Dielectric strength

Insulating fluids, usually referred to as transformer oils in common parlance, have to perform special tasks compared to engine lubricants or hydraulic fluids.

The extremely low-viscosity (ISO VG 7 to ISO VG 10), mostly naphthenic-based mineral oils must prevent flashovers between the live parts as dielectrics in transformers, switches and converters at voltages of up to 1100 kV. They insulate the high potential differences, impregnate fibrous insulating materials, usually special papers, stable to oxidation or contribute to arc quenching. In addition, they must safely dissipate the dissipated heat that occurs in the copper windings to the outside. The service life of insulating oils does not necessarily correspond to the service life of the transformers, which can be over 50 years.

Transformer oils age depending on temperature and time, becoming "acidic" and polar. Water from the air humidity or decomposition products from the paper also change the dielectric strength. Therefore, the performance of these oils must be checked again and again. The determination of dielectric strength is thus the central criterion for assessing the service life of transformer oils. This is defined as the voltage up to which no spark discharge occurs under well-defined conditions.

In the OELCHECK laboratory, a special test device is installed for testing dielectric strength according to IEC 156 or DIN VDE 0370/5. For the test, approx. 400 ml of transformer oil is poured into a test cup up to the upper edge without bubbles. 40 mm below the liquid surface there are two brass spherical caps. The distance between these electrodes is set to 2.5 mm using a spacing template.

Now the test cup is closed and the sample is first homogenized for 5 min at room temperature. A sinusoidal AC voltage is then applied to the electrodes and increased at a rate of 2 kV/s until the first spark discharge occurs between the electrodes. The voltage reached up to this discharge represents the result of the test.

The experiment is performed six times with the same filling of the cup. The arithmetic mean of the individual measurements is given as the test result. If the value of the breakdown voltage falls below the specified limits, an alarm is triggered immediately. In connection with the values for contamination by water (Karl Fischer water), neutralization number, dielectric loss factor and interfacial tension, the experienced OELCHECK tribologists then recommend a complete oil replacement, vacuum drying to reduce moisture or complete oil reconditioning by a specialist company.

The dielectric strength test for insulating oils is included from the "ISO 2" analysis kit on.