A analysis of PD in the void of three insulators; air, oil and resin. The relative permittivity (¦ r) of insulator is higher than void as shown in Fig.2. The electric field intensity in the void is higher than in the solid insulator. By this reason, the PD will be increased in the insulator after that the electric treeing occurred. The life time of insulator is reduced. The equivalent circuit of void is represented in Fig. 2. The capacitance at point C is connected in parallel with discharge gap. The capacitances at point A and point B are no void that have the drop voltage of V. The drop voltage can be calculated as equation (2)