Nowadays, the presence of nonlinear devices cannot be neglected either for their growing numbers or for their effect on the distribution network. In particular, these devices will produce relevant harmonics, which can cause problems such as increase of losses, over-heating in the power transformer malfunction of the circuit switch and, generally, an untimely aging process of connected devices. As a matter of fact, the presence of harmonics is a power quality issue and this distortion phenomena needs to be strictly monitored. The voltage transformer (VT) is the most popular transducer in the power grid due to its robustness and low cost. The frequency response of these transducers is usually characterized by sweep sine test, which can assure their performance under sinusoidal condition, but not under distortion condition. Based on the standard EN 50160, a new characterization method is proposed in this paper, in order to test the VTs under distortion condition. Several VTs with different rated values have been tested. As a direct consequence, it will be shown that, in working conditions similar to the real ones, the use of VT for the measurement of power quality (PQ) index can lead to wrong interpretations and decisions.