Abstract The aim of this paper is to present results explaining the phenomenon of inductive current transformers (CT) additional secondary current distortion in condition of variation of their primary current rms value caused by dips and interruptions of the power line's voltage. To assess its accuracy of current transformation for such transients, harmonics spectrums’ of primary and secondary currents are compared. To ensure repeatability of measurement results with the usage of FFT algorithm, regardless of the beginning of the sampling process, the window of 16 fundamental periods consisting 4 dips/interruptions cycles is used. Additional CT's secondary current distortion caused by repeatable transients results in increased error of indirect measurement of rms values of power line's current higher harmonics and additional inaccuracy of power quality estimation.