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A series resonance, between L and C, appears.As opposed to this resonance, which gives aminimum impedance, the parallel resonance isoften referred to an anti-resonance.The equation giving the frequency of the antiresonanceis: far =12 π (Lsc + L) CLsc generally being small compared to L, theequation shows that the presence of reactor L,connected in series with the capacitors, rendersthe frequency f ar less sensitive to the variationsof the short-circuit inductance Lsc (from theconnection points = busbars A).Series resonanceThe branch made up of reactor L and capacitorC (see fig. 18 ), form a series resonance systemof impedance: Z =r+j (Lω- 1/Cω) with:c a minimum resistive value r (resistance of theinductance coil) for the resonance frequency f r ,c a capacitive reactance below the resonancefrequency f r ,c an inductive reactance above the resonancefrequency f r , wheref r=12 π L CThe curves in figure 19 show the shape of thenetwork impedance, including the short-circuitimpedance and that of the LC branch, seen frombusbars A.The choice of f ar depends on Lsc, L and C, whilethat of f r depends only on L and C; f ar and f rtherefore become closer as Lsc becomes smallwith respect to L. The level of reactive powercompensation, and the voltage applied to thecapacitors, depend partly on L and C.The reactor L can be added in two differentmanners, depending on the position of the seriesresonance with respect to the spectrum. The twoforms of equipment are:c anti-harmonic reactors (for series resonanceoutside the spectrum lines);c filters (for series resonance on a spectrumline).XΩXΩPh1InductiveInductiverCapacitive0f (Hz)Capacitive0f (Hz)f rf rf arf arLIZIΩIZIΩCNeutral0rf (Hz)0~rf rf (Hz)f rFig. 18 : impedance of the rejector circuit. Fig. 19 : network impedance at point A.Cahier Technique Schneider Electric no. 152 / p.16