In order to investigate the characteristics of the partial discharge（PD） defects on the Nomex paper used for mining dry-type transformer insulation with cavity

we theoretically analyzed the relationship between the electric field strength or partial discharge inception voltage of Nomex insulation and the type and dimension of cavity. The electric field and space charge distribution of cavity within Nomex insulation were simulated before and after partial discharge occurred in COMSOL Multiphysics. Then the model with artificial cavity in Nomex insulation was prepared. Based on this model

continuous tests using the constant voltage method were developed. The partial discharge characteristics were measured in the laboratory. Finally

the cavity discharge mechanism was analyzed. The results reveal that the electric field strength distribution of cavity in the Nomex

as well as the inception voltage of partial discharge

depends on the type and dimension of cavity. Moreover

partial discharge in the cavity is closely related to the distribution of both the electric field and the space charge. In addition

It is also concluded that the accumulative total discharge and average discharge energy per second in a stable trend and followed consistent rules are best indicators that are able to characterize the severity of the defects. This results has been applied to the pattern recognition and insulation life assessment of insulation for a 10 kV dry-type transformer in a coal mine and it has great application significance.