Can new oil fail a dielectric breakdown test?
The simple answer is yes, new oil can fail a breakdown test. Sometimes users suspect that their test set is faulty because it is failing new oil. When the test set is checked however, almost invariably no fault is found. The true situation is that the oil being tested really does have a low breakdown value, so the test results are actually correct, even if they are hard to accept.
The problem usually relates to the way the oil has been stored. Often oil is delivered in drums that are stacked on site in readiness for filling a transformer or other equipment. In hot climates, pressure rises in the drums when they are exposed to the heat of the sun, but falls at night when the ambient temperature drops. Sometimes this pressure cycling damages the seals on the drums, and they start to allow the penetration of moisture. It only takes a very small amount of moisture to degrade the properties of the oil to the extent that it fails the test.
To give an idea how quickly oil can deteriorate, after a drum of new oil is opened it is often less than three weeks before it fails a breakdown test.
In circumstances where such problems may occur, it is useful to verify the performance of the oil test set, so that the real situation can be quickly determined. One approach is to use a voltage check meter to ensure that the test set is supplying the correct voltage. An alternative method is to perform a breakdown test in air with the instrument electrode gap set as wide as possible. If the value obtained when the instrument is new or when it has recently been calibrated is recorded, this can be used as a reference value to confirm that it is still working correctly at a later date.
Note that some variation in the air breakdown test results are to be expected, due to changes in atmospheric pressure and humidity, but this is a good "quick check" that has the advantage of requiring no additional equipment. As a guide, some oil dielectric breakdown voltage test sets typically give a breakdown voltage of around 37 kV in air, while some test sets typically give a result of approximately 19 kV.
