Control schemes and compensation characteristics of an active filter connected in series with a passive filter.

Conventional passive filters have been broadly used to suppress the harmonics in power systems. However, they have the following disadvantages; (1) Source impedance affects the compensation characteristics. (2) Antiresonance between the source and the passive filter amplifies the harmonic current of the source at the specific frequency. (3) No harmonic compensation is performed at the frequency to which the passive filter is not tuned. On the other hand, active filters which have been studied and put into practical use have the ability to overcome the disadvantages mentioned above, but their initial and running costs are high, because they need voltage-or current-source PWM converters having high speed current response.This paper presents a novel harmonic compensator consisting of an active filter and a passive filter which are connected in series with each other. The passive filter removes the harmonics of the load, and the active filter improves the compensation characteristics of the passive filter. So the proposed filter system can eliminate the problems of the passive filter used alone and needs a small rated active filter, compared with a conventional active filter used alone. Control schemes and compensation characteristics are discussed, and experimental results of a laboratory model are shown.