbacks:
1) Filtering characteristics are strongly affected by the source impedance
2) Amplification of currents on the source side at specific frequencies can appear due to the parallel resonance between the source and the passive filter
3) Excessive harmonic currents flow into the passive filter due to the voltage distortion caused by the possible series resonance with the source.
And with respect to active filters:
1) It is difficult to construct a large-rated current source with a rapid current response.
2) There are high initial and running costs.
And wrt hyrbrid active/passive combination:
1) The size of the passive filter is large due to the bulky ac capacitors (the rating of this filter represents about half of the rating of harmonic-producing load).
2) The control scheme is very complex and the safety in operation is reduced. When the active filter is not working, there are certain conditions when the passive filter can be
3) There are high initial and running costs.
My ulterior motive for posting this is to see what others think of this author's (Alexa, IEEE Trans. on Industrial Electronics V48 N6, Dec 01) take on active and passive filtering.