This paper introduces a new measurement method for supraharmonic distortion with improved robustness against power frequency deviations. The proposed method is based on wavelet analysis and it is designed to analyze a 10 cycles measurement interval. Fourier-based methods, instead, must analyze a fixed 200 ms window in order to avoid apparent shifts of the supraharmonic frequency components. The results of the experimental measurements presented in the paper show that the Fourier-based method suggested in IEC 61000-4-7 standard produces results affected by the value of the power frequency. The proposed method, instead, provides results completely independent of the power frequency. This feature also removes the need of a previous high pass filtering stage, required for the IEC method. Moreover, the proposed method has the additional advantage of working on the same measurement interval as traditional harmonics (below 2 kHz), feature that can reduce the complexity of measurement instruments.