HYBRID ACTIVE PASSIVE FILTERS

Figures 24.6, 24.7, and 24.8 show three types of hybrid active/passive filters, the main purpose of which is to reduce initial costs and to improve efficiency. The shunt passive filter consists of one or more tuned LC filters and/ or a high-pass filter. Table 24.2 shows comparisons among the three hybrid filters in which the active filters are different in function from the passive filters. Note that the hybrid filters are applicable to any current harmonic source, although a harmonic-producing load is represented by a thyristor rectifier with a DC link inductor in Figs. 24.6, 24.7, and 24.8.

Such a combination of a shunt active filter and a shunt passive filter as shown in Fig. 24.6 has already been applied to harmonic compensation of naturally-commutated twelve-pulse cycloconverters for steel mill drives. The passive filters absorbs 11th and 13th harmonic currents while the active filter compensates for 5th and 7th harmonic currents and achieves damping of harmonic resonance between the supply and the passive filter. One of the most import ant considerations in system design is to avoid competition for compensation between the passive filter and the active filter.

The hybrid active filters, shown in Fig. 24.7 and in Fig. 24.8 are rig ht now on the commercial stage, not only for harmonic compensation but also for harmonic isolation between supply and load, and for voltage regulation and imbalance compensation. They are considered prospective alternatives to pure active filters used alone.