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To assess the quality of electricity supply through the power networks it is necessary to use instrument transformers or other devices for measuring voltages and currents of considerable values. In many terminals the quality of electric energy is significantly deteriorated and this result in the occurrence of the conducted disturbances in the primary signal of the instrument transformer which cause deterioration of its metrological characteristics. The main indicators to determine the level of periodic waveforms distortion are RMS and average values, total harmonic distortion (THD), crest and shape factors. Laboratory tests consisted of determining these parameters in the system with and without instrument transformers for pre-defined and repeatable distorted waveforms of instrument transformers primary windings supply voltage. The tests were performed for voltage (1000V / 100V) and current (5A / 5A) transformers. On the basis of comparison between values of designated parameters used in estimation of the power quality, obtained for measurements of the same distorted waveforms, in measuring circuits with and without instrument transformers additional errors caused by their operation is determine. It was found that higher harmonic from disturbance signal in the primary circuits of both voltage and current transformers causes increase of errors during the transfer of the signal basic harmonic (50 Hz). Furthermore, in the case of disturbance signals which frequencies are higher than 50 Hz the decrease in transfer ratio and increase of the higher harmonic phase displacement after transfer by the instrument transformer is noticeable. It must therefore be concluded that during the transfer of distorted signals through the instrument transformers their errors are increased in relation to the value defined for sinusoidal signals which frequency is 50 Hz. This leads to additional error in determining the parameters of voltages and currents used to define the power quality.