In partial discharge phenomenon; because the grading material is non-linear it is expected that the current in the grading is significantly non-sinusoidal, containing higher harmonics as well as the fundamental component. Since the material can reasonably be taken to have properties independent of the polarity of the voltage, the current waveform must have positive and negative parts that are simply negations of each other. According to basic Fourier analysis in this situation (equalizing of positive and negative portion of periodic variation signal) the odd harmonics are therefore the only expected components of the harmonic spectrum, and this was confirmed by the measurements.