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Figure 24 compares the impedance of a networkwith a 2 nd order damped filter to that of a networkwith a resonant shunt.Other types of damped filtersAlthough more rarely used, other damped filtershave been derived from the 2 nd order filter.c 3 rd order damped filter (see fig. 25a )Of a more complex design than the 2nd orderfilter, the 3 rd order filter is intended particularly forhigh compensation powers.The 3 rd order filter is derived from a 2 nd orderfilter by adding another capacitor bank C2 inseries with the resistor R, thereby reducing thelosses due to the fundamental.C2 can be chosen to improve the behaviour ofthe filter below the tuning frequency as well,which favours the reduction of anti-resonance.The 3 rd order filter should be tuned to the lowestfrequencies of the spectrum.Given the complexity of the 3 rd order filter, andthe resulting high cost, a 2 nd order filter is oftenpreferred for industrial applications.c Type C damped filter (see fig. 25b )In this filter, the additional capacitor bank C2 isconnected in series with the reactor. This filteroffers characteristics roughly the same as thoseof the 3 rd order filter.c Damped double filter (see fig. 25c )Made up of two resonant shunts connected by aresistor R, this filter is specially suited to thedamping of the anti-resonance between the twotuning frequencies.c Low q resonant shuntThis filter, which behaves like a damped widebandfilter, is designed especially for very smallinstallations not requiring reactive powercompensation. The reactor, with a very highresistance (often due to the addition of a seriesresistor) results in losses which are prohibitivefor industrial applications.IZIΩPhaseWith resonant shuntrZ networkRLWith 2nd order damped filterf (Hz)CNeutralFig. 24 : the impedance, seen from point A, of a network equipped with either a 2 nd order damped filter or aresonant shunt.a Phaseb PhasecPhaserrrarbRRLLLaLbC2C2CCCaCbNeutralNeutralNeutralNeutralFig. 25 : other types of damped filters: [a] - 3 rd order; [b] - C type; [c] - double filter.7.3 Active filtersAn active filter enables neutralizing the effect ofa disturbance by the injection of an equal signalbut of opposite sign. Active filters are often usedin complement with passive filters described inthis chapter, thus forming hybrid filtering.Such filters are described in Cahier Techniqueno. 183 entitled "Active harmonic conditionersand unity power factor rectifiers".Cahier Technique Schneider Electric no. 152 / p.21