Insulation on the transformer is a very important component. So that the quality of the prisoner in the transformer insulation is very important. To find out the condition of the isolation transformer must do some testing, namely poalarization index, delta tangent and break down voltage. At the writing of this thesis, the three tests were carried out at Transformer 1 at the 150 kV Substation of Bantul. Polarization index testing itself is a test of leakage current in the transformer insulation by comparing the test results for 1 minute with 10 minutes diving test results. While the tangent delta is a test of capacitive leakage current on the transformer isolation. And break down voltage is a test of the breakdown voltage capability in transformer insulation oil. Based on the results of the analysis, it can be seen that the transformer polarization index value in 2016 and 2018 has an average rating between 1.25-2.0. While for the delta tangent test results in 2016 and 2018 the value between the capacitive parts is already at a rating of 0.18% -0.36%. And for the break down voltage test results in 2016 and 2018 the average breakdown voltage is 43.9 kV / mm-65.5 kV / mm. From these data it can be concluded that the conditions of transformer isolation in 2016 and 2018 are in good condition. Although there was a decline in the condition of transformer insulation in 2016. However, the PLN has made repairs and maintenance of the transformer isolation. This is seen because of the increase in the quality of the transformer insulation in 2018. The repair and maintenance of the transformer insulation is done to maximize the working system and the life of the transformer.