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During a fault, protection relays of the faulty line as well as adjacent lines react and record measurement data. The records of an adjacent healthy line can be used for line parameter identification. The special feature of the proposed method is the consideration of unsymmetrical transmission lines and the application of a high-accuracy signal modeling technique. Derived from a π line model, an estimation equation for determining the different impedances is developed. This equation contains the signal models of measurements as well as their time derivatives, which can be estimated very precisely compared to the classical approach based on filtering. By means of a simple simulation model, the accuracy of the proposed algorithm is compared to an approach previously proposed which is based on deconvolution and filtering methods as well as on the representation of a transmission line using time-varying phasors.