Corona Detection from Distortion of Source Voltage Waveform

Recently I conduct a research on corona detection from the voltage and current waveform distortion. So far what I got is that there is total harmonic distortion on voltage waveform of source voltage that caused by corona but the difference is only small amount less that 2 %. Is there was something wrong with my procedure? Because I have hypothesis that corona could effect the source voltage waveform bigger than that.

thank you very much..