DC insulator has longer creepage distance than AC insulator. It has higher resistivity for insulation condition. There is big difference in design and manufacturing between DC insulator and AC insulator.
      Under effect of DC electric field fixed direction, ion inside insulation material moves, which changes the material and degrades electromechanical performance. In addition, flashover under DC voltage is mainly pollution flashover. It’s caused by uneven rain. Dust collection effect under DC condition is severer than under AC condition. And AC automatic arc extinction may not appear on insulator local arc, which may cause long duration of arc extinction. Arc may be bridge-connected easily. Therefore structural shape of DC insulator is different from AC insulator’s.