In this paper, a state estimation based method is proposed to determine harmonic current contributions of plants supplied from a Point of Common Coupling (PCC). By using Weighted Least Squares (WLS) state estimation, it is aimed to improve harmonic voltage measurements acquired from conventional voltage transformers, which are employed commonly in the power systems. Using measured harmonic currents and estimated harmonic voltages, harmonic contributions of all plants have been determined by employing the Norton equivalent circuits of the plants and the superposition theorem. Absence of reliable harmonic voltage measurements necessitates employing harmonic filter current measured from at least one of the plants. Using the new measurement set built after the estimation process, a model to determine the harmonic current contribution of plants supplied from a PCC has been developed.