The most recent IEC standards about voltage transformers warn about nonlinearity, which may significantly impact the harmonic measurement performance. However, there is a lack of scientific literature about this topic: usually their characterization consists of frequency response measurements that are clearly not able to capture the nonlinear behavior. In this paper, an innovative approach based on recently developed simplified frequency-domain polynomial models is proposed. The method is applied to two different medium-voltage inductive transformers. Models are identified and validated with a large set of realistic primary waveforms injected by a proper setup. Experimental results confirm the remarkable accuracy of the proposed models especially for low-order harmonics, which are the most affected by nonlinearity.