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4.3 Arc furnacesArc furnaces used in the steel industry may be ofthe AC or DC type.AC arc furnaces (cf. fig. 8 )The arc is non-linear, asymmetric and unstable.It generates a spectrum including odd and evenharmonics as was well as a continuous spectrum(background noise at all frequencies).The spectrum depends on the type of furnace, itspower rating and the operation considered (e.g.melting, refining). Measurements are thereforerequired to determine the exact spectrum(see fig. 9 ).DC arc furnaces (cf. fig. 10 )The arc is supplied via a rectifier and is morestable than the arc in AC furnaces.The current drawn can be broken down into:c a spectrum similar to that of a rectifier,c a continuous spectrum lower than that of anAC arc furnace.I nI 11001010,1in %1004Continuous spectrum3,21,30,51 3 5 7 9 OrderFig. 9 : current spectrum for an arc furnace supplied byAC power.HVHVTransformerTransformerCableRectifierCableCableFurnaceFurnaceFig. 8 : arc furnace supplied by AC power.Fig. 10 : arc furnace supplied by DC power.4.4 Saturated reactorsThe impedance of a saturable reactor is varyingwith the current flowing through it, resulting inconsiderable current distortion.This is, for instance, the case for transformersat no load, subjected to a continuousovervoltage.Cahier Technique Schneider Electric no. 152 / p.11