DC breakdown properties in large oil gaps with and without pressboard interface

Dielectric breakdown values were measured in 15-100mm gaps in mineral oil and two types of natural esters. The test included breakdown properties in oils and creeping discharge over pressboard immersed in oils under negative and positive direct current with a boost speed of 2kV/s. The test used point-plane electrode system. It has been found that except the positive breakdown voltage at small gaps under the experimental gap, the insulating properties of mineral oils are superior to the two natural esters. The DC properties of the two natural esters are almost the same, and each has its own advantages.