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The increase of nonlinear loads (NLLs) in supply networks has led to an increase of harmonic content in supply currents. To keep current distortion within the limits fixed by international standards, several filtering solutions have been proposed, the most advanced consisting of active power-line conditioners (APLCs) which cancel the reactive components and harmonic currents. The control and sizing of APLC equipment are based on the hypothesis that NLLs behave as “current sources” whose ripple may be canceled by injecting a counter-phase set of harmonic currents. Nevertheless, the internal impedance of the distribution network influences the real behavior of NLLs, so that when the APLC controller tries to cancel harmonic currents at a global level, harmonics generated by the NLLs increase. This paper is devoted to the analysis and quantification of this phenomenon related to the attenuation phenomenon and the attenuation factor.