The testing result shows that crude oil indicates D 8,0 (black colour), with moisture content TRACE () which is 4 ppm. The value of 6-time breakdown voltage test in the temperature of 22°C indicates: kV, kV, kV, kV, 09,5 kV, kV with dielectric power kV/mm. | The nynas nitro libra oil testing as an isolator on the 150 KV transformer using colour water content and breakdown voltage testing method in bantul substation