A New Combined System of Active and Passive Filters Aiming at Harmonic Compensation for Large Capacity Thyristor Converters.

Shunt passive filters, which are connected in parallel to harmonic-producing loads, have been widely used for harmonics suppression in power systems because of low costs and high efficiency, although they have inherent problems such as series and parallel resonance. On the other hand, shunt active filters, which are also placed in parallel, have such an inherent problem as initial and running costs are high, compared with those of shunt passive filters. To solve these inherent problems in shunt passive and shunt active filters, the authors have proposed a combined system of a shunt passive filter and a small rated series active filter.The purpose of this paper is to reduce the required rating of the series active filter, considering a practical application of the combined power filter to harmonic compensation for large capacity thyristor converters. The points are summarized as follows:(1) The required rating of the series active filter can be reduced less than one-tenth by connecting a series resonant circuit in parallel to the secondary of current transformer.(2) It is clarified theoretically and verified experimentally that compensation characteristics of the new combined system are equivalent to those of the already proposed system with no series resonant circuit.