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This paper presents the investigation results of voltage harmonic transfer accuracy problems through voltage transformers which are widely used in power quality monitoring systems in medium and high voltage grids. A simplified lumped parameters circuit model of the voltage transformer is presented and verified by simulation and experimental investigations. A number voltage transformers typically used in medium voltage grid has been tested in the conducted electromagnetic interference frequency range up to 30 MHz. The obtained results confirm that broadband voltage transfer function of the voltage transformer usually exhibits various irregularities which are primarily associated with voltage transformer winding parasitic capacitances.