Ultra-low-frequency response of FDS and PDC method is useful for diagnosing moisture or ageing of oilpaper insulation

and studies have shown that dispersion exists in dielectric response curves under ultra-lowfrequency excitation. But there is not yet unified conclusion about the mechanism of low-frequency response. In this paper

by analyzing concentration polarization caused by carrier migration

function of polarization current under ultra-low frequency excitation is derived. Then the stochastic statistical model of carrier mobility inducing low frequency dispersion is established to explain low-frequency dispersion. By comparing with the experimental PDC data

it is found that the model conforms to the actual situation

show that:the low-frequency dispersion of oil-paper insulation are mainly caused by statistical concentration polarization.