Phasor Measurement Units (PMUs) may improve the line parameters estimation process, but the accuracy of the result suffers from all the elements of the PMU-based measurement chain. Systematic errors of instrument transformers can be included in the estimation model, but, in particular, current transformers are nonlinear and thus their modelling is not straightforward. In this regard, the paper analyzes the impact of current transformers on the line parameters estimation performance using a detailed description of the transformer behavior under different operating conditions.