DESIGN OF AN ACTIVE DC FILTER FOR A HVDC SYSTEM: INTRODUCTION(part 4)

5.3 Limitations of Passive Filters

• The source impedance influences the compensation characteristics of the passive filters, which is not known and varies with the system configuration.

• The Passive filter act as a sink to the harmonics of the source voltage and harmonics generated elsewhere on the ac system.

• At specific frequency there is always an anti resonance between the source impedance and the passive filter, called load harmonics amplifying phenomenon.

• Designing is a trivial task so that sometimes it became so complex that practical meaning of using it will be lost.

• Constraint on the free choice of the values of the filter component there by making the design complex.

• Deviation of the ac fundamental frequency leads to the considerable mistuning of the filter circuit.

5.4 Active filter

The concept of shunt active filter was first introduced by Gyugyi and Strycula in 1976.Now a days a shunt active filter is not a dream but a reality and many shunt active filters are in commercial operation all over the world. Their controllers determine in the real time the compensating the current reference and force a power converter to synthesize it accurately. In this way the active filtering can be selective and adaptive. In other words a shunt active filter can compensate only for harmonics current of a selective harmonics, linear and non linear loads and also can continuously track changes in the harmonic content. A shunt active filter can be properly controlled to present selective compensation characteristics. In other words it is possible to select what current is to be compensated.

A shunt active filter generally consists of two distinct main blocks:

a. The PWM converter

b. The active filter controller

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