This article presents how the analysis an insulation of high voltage transformer bushing. The purpose is to analysis the moisture in the bushing to evaluate the approach for maintenance. The analysis rely on the principle of the Polarization and Depolarization Current (PDC) technique which is a technique for analyzing the insulation of high voltage equipment to test high voltage transformer bushing for a case study. This method is performed by energize 1000 Vdc into a bushing in period of time and then switch off the circuit for discharge current. Finally we calculate the moisture of main insulation of high volt bushing as well compare with standard moisture of criteria. The method is useful of preventive maintenance plane of high volt bushing and reduce the failure in the power system.

The analysis of the grounding system overvoltage in High voltage substation will be held at Bumnetnarong substation Electricity Generating Authority of Thailand ( EGAT ) by using ATP draw for grounding system’s model of substation also to model overvoltage from lightning and to read the result of overvoltage from differences positions. The advantages of the experiment are to be educated and to who design Grounding system in high voltage substation EGAT. The Study’s result of the over voltage level on Ground Potential Rise: GPR model shows that the pie will show close result to the result from the calculation by standard IEEE Std 80-2000 and the result of the Fault Current at the middle position of the grounding system will result in higher overvoltage than at the result at the corner position of Grounding system. The result of Ground Resistance: Rg shows that the increasing of overvoltage the increasing of Ground Resistance: Rg