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In a liberalized electric market, the knowledge of the system status is required to deal with management and control issues, such as energy dispatching and protection coordination. In this paper, a procedure for the harmonic source estimation aimed at giving distribution network operators information about the possible presence of harmonic producing loads, along with an indication about the reliability of such information, is studied in detail. The methodology exploits the few real-time measurements existing in distribution systems and all the other available information to provide an initial estimation of the harmonic currents injected by the nonlinear loads. A great attention has been paid to correctly model, in terms of probability distributions, the a-priori knowledge about the harmonic behavior of the loads. This is because the validity of the results strictly depends on the validity of the input data. The practical applicability of the method is discussed with the help of both computer simulations relevant to an IEEE benchmark grid and experimental tests performed on a small low-voltage distribution network in the Measurement Laboratory at the Department of Electrical and Electronic Engineering, University of Cagliari.