With the improvement of voltage level

the insulation problem of power electronic devices in power electronic transformer（PET） is becoming increasingly prominent.If effective isolation and protection measures are not taken

serious partial discharge will occur.Long-term partial discharge will lead to insulation breakdown failure

which will affect the normal operation of electrical equipment and the safety of personnel.Therefore

a double shielded insulation shell is designed.The insulation structure can not only suppress the partial discharge generated by the power electronic device tips in the medium and high voltage modules

but also reduce the insulation distance to achieve compact layout

and also provide safety guarantee for operators.The equivalent model of the double shielded insulation shell is established.The electric field is calculated and the electrical performance is analyzed by combining simulation and experiment.The key parts that affect the partial discharge characteristic are determined

that is

the shape of the end of the metal mesh and the structure of the end of the zinc-sprayed layer.The partial discharge voltage level of the double shielded insulation shell is improved by using two optimization measures:making arc and building groove at the end of the zinc-sprayed layer.